Tl,dr… let’s get straight into it:
- If you want to run appliances, devices and lighting in your campsite you’ll need a deep cycle battery, it’s not recommended to use your vehicle’s starter battery.
- To keep your drinks cold you’ll need to connect your fridge to a battery which will also need to be connected to a charging system i.e. a solar panel or running vehicle… you can’t skip the battery part, it needs to be in the system.
- There is some basic arithmetic you can use to work out how much battery power you need to power your devices, and how much solar or vehicle power you need to keep that battery topped up.
With a bit of knowledge, you’ll be powering your campsite in no time! Image: Hard Korr
Common questions about a 12V and solar setup:
What size battery do you need to run your fridge & how long will it run for?
How do I connect my solar panel to my fridge?
How many solar panels do you need for a caravan? And…
Can I run my fridge from my car’s starter battery?
All valid questions that we receive almost every day from customers starting out with 12V and solar power.
The information below answers all these questions, so by the end you’ll be armed with enough knowledge to label yourself a 12V ninja!
But first…
Solar panels are a key part of your setup. Image: Hard Korr
What’s the difference between volts, watts and amps
These are the three electrical ratings you’ll find stamped on the compliance stickers of electrical appliances.
You’ll need to understand the basics of these three ratings, so here’s a quick analogy:
A battery (volts) forces an electrical flow (amps) through a cable which produces power (watts) to run a device.
For the purpose of this article, all you really need to know is:
Volts (V) – 12V the amount of force your car battery and alternator delivers.
Amps (A) – Think of this as the volume of electricity the battery is pushing through a cable.
Watts (W) – A measure of power that is easily converted to Amps with the following formula: Watts ÷ Volts = Amps
Amp-hours (Ah) – Calculated by multiplying Amps by hours to represent consumption over time. Deep cycle batteries have a rating that specifies how many Amp-hours (Ah) they contain.
So, what do you need for a 12V solar-charged setup?
There are three components to a basic 12-volt solar-charged setup:
1. The devices you want to run.
2. A battery to power the devices you want to run (generally a deep cycle battery).
3. Something to charge the battery that is powering the devices you want to run.
You can’t skip step 2… you need a battery in your system!
You’ll also need a solar controller between the solar panel and battery to regulate the charge going into the battery. These are generally integrated into or form part of the wiring that came with your solar panel.
The factory-installed cigarette outlet isn’t heavy-duty enough for high currents. Image: Companion
Can I run these from a factory cigarette outlet in my car?
Good question!
Factory-installed cigarette outlets are often made with lightweight housings and cables that won’t handle high currents or even medium currents for long periods without becoming hot.
These outlets are often rated to handle a maximum of 120W or 10 amps (using our formula from earlier – 120W ÷ 12V = 10 Amps) at any given time and are also generally only powered when your vehicle is on, making them useless at the campsite.
They are also wired to your vehicles starter battery, which leads us to our next common question…
How long will my fridge run from my vehicle’s starter battery?
Not nearly as long as a deep cycle auxiliary battery or power pack.
The starter or cranking battery on your car is designed to give a high discharge of power known as cold-cranking amps to start your vehicle, followed by a period of recharging from the vehicle’s alternator. A deep cycle battery, however, is much happier with a prolonged and slow discharge.
Whilst you could power your fridge for short periods from a cranking battery, we recommend only doing so whilst your vehicle is running, otherwise, you’ll be potentially calling for road service.
A battery box is an option that you have. Image: Hard Korr
Is the best solution to install an auxiliary battery or use a portable power pack?
In this article, I will refer primarily to a portable power pack, but the basics are the same for both of the following setups.
A portable power pack is generally not dependent on a vehicle so you can locate your solar panel and power requirements wherever you need it i.e. in a tent or camp kitchen. This is versatile and ideal if you don’t have space for a second battery under the bonnet.
Another option is to install a second, or auxiliary, deep cycle battery into your vehicle where you use your vehicle’s alternator to charge the battery. This hard-wired setup is ideal for those on the move every day. It can also be beneficial for those wanting to stay in one location for a period of time as it facilitates batteries with larger power storage, but you’ll probably need to include solar charging in your setup.
Setting up an auxiliary battery in your vehicle requires a little knowledge if you are doing it yourself, or an auto electrician will be able to install one professionally for you.
If you don’t have space for a second battery, a portable power pack might work for you. Image: Goal Zero
How do I work out my 12v power requirements?
Grab yourself a notepad and pen and write down all the Amp ratings for each of the appliances you want to run in your campsite, converting watts to amps with our formula from earlier.
Let’s assume (I’m making up figures here) you want to run a 12V fridge rated at 2.7A max, and 2 camping lights each rated at 0.6A.
With all of these appliances running non-stop they will be consuming 3.9 Amps. (2.7A+0.6A+0.6A = 3.9 Amps).
Consider though, that your lights will only be consuming power for a few hours in the evening and your fridge will only consume power whilst its compressor is running which will be cutting in and out to maintain temperature.
So, hourly power consumption will, for the most part, be far less than the maximum and will vary over 24 hours depending on the time of day, ambient temperature and how often you open your fridge.
Let’s look at three different times of the day, and just as a disclaimer I’ve made the figures below up for simple calculations:
Daytime – fridge only
Daytime running fridge only: 7am – 5pm = 10 hours
Estimate that fridge cycles: 15 mins on/ 15 mins off = 30 mins/hour – 2.7/2 = 1.35 Amps
Total consumption over 10 hours = 13.5 Amp-hours
In the day, with just your fridge running.
Evening – fridge and lights
Evening running fridge and lights: 5pm – 9pm = 4 hours,
Estimate that fridge cycles: 10 mins on/ 20 mins off = 20 mins/hour = 2.7/3 = 0.9 Amps
Camping lights: 0.6A each = 0.6A x 2 = 1.2 Amps
Total consumption of 2.1 Amps over 4 hours = 8.4 Amp-hours
In the evening, with both fridge and lights in use.
Nightime – fridge only
Nighttime running fridge only: 9pm – 7am = 10 hours
Estimate that fridge cycles: 5 minutes on / 25 minutes off = 10 minutes per hour + 2.7/6 = 0.45 Amps
Total consumption over 10 hours = 4.5 Amp-hours
At night, with just your fridge running.
If we now add the daytime, evening and nighttime consumptions together, we get our total approximate consumption for the full day.
Total – 13.5 Amp-hours + 8.4 Amp-hours + 4.5 Amp-hours = 26.4 Amp-hours
What size 12V battery do I need?
The standard Sealed Lead Acid (SLA) or Absorbed Glass Mat (AGM) batteries, which are the most common and affordable portable deep cycle batteries, are rated to 44 Amp Hours (44Ah). An in-vehicle or hard-wired setup is often around 100 Ah.
But, these batteries can only be discharged to 50% of their rated capacity which gives you 22Ah of usable power if we use a 44 Ah battery as an example.
Given that in our example above, our appliances are consuming 26.4 Ah over 24 hours, giving us less than a day of power from a 44 Ah battery and a little under 2 days out from a 100 Ah battery. So, we need some way to charge the battery after these timeframes.
Note that these figures are never exact. There are inefficiencies that need to be allowed for, so use these calculations as approximations.
Solar panels will keep your battery charged. Image: Hard Korr
How to keep your battery charged up?
If your battery is hard wired to your vehicles charging system then driving your vehicle every day will keep your battery topped up. Also, if you’ve also got solar panels hooked into your vehicle setup then you’re as good as ‘set and forget’.
If you’re using a portable power pack then you’ll need to plug it into your vehicle’s charging system whilst driving or, if you are staying at camp for the day you can use a solar panel.
Solar panels are generally rated in Watts, the higher the wattage the quicker they charge your battery. They are not perfectly efficient at converting the sun’s rays into power so we need to factor in a rough figure of 20% inefficiency rate.
Use the formula to figure out how to keep your battery charged.
Let’s do the maths
Referring to the diagram above, let’s assume you have a 120W Solar Panel. We go back to our formula again and divide watts by volts which gives us 10 Amps (120W ÷ 12V = 10A). If we factor in our inefficiency rate of 20% we are down to 8 Amps that your solar panel is delivering to the battery (10Amps – 20% = 8Amps).
But, remember that our fridge is still consuming 1.35Amps from the battery at the same time. The rate at which the battery is actually being charged in this scenario is the difference between input amps and output amps, which in this case is about 6.65Amps (8A – 1.35A = 6.65 Amps).
A solar panel is only going to deliver charge during the sunlit hours and when not in shade, so let’s assume that we chase the sun all day by moving the solar panel around and get 8 hours of sunlight.
So at 6.65 Amps for 8 hours, a 120W solar panel can potentially deliver 53.2Ah of chargeback into our battery (6.65Amps x 8hours = 53.2Ah). This is far more than the 22Ah we need to charge a depleted battery.
If we work backwards, we can work out roughly how many actual hours of sunlight we need to charge a 50% depleted battery whilst our fridge is still running by dividing the charge needed by the consumption.
22Ah ÷ 6.65A = 3.3 hours approximately to reach full charge whilst your fridge is running.
Make allowances for overcast days
The angle of the sun and the atmosphere all impact the amount of sunlight reaching a solar panel and reduce the output. In reality if your solar panel is in full sun for 8 hours you are likely only getting 4-6 hours of it’s potential capacity per day. Furthermore the output of the solar panel may be reduced to almost nothing on cloudy days, so it’s worth making sure you have a few Amp-hours in reserve.
Long story short, in the above situation, if the weather is clear and sunny and you are chasing the sun with your solar panel each day, you’d theoretically be set to stay in the one spot for as long as you want. If you are relying on your vehicle, then charge times will be much less as a vehicle’s alternator generally delivers a much higher current. You can usually find details on your alternators Amp output in your vehicle manual.
Once your 12V system is set up, it will serve you well. Image: Hard Korr
One last thing to note
One last thing to note is that solar and your vehicle’s alternator don’t always deliver a full charge in the same way a good 240V battery charger can. So, it’s worth hooking them up to one of these and giving them a full charge once every 6-12 months.
That may seem like a whole lot of information but in reality, it’s just the basics, there’s a lot more to learn about when it comes to setting up a 12V system in your vehicle.
For now… at least you can make sure your drinks stay cold!
How long has it taken you to come up with your ideal 12V power setup?
About the writer...
Trekker, surfer, climber, mountain biker, runner, camper. Participator in most things… master of none.
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Great read, thank-you.
I tend to stay in one spot for several days when I go camping, so a DC-DC charger doesn’t make much sense to me.
I will likely be relying 100% on solar to recharge batteries.
I camp year round and often in cold, overcast weather.
Would using 600w of panels and a 40A mppt work?
In ideal conditions I’d be making 50A – but do I then apply the 20% loss?
My reasoning is that on an overcast day I’d still make some decent power if I had “too much” panel.
Or do I risk burning out the mppt or controllers doing this?
Hi Gard,
I’m by no means an expert when it comes to the capabilities of DC to DC chargers and MPPT controllers, it would be worth consulting with the manufacturers or an auto electrician for some of the specifics. That being said, these are the things that do come to mind.
Some DC to DC controllers allow for solar input, Redarc in particular, comes to mind. That way you can use the same device to get the most from both DC and solar charge. Regarding your MPPT question, I’d probably work to the stated output of your panels and you’d be best consulting the requirements of the controller itself regarding what input it can handle, you may find it limits it to 40A even if there is more available but I can’t be certain of this.
Your theory on having larger panels to still get power on an overcast day is a good one though.
I hope that helps.
~ Ben
Hi Ben,
Great article, thanks, wonder if you could answer a question, I have 2 batteries in parallel isolated from the starter via a redarc. I want to add solar, do you reckon I need a DC to DC charger? I just don’t really see why as the MPPT regulator will cut out once the batteries are charged?
It’s a bit hard to comment here without know just how your system is set up, and even then I’m not sure I’d have the best answer sorry. If you have both a DC-DC charger and MPPT in your system whilst driving, then you may need to be careful about what voltages each is sensing before they cut out. I.e. if your solar is maintaining a high voltage at the MPPT then the DC-DC charger may cut out or vice versa, rather than actually detecting the battery voltage… I’m kind of thinking out loud here and questioning myself as I go.
If you just want portable panels that you can put out at the campsite, then I’d suggest just attaching these straight to your auxiliary battery when parked up, but if you are looking at a permanent solar system then it would be best to consult an auto electrician I reckon.
Sorry I couldn’t be of much more help.
Hi, thank you for this informative and accessible article! I have a question, however.
Assuming that I only need my portable fridge to work when there is sunlight (I know, an unusual setup), why is connectng my solar panel directly to the portable fridge “bad.” You mention that a number of times, but I didn’t quite see an explanation.
In doing this setup, I would ensure that the voltage and wattage coming from the panels (accounting for inefficiency) are appropriate for the specs of the fridge. I’m interested in doing a direct drive system that does not depend on batteries and only functions when there is sun. While this may be irregular, I’m very keen to understand if there are any particular reasons (other then a potential for sub-optimal freezing) why this is a bad idea.
Thanks!
I’m glad you liked our article, RTHM.
The output of a solar panel fluctuates, the open cell voltage can reach around 25V for a 200W panel which is much higher than batteries or appliances can generally handle. Having solar go through a solar controller and then stored in a battery means that there is always an appropriate and reliable power supply for the appliance.
Whether you can use the power directly from a solar panel to a device is not something I know anything about sorry. It’s an unconventional approach and you wouldn’t be using stored energy. In the instance the sun disappeared behind the clouds, your appliances would likely stop working.
I can’t really shed much more light on this for you sorry, it’s maybe a question for an auto electrician.
Hi Ben,
Ive just had a dual battery setup installed with a DCDC charger and will be using a 200w solar blanket to charge my battery when camped. I note that on the DCDC charger there is no voltage meter for the auxiliary battery as is sometimes present on other regulators. Is keeping an eye on my auxiliary batterie’s level of charge a matter of using my multimeter to manually check the voltage each day, to ensure the voltage is not dropping too low?
If your DC to DC charger doesn’t have any sort of indicator, then you may need to use a multimeter to check this, Toby.
However, if you are using a 200W solar blanket all day each day and getting good sun then it’s probably not something you need to check regularly as the blanket should provide ample power to keep it topped up… that is assuming you don’t have a super power-hungry setup.
I wired up a small voltmeter in the cabin of my car that you can get from most auto shops, it has a readout for both main and Aux battery.
I pick up the voltage from the auxiliary battery terminal and from the main battery side of a Redarc isolator (I don’t have a DC DC charger), then a good earth point inside the cabin. It may not be as accurate as a multimeter directly on the battery terminals, but it’s pretty close and gives me a quick reference to make sure the system is working as I am expecting it to.
Hi, just wondering if a 110w solar panel will keep a 100ah battery charged in order to power a 60l waeco fridge
G’day Julie, I’ve had a chat with Ben who’s not able to get back to you right away but we reckon it’s a bit touch and go. Yes, it could work but you’d have to commit to moving your panel around throughout the day to make sure it’s getting optimal exposure to the sun, plus you’d need to manage your fridge in the most efficient way possible. Ie. Use a cover, keep it shaded and in a cool position, and only open it when absolutely necessary. We’ve recently launched our Snowys Camping Show podcast with Ben and Lauren as hosts and in this week’s episode, they discuss ‘Staying Powered Up When Camping’. There might be some points of interest in there for you and you are welcome to jump into our podcast Facebook group where Ben or Lauren might be able to go onto more detail specific to your setup. Hope that helps, cheers!
https://www.snowys.com.au/blog/podcast-staying-powered-up-when-camping/
This is a great article!! Thanks so much.
What role does a battery box play? Are they worth it?
Cheers
There is no reason why agm type batteries can’t be discharged past 50%. That is a myth of folklore proportions.
Unfortunately this is not a myth. Yes you can run down your AGM batteries below 50%, it is just not advised, for two reasons:
(a) an AGM battery discharged to only 50% will last 500 recharges. If fully discharged it will last only 250 recharges, ie half the life. If you are a weekend camper only you probably will not notice the difference. If you travel full time, you will. We travel nearly full time in our Troopy fitted out as a camper van, initially replacing our deep cycle batteries each year because an ignorant salesman had told us that deep cycle batteries could be fully discharged. It took four battery replacements before we discovered the truth.
(b) As the % charge remaining drops, so does the voltage. At 100% charge the voltage will be 12.8V. At 50% charge it will be down to 12.1V. At 30% it will be 11.7V, which is the lowest you can operate some 12V devices. At 0% it will be down to 10.7V, essentially useless.
Here is another suggestion; run a 24v aux power system and you can use a lot smaller cabling due to v=i^2*r. And these days you can get dc to dc chargers that will go go from 12v to 24v
Sounds like you know what you’re talking about, Bob! Cheers
Thanks Ben for very informative blog. I am getting dual battery system installed after reading this blog, with DCDC charger for my Prado. Could you please advice on the solar panels I should go for?
Deep cycle battery of 102ah aiming to run waeco fridge and a couple of lights at evening and charge my 5ah Ryobi batteries in the day for portable devices.
Cheers!
Hi Shaf, I’ll start off by mentioning that I’m by no way an expert here, simply an enthusiast and the knowledge I’m sharing is what I’ve learnt along the way. It would be worth asking the opinions of the auto electrician installing your battery system too.
That being said I’ll still give you my two cents worth.
Someone once said to me to buy as much solar as your money can afford, a good quality 200W solar panel would be a good solution. I use the Hard Korr 200W panels (https://www.snowys.com.au/200w-heavy-duty-portable-solar-panels-with-crocskin-cell-armour) and I can easily keep a 100Ah battery charged enough to run a fridge, lights and charge devices day after day, even late into the afternoon when the light isn’t the best. You may be able to plug these panels directly into your DCDC charger too, making setup fast and easy.
In good conditions, you will probably find that a 120W solar panel will also keep you powered up, but the output from the panels may drop too low to give good charge early in the morning and late in the afternoon, which means you need to make sure you have your solar panel set up in the sun during the middle of the day, this may limit your daytime ‘out of camp’ adventures unless you’re happy to leave your panels set up next to your vehicle while you go for a hike.
You could also consider fixed panels on your vehicle, but you can’t angle/move them as the sun moves and they won’t be much good if you’ve found a shady spot to park your car in the campsite.
I love my 200W panels, and we’ve had loads of other good feedback on them too.
i have a dual battery system on my unit. Can i also safely connect a solar panel to keep the second battery charged when the vehicle is not running or do i need to add an isolator so the charge from running vehicle does not back feed into the panel
G’day John, we’re keen to get Ben’s thoughts on this one but he’s on holiday with his family so we’ll get back to you asap. Cheers
Hi John, I’m not an auto electrician just an enthusiast, so it would be worth having your system looked at professionally to confirm my following thoughts.
My understanding is that if you were to connect a solar panel to your second (auxiliary) battery via a solar controller then power from the alternator will not feed back into the solar panel (good solar panels also have blocking diodes) and the secondary battery will simply take charge from one or both charge sources. Both the alternator and the solar panel are regulated so the charge voltage will remain at a safe level for your batter. When the sun is low the alternator will be doing the work but when the sun is high the solar panels will chip in their fair share if required.
The ideal setup though is to use a DC to DC charger of which Redarc make a few really good units – https://www.redarc.com.au/battery-chargers/in-vehicle-chargers. These go between your alternator and auxiliary battery as well as providing a direct solar input which then delivers a better charge pattern from both sources, extending the life of your batteries and creating a set and forget setup for your 12V power requirements… but it is a more expensive option.
As I mentioned though, it would be worth speaking to an auto electrician or 12V expert to confirm your specific setup.
Hope that helps.
Hi just starting out on our trip. have a lithium battery/solar connected to our fridge/freezer in car. Works when plugged into power source, but, when switched over to battery/solar does not work nothing shows on screen panel when switched over. Getting very frustrated as everything is new
I’d need to know more about your setup in order to understand what could be happening here, Tonny. I’d suggest taking it back to the place where you had everything installed in your vehicle, or to an auto electrician so they can troubleshoot for you.
Thanks Ben, great article. A question on charging a stand-alone deep cycle battery- Can the battery be connected to the in-car cigarette lighter to charge it up on the travel journey; does this facility exist?
Thanks
Hi Peter, I’m not 100% on this, but I’ll mention what I do know and the questions you should probably ask youself.
Firstly, if you want to get the most life out of your battery nothing beats good 240V or 12V DC to DC chargers, these deliver charge profiles that ensure your battery is 100% charged every time. That being said, you can also top up the batteries whilst in the field from the charging system in your vehicle, the best way to do this if you are not using a DC to DC charger is with an isolator, this is going down the dual battery setup path.
Many smaller portable power packs come with a cigarette plug and are suitable for ‘top-up’ charging whilst you are driving your vehicle, the Companion Rover Lithium, for example, limits charge at 5A which is well within the 10A rating on cigarette sockets.
If, however, you are referring to a regular AGM or SLA car battery, then charging from a cigarette socket may result in more problems than the small amount of charge it will deliver is worth. The battery is likely to want to draw more current than the socket is capable of offering so you will blow fuses. You’d also need a suitable cable made up to go from cigarette socket to battery, on top of this you probably don’t want to charge these in the cabin of your vehicle. If you want to go down this path then I’d suggest speaking to an auto electrician or battery specialist first.
One last thing, keep in mind that if you treat your battery well by providing it with good charge profiles and keeping it charged up all the time, you’ll get better life out of it. So a bit of money spent on setup now may save you in the long run when your batteries don’t last as long as they could have.
I hope that’s helpful.
If I have a solar panel, deep cycle battery to power a portable fridge for camping, can I charge the battery from the solar panel at the same time it is providing power for the fridge? Thanks
Hi Jane, this will certainly work. You’ll need to make sure you connect your fridge and solar panel directly to the battery, don’t connect the solar panel directly to the fridge.
What will happen is that the fridge will be draining power from the battery whilst the solar panel is putting power back into the battery. On a bright sunny day it is likely that the solar panel will recharge your battery at a faster rate than the fridge will drain it, but at night the fridge will be draining the battery while the solar panel will deliver no charge.
It’s a bit like a tap on a rainwater tank; if the tap is running the tank will be emptying, but if the tap is running while it is raining it will be both emptying and filling at the same time, most likely at different rates.
It then becomes a balancing act to make sure you get enough charge during sunlit hours to power your fridge for the full 24 hours.
I hope that makes sense.
~ Ben
Yep. I don’t what type/brand of solar controller you have, but with my Victron MPPT 75/10 I have the 12v fridge connected to the load output, with a 100ah AGM batt & 160w of solar. In this setup, the fridge will place no load on the batt during daylight & whilst the batt is still receiving a charge. My controller has a max constant load output of 15a. (This setup has been running non-stop for 3 months now).
My point in all of this is: if you have a big enough solar setup, a decent controller (MPPT not PWM) with a good load output, you’ll be fine.
Ie:
Really, really helpful.
Easy to read & absorb.
Thank you very much.
That is a great article to read for a newbie. Thank you
Quick question, can I connect a second battery to my start battery (that’s connected to panels through a prostar 30 ) or do I need to connect a volt controller in between the two batteries???? cheers in advance
This is a little more technical than I’m comfortable giving advice on sorry, Kevin. My initial thought is that this would be ok as the solar panel would just charge both batteries at the same time, assuming they were connected in parallel. I’d suggest consulting an auto electrician as I’m not familiar with the Prostar 30 controllers, how your batteries are connected or what type of batteries you are using.
Really useful article, and lovely to see some constructive work in the comments.
Thanks to the author and the editors!
Great to hear you found it useful, Wayne! We agree – the feedback and input for other readers in the comments has been so valuable. Cheers
Nice rundown on charge replacement times…easily undetandable and practical ..very handy to me as I’m increasing pannels..
Having beeing set up in the car r 14 years ive developed an instinct to what’s being replaced and what’s bing used….this was real handy To confirm what I need watt wise to replace usage quickly in inclement Vic high country weather fast
Thumbs up
Regards mark
Thanks Mark! Great to hear it was helpful.
Cheers for the article dude. I’ve been looking into a setup for a little while now, and it seems like most people’s opinions are just that, opinions. A lot of people say “You need this battery” or “You need this solar panel”, but having this written out in an easy to follow way with example calculations and some decent theory to back it up has really helped me figure out how to build my setup.
So thanks man, you’re a legend.
You’re welcome Shane! We wanted to break it all down and simplify the ‘science’ so people building their setup could have a head start. Great to hear it helped you out. Cheers
Hi Ben
The article looks great now. What I need to do is to get out with a fridge and some meters to see how many amp-hours it actually uses per day in the real world. Of course, I’ll have to make sure the fridge is full of beer for it to be an accurate test ……
Lindsay
Hi Ben
In the interests of making it a really useful article, may I make three other comments.
I’d question the accuracy of calculating a solar panel’s output as 8 hours’ worth of its rated capacity per day. When the sun is low, the panel’s output will be much less than this due to the glancing angle it hits the panel and atmospheric absorption (and the fact that many panels’ rated capacity is “optimistic”).
Based on my experience with an off-grid beach house, a fixed north-facing panel generates about 4-5 hours worth of its rated capacity per day. Moving the panel to chase the sun might make that 6 – 6.5 hours.
Next, I wonder whether the overnight calculation assuming the fridge does 5 mins on, 25 off (a 16.7% duty cycle) is accurate. I haven’t tested this, but it seems rather low.
Finally, one other consideration needs to be emphasized. What happens if it’s cloudy? You need enough capacity to cope with this, or an another way of charging the battery.
One guide for off-grid houses suggests that ideally, you should have 5 days of reserve. In your example, that would need 26.4 x 5 = 132 Ah, and because the battery should only be discharged to 50%, it will need a rated capacity twice that, or 264 Ah. A cost-conscious weekend camper may well decide to limit this to 2 days, say, which would require a 105 Ah battery. If you can charge the battery from your vehicle if needed, you could use an even smaller battery.
Looking forward to an updated article, as I’ve long looked for a page I can direct people to when they ask about solar and batteries.
Regards
Lindsay
Noted, thanks Lindsay. I’ll make some additional comments in the articles based on what you’ve provided here. I won’t go into too much detail as this was really aimed at people starting out and understanding the basic requirements. The figures I used were purely arbitrary for the sake of simple calculations and were not tested in real life. I’ll make some additions as soon as I’m done responding to you here.
The numbers look right but the units are a confusing mess.
“Camping lights: 0.6A each = 0.6A x 2 = 1.2Ah/h”
If you have two camping lights which draw 0.6A each then together they draw 1.2A. Don’t invent a unit Ah/h it doesn’t make any sense, there was no time unit involved in this calculation. Ah/h is just A (Amperes).
-> Camping lights: 0.6A each = 0.6A x 2 = 1.2A
“Total of 2.1 Ah/h over 4 hours = 8.4 Amps”
We have a mean draw of 2.1A for 4 hours. Ah is Amps times hours, so we multiply Amps by hours and get Ah.
-> “Total of 2.1 A for 4 hours = 8.4 Ah”
Since the battery is rated in Ah this unit is now directly comparable.
These errors in units permeate the whole article and make it potentially quite confusing, which is unfortunate as otherwise I think it’s a well presented worked example which could really help the reader understand. There is so much confusing wrong information online it would be nice to see something clear it up correctly.
A few other terminology issues crept in:
“Volts (V) – 12V power is what your car battery and alternator delivers.”
Volt does not measure power, Watt measures power. Volt measures potential difference, or electromotive force. Force is probably the best way to understand it in layman’s terms, the greater the force (Volts) the more current will flow (Amps). Like higher water pressure pushing more water through a hose.
Good points, Martin. I’ve tried to write it so anyone can understand without getting too technical, I think I’ve been staring at it for too long now as what you mention makes perfect sense. I’ll look over it and fix up the points you make. I appreciate your feedback.
Should be all fixed, Martin. I’ll admit that I’m no auto electrician, I’m just trying to create an article based on what I’ve learnt over the years to help newbies understand the basics. It sounds like you are in the know though, so hit me up if I’ve made any other blunders, I just want to make sure it is clear.
Ben, fixes look good now. It’s a good article and will hopefully help many people out.
[For what it’s worth you are right: BEng (Electronics).]
Hi Martin,,
A camping newbie, can you tell me what you would do if you wanted to get a 200-250 solar panel, blanket or fold out? What is best wise,? Would lithium be better than lead battery? If so what size would I need to go with the solar?
I have a 2016 ranger which has a connector built in in the ute area, I have a 95 lt brass monkey fridge freezer not sure of how much power that would draw? :/ lighting? What would be best way forward to supplying that?
Complete novice lol take pity or a gentle nudge to the right direction lol
Cheers Claudia
Ps I have a camper trailer, wanting to give my young grandkids the opportunity to enjoy the great outdoor lifestyle that their overworked parents don’t have the time or means to provide :/
Hi Ben
That’s the best article I’ve seen explaining the ins and outs of solar + battery power for campers. Many authors get hopelessly confused about Amps, Amp-hours, and fridge duty cycles. However, there is one error in the article. Where you give the calculations for how much power you’ll use, you confuse Amps and Amp-hours. For example, in the “Daytime – fridge only” section, you say:
“Daytime running fridge only: 7am – 5pm = 10 hours
Estimate that fridge cycles: 15 mins on/ 15 mins off = 30 mins/hour – 2.7/2 = 1.35Ah/h
Total over 10 hours = 13.5 Amps”
The last line should say:
“Total over 10 hours = 1.35 Ah/h x 10 hours = 13.5 Amp-hours”
Cheers
Lindsay
Thanks, Lindsay, I think there are a few areas I need to fix up now that I look over it again, I’ll fix it up now. Thanks for your feedback.