How Much Solar?
The size of solar panels and the size of batteries are determined pretty much independently of each other, but because they work together in any 12Volt system, if we get one wrong then it will probably affect the other. Also, if you’ve already got a 12Volt system but are unsure whether to increase your battery size or solar size, using the examples here and in How much Battery? should sort that out pretty quickly.
But before we can work out how much Solar we’re going to need, there is one key figure we need know.
First, how much am I using each day?
To get to the how-much question for Solar (and for Battery) we need to back up a bit and ask how much energy I’m going to use each day. The more I use, the more Solar I need, so my daily usage has to be our starting point. For this you can also refer to the article on How much power does 12Volt stuff use?
Totalling up your daily usage is the hard part, but it’s also the key to getting the sizing right. So don’t try and skip this step – it takes a bit of time but it’s worth it!
Example
To give us an example to work with for our solar calcs, we will use a small setup consisting of a 40 litre fridge, a TV and a strip of LED lights. This is the same example used in How much Power does 12Volt stuff use? and we worked out that it will take 45Ah per day to run the system, assuming winter conditions for both the fridge and solar.
How much sun are we getting?
Now that we know how much we’re using, we have to work out how much input we can get from the sun, and that’s going to depend on whether you’re in Darwin or Adelaide, Hobart or Cairns. Fortunately the Bureau of Meteorology (BoM) has some excellent climate data on tap, so we can use that. I’ve summarised some of that for typical towns around Australia in the article on Solar Insolation[*] in Australia so I’ll assume you know what I’m talking about in the next para.
For our example let’s take Perth’s weather conditions – for no other reason than I live there. BoM’s figures show that on average we only get 2.6 hours of peak sun in June and 8.3 hours in December, with an annual average of 5.4 hours. I always go for the winter hours – that way I’m doing a worst-case scenario and I know my system will work no matter what time of year I go travelling. If your usage of solar is very specific e.g. a summer-only beach shack, you could adjust the hours a bit.
Solar Panel Sizing
Now it’s time to look at those panels that are going to supply the daily power that we are using. So in this example we need to put back at least 45Ah into our batteries plus a bit extra for losses in the battery, cables, etc. So on a winter’s day in June we have 2.6 hours of peak sun, so the current we need from the panels is (45Ah/2.6hrs) = 17.31 Amps. Now 12Volt panels typically have a peak voltage (Vpk) of around 17 volts, and that’s the voltage at which their power (Watts) is specified. So to get the Wattage of the panels we need, just multiply 17Volts by 17.31 Amps and you should get 294 Watts. This is the solar power you need to keep your 12Volt system going reliably in worst-case winter conditions.
Solar Calculator
The Solar Calculator can work all this out for you if you like. Just put in the solar hours for your location, and the daily usage in Ah. It will then spit out a figure in Watts which is the minimum size of panel we need to support the 12Volt equipment we have.
Solar Power required: Watts
Other power sources
The above figure of course assumes that solar is your only source of power. If you're at a beachside weekender, you might have some wind power, or maybe a gennie that has to run a few hours a day anyway. In that case you can start reducing the amount of solar.
If you have the space, it's always best to stick with this figure, or even go a little bigger – things always grow, especially with 12Volt stuff. Having too much solar is never a problem, it's having too little that causes those little "disagreements" we all have – and who wants that when you're on holiday?!
There's a specific blog post on Solar and Regulators if you're wanting to understand it all a bit better, and if you're wanting to move your solar power around, you can have a look at the one on Portable Solar Panels too.
Solar and Dual-Battery Systems:
If you have a vehicle with a dual-battery system which charges the battery while you're driving along, then you can again reduce the solar. But remember that it's only charging while you're driving, so if you plan to drive to the spot and stay put for a week or so, then solar is going to be your only source of power once you park up.
However if the dual-battery system is in a motorhome and you're unlikely to park up for more than a night or two at a time, then the batteries will be kept charged most of the time from the alternator. In this case you could reduce the amount of solar quite appreciably.
Limited space:
There are times when the space available for solar panels is limited – this is typically the case for small boats for example. In that case it's simple – just fill the available space with as much solar as will fit, and in that case an MPPT regulator might be worth the extra cost, just to squeeze the last drop of power out of those panels.
[*] Insolation is just a long word meaning the power-per-area received on a horizontal surface on planet Earth.
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14 Comments
Hi Alister, great blog mate, best one I’ve found! I am such a newbie to all this 12V solar stuff. I have a Toyota coaster that I am converting to a motor home. We are just trying to work out the best 12V setup for out situation. We will have a 220ltr fridge, 12v pump and some GPOs powering just phone chargers and laptops etc. we may have a 2 burner induction cooktop for inside cooking. Worst case scenario I think about 100Ah per day. I plan to get two 100AH batteries, with a 2000W inverter. I will have a DC to DC charger and have 1x 375W house solar panel. Do you think this is sufficient? We won’t be driving every day and free camping most of the time. Any advice would be appreciated. Thanks again mate.
Matt
Hi Matt,
Thanks for the kind words, and glad you’re finding the 12Volt Blog handy!
You’ve made a great start by coming up with a figure of 100Ah per day and your solar sizing seems good too. If you plan on travelling in winter, the battery could be a bit bigger (3 days = 300Ah) if you have space, and playing around with price and capacity (Ah) will get you the lowest cost (e.g. 2x150Ah vs 3x100Ah). I’m hoping the fridge is 12Volt, that way it’s nice and efficient and your battery will thank you by lasting much longer. For phone charging, you’d be better off installing a USB outlet or two – they have a 12Volt input and use much less power than 240V chargers. Same goes for the laptops – have a quick internet search for “12volt laptop power” – they come with a range of plugs that fit most laptops – top tip: take your laptops into the shop selling the power supply and try it out to make sure it actually works on your laptops.
The reason you want to use 12Volt chargers for phone and laptop, is to avoid firing up the hungry 2000W inverter for tiny 240V loads for a long time (very inefficient!). Talking of the inverter, an induction cooktop is a really great option, but you might have to look at Lithium batteries which can supply the 200Amps needed by the inverter to supply the cooktop or a microwave. (sealed lead-acid batteries don’t like high currents).
Sounds like a great project – hope these pointers help – have fun!
Cheers,
Alistair
Hi Alistair – I am setting up a camper and a vehicle and an off grid shed/garage so I really appreciate your website. Cheers
Thanks Steve – good to hear you’re finding the 12Volt Blog useful!
Cheers, Alistair
Hi Alistair, very helpful website.
I’m hoping you can push me in the right direction. We have just bought a 65ltr fridge/freezer which has an average 40watt power consumption. Am I right in saying that draws approx 3.33ah? We are now in the market for a suitable solar panel to run the fridge from a 100ah battery. This is where I need your assistance, what size solar panel would best suit our needs? At this stage it’s just the fridge we need to power but in the future I would like to add some lights. We were looking at a 160 watt panel but not sure this will be big enough. Cheers for your help
Hi Karl,
Glad you’re finding the 12Volt Blog useful!
There are heaps of different types of fridge, so to answer your question properly, I need a bit more info. Is it a compressor fridge? (I think so, given you describe it as a fridge/freezer).
The 40W power draw is certainly 3.33Amps (12Volts) but a compressor fridge only uses this much when it’s running, and it cycles on an off to keep the temperature constant.
So what we need is the average, over a 24hr period, and for that we can use the figures from the blogpost “How much Power does 12Volt stuff use?”
For a 50-70 litre chest fridge it uses 45Ah per day – obviously this is an average, and depends on things like how hot it is outside, how many warm beers we just put into it, etc.
Now to the solar – that’s going to depend on where you are, the season, etc – so I’ll use the figures for Perth for this calc, which is 5.4hrs of sun (annual average).
So if we put 45Ah into the calculator above, it says we need 146 Watts of solar to keep things going, so your 160 Watt panel should be fine, with a bit left over for lights, etc.
In terms of the battery, 100Ah would give you a squeak more than 2 days running time – 3 days is preferable, so a bit bigger would be better – but if you’ve already got the battery – hey, it’ll work fine.
With the solar too – if you have the choice to go bigger than 160W – no problem!
So, short answer: you’re in the ballpark, you’re on the money – going bigger is only a problem once (when paying!) – going too small is going to be a headache much more often… 😉
Hope this helps.
Cheers,
Alistair
Hi Alistair, thanks very much for putting the time in to write out clear and easy-to-understand blogs. I have a question on solar efficiency. I recently purchased 160watt panel (80w panels) with a max of 9.4 a/hr but thats assuming i will harness the full 160 watts in perfect conditions. The solar efficiency is 16.8% at 1000W/m2. Can you please explain to me how i can calculate the true output of the panels after the loss is assumed . My calculations bring me to 135watt of actual output in good conditions with a return of 7.9 a/hr. Is this correct? Thanks in advanced for you reply
Hi Steve,
Thanks for the positive comments – glad you’re having fun reading the blogs!
Your solar efficiency of 16.8% sounds spot-on – it’s in the ballpark that we can call “believable” (agrees with the science) – all good!
Now, to work out the actual current you’re getting into your battery, we’re going to need the IV-curve for that panel. Have a look at the blog “A Guide to using Solar Panels and Regulators” for an example of an IV-curve, and how to work out the actual current that’s going to the battery.
If you can’t get hold of the IV-curve, then the other (more fun?) option is to measure the actual current with a meter – you can get a multimeter for about $15 these days, so that might be something to think about – and then you can accurately measure volts and amps of all sorts of things as well.
Hope this helps.
Cheers,
Alistair
Hi Alistair
I really love this blog. Great job. I have a question regarding the sizing of a 12v system.
I have a bush block with a shack where I go there probably 16 days per year spread evenly over the year. Usually 4 day trips.
My ultimate amp-hours per day would be around 100ah/d in the winter. Substantially less in the summer.
Daily peak hours of solar gets down to 2.2hr.
So using your blog’s calculators I am getting about 800W of solar and a 300ah battery.
Considering how infrequently I go there, is there the opportunity of using a smaller solar array and a larger or different type of battery? Over the 4 days the system would dig deeper into the battery but smaller panels say 100W would charge it up over the next 2-3 months. I realise this is not ideal but it all seems like a lot of overkill to me for my requirements.
If I lived there or spent weeks there I could justify it. I am just interested to know if there is an alternative for the short visits I take.
Thank you very much
Mark
Hi Mark,
You got it in one! You’re absolutely right about going smaller in solar – because in your case it’s got so much longer to charge up the batteries.
So let’s go with your ideal of 100Ah/day – over 4 days that would mean 400Ah, so if your battery was 400Ah then you’d make it even if it was cloudy all the time and you got no solar input. So putting in that much battery might be the best investment for you.
As for the solar, yep – just a small panel would do it. Let’s say you’d completely flattened the battery after 4 days, then the solar would have to put back 400Ah to charge the battery to full. Let’s be pessimistic and say there’s only 2.2hrs of sun, and you’ve got 100W of solar. A 100W panel would give you just shy of 6Amps, so each day you’d get 6Amps x 2.2hrs = 13.2Ah. So to charge your 400Ah battery would take 400Ah/13.2Ah = 30 days.
So as long as there was more than a month’s gap between your visits, your 100W solar and 400Ah batteries would be just fine – and you’d get 100Ah/day for at least 4 days – all good!
Hope this makes sense – otherwise let’s chat some more.
Cheers,
Alistair
Hi, I build small campers in Qld and normally fit them out with 130 watt solar panel plus a 100 AH Deep Cycle battery. This is running 2 LED 2.5 watt lights and a small fridge which uses 45 watts of power. With the average sunlight in Qld during the summer, will this combination of panel and battery be enough to provide enough power?
Regards
Jim
Hi Jim,
The LED lights are just fine – no problem there at all.
But the fridge seems to be a cooler/ warmer and they are pretty hungry beasts. It draws that 45 Watts 24/7 – so that’s a current of 3.75Amps and in 24 hours 90Ah will be drained from the battery – so it will last just a day. The 130 Watt panel won’t be able to replace that on a daily basis either – to replace that amount would take over 350W of solar!
The cooler/ warmer certainly has affordability on its side, but unfortunately it takes a lot of power.
As you can see from the tables in “How much power does 12Volt stuff use?” the compressor fridges use much less power, and the 130Watt of solar will keep up most of the time.
Sorry – not the best news, but at least knowing it now means you can plan out some options.
Cheers,
Alistair
Hello, nice website, useful calculator…
Thanks John, much appreciated