How do you choose the right size solar panel and battery?

I was often asked to help people choose the most appropriate size of solar panel and battery to power the fan in an off-grid compost toilet (although my methods and maths applies to anything that could use electricity, such as a light bulb). This articles explains my thinking, and I’ve not been proved wrong so far!

The main difference between compost toilet fans and other electrical stuff like LED bulbs, is although these fans tend to be low-power (1-2 watts), they are often on 24-hours a day, which can add considerably to the electricity requirement.

Where fitted, the fan in a compost toilet is used to extract odours and to assist in moisture control (an important and often overlooked aspect where toilets are used in smaller spaces), and in some cases can replace the need to use cover material (but not always).

In most instances, the fans in compost toilets can be run from a 12-volt DC electricity source, such as a leisure battery or deep-cycle battery.

But what size solar panel and battery will be right for you? Here, I’ll explain how to work out the ideal size panel and battery capacity.

It’s easy to under-specify both the panel size and battery. Whilst the system may work well from March to October, winter puts additional pressure on the battery and an under-sized solar panel can dramatically shorten battery life expectancy, leading to an unexpected loss of power and the expense of having to replace a battery! Relatively speaking, solar panels are not that expensive, so it’s best to err on the side of caution and go bigger than you need.

What size solar panel will you need?

The 12-volt fans in many compost toilets, draw between 1 and 2.5 watts at 12 volts DC depending on the make and model (check with your supplier if you haven’t bought it yet, or look at the fan itself for information).

Because they run 24 hours a day, you take the power requirement (for example 2.5 watts) and multiply this by 24 (the hours in a day) and you get a daily power requirement of 60 watt-hours (for a 2.5-watt fan).

20-watt solar panel, operating at 100% efficiency (i.e. assuming the sun is shining brightly and directly onto your solar panel with no clouds, no shadows etc) would therefore be able to generate 60 watt-hours in 3 hours of full sun (ie 3 hours x 20 watts = 60 watt-hours).

That’s fine for perfect summer days, but winter brings with it the triple whammy of lower sun angles, shorter daylight hours, and greater chance of cloud cover. So, how do you factor this in when it’s impossible to know what the weather is going to be like? You have to use ‘rules of thumb’ together with some good old guesswork!

The rule of thumb I generally use is to assume you’ll have on average, one hour of full sun per day (of course, some days there will be negligible sun and on others there will be more, but we’re working on averages over several days, not specific conditions day to day). In other words, I’d suggest around a 60-watt solar panel to supply on average, 60 watt-hours per day during all seasons. Of course, if you want to run some 12-volt lighting and other things too, you’ll need to factor them in and increase panel capacity to cover that.

In addition, 100% of the power generated at the panel will not get into the battery – there are inefficiencies in the panel, wiring and the charge controller which mean that some power will be lost along the way. It’s hard to say how much, but I assume that about 90% of the power generated by the panel will actually make it into the battery.

If in doubt, over specify the panel capacity – you can never have too much!

If you don’t intend or need to have your system running over winter, then you’ll probably get away with a 20 or 30 watt solar panel, assuming it’s optimally positioned, but as the price difference between panels of this size is negligible, going bigger seems a sensible route.

Battery sizes and types

The other significant component in a solar system is the battery. This takes the electricity from the solar panel and stores it ready for use when needed. The best type of battery for a solar installation is a ‘deep cycle’ battery – so-called because they can be ‘cycled’ ie have power drained from them and then recharged day in, day out. However, true deep cycle batteries are quite expensive.

You might also come across ‘leisure’ batteries. Often used in boats, caravans and motorhomes, these are also good for solar installations. Although not to be as long-lasting or durable as ‘deep cycle’ batteries, they are more reasonably priced.

Recently, 12-volt lithium batteries have started to come onto the market, but at the time of writing (2022), they are typically 4-6 times more expensive than an equivalent lead-acid leisure battery, will need special charge controllers, and are not as recyclable as other technologies, so for this example, I’ll be ignoring them.

Although we measure solar panels in watts, batteries have capacities rated in amps or amp hours.

Amps can be thought of as the volume or quantity of electric power. The flow of amps is called the current, as in the flow of a river. Unlike a river, though, the speed of the current is fixed – only the volume varies.

To calculate the ‘volume’ or Amps of electricity required to run the fan, there’s a simple calculation you can apply – sorry if this gets too technical, but stick with me! 

Amps = Watts / Volts

So in our example, the Amps would be: 2.5 (watts) / 12 (volts) which is around 0.21 Amps.

Every battery has a stated capacity, and in this case, ours states it is 80 Ah (Amp-hours). Assuming it’s fully charged and in perfect condition, it could supply 1 Amp for 80 hours, or 80 Amps for 1 hour (or anything in between those amounts). In reality, you never run a battery completely flat as it will fail very quickly. I’d suggest that no more than 50% of the battery be discharged, which gives around 40 Ah of useable electricity.

Remember, I calculated that the 2.5-watt fan consumes 0.21 Amps? Working on 40 Ah realistic availability from the battery, we take that and divide it by the consumption ie 40 / 0.21 and we get 190.5, which is the number of hours the battery could run the fan, assuming no charge was coming in.

If you divide 190.5 by 24 (number of hours in a day), you get 7.93 – let’s call that 8 days worth of power. So a fully charged, 80Ah battery, in perfect condition, gives you sufficient capacity to get over nearly eight days without any sun, on the basis that sooner or later, there will be some light falling on the solar panel and the battery will start recharging once more.

You still with me, or did I loose you in the sums?

Charge Controllers & Wiring

Renogy 10A PWM Charge Controller
Typical PWM Charge Controller

The last component you’ll have to consider is a ‘charge controller’. This is a little box of electronics that ensures the right amount of charge is going into the battery, and when the battery is full, it reduces to a trickle. If you were to continue to try to charge an already full battery, you’d quickly destroy it, especially with a large solar panel on a sunny day. Many charge controllers also act a wiring block to connect the panel, battery and the load (in our case, the fan on the toilet) together in one convenient place. It will also monitor the battery charge levels, often giving you a visual indication of the State of Charge and shut the system down if the battery drains too much (preventing damage to the battery).

Wiring is something that people tend not to think about on 12-volt systems, but choosing the right wire is important. If it’s too thin, you can get power loss and potentially overheating, and the last you want is to throw away your precious power or start a fire. I typically use 2.5mm diameter cable which is fine for short to medium cable runs.

This is just an introduction – there’s a lot more to solar power! If you are serious, then you might also want to look at different types of charge controllers – basic types are fine (they are sometimes referred to as PWM charge controllers – Pulse Width Modulation – due to way they charge the battery), but you want to squeeze even more power from your solar panels, then there are MPPT – Maximum Power Point Tracking – controllers which match the power requirements of the battery with the available power and by doing some fancy voltage to voltage conversions, they can sometimes get up to 20% more useable energy from your panels – useful if you’re near the limit of what you need, but they are more expensive.

Getting advice from an expert

Solar systems are not too complicated provided you understand some of the basics, and you can always add more panels or more/bigger batteries later on, but it’s best to get it right from day one if you can. So if the numbers and calculations confuse you, don’t fret, get advice!

Some companies offer excellent advice, either over the phone, via online chat or email, and they can help you make sure you size the system according to your needs.

Where to buy

If you’re reasonably confident about what you need, then we’ve included some suggested kits that you can buy from Amazon:

Renogy 50 watt kit. This is a good value 50-watt solar kit from Renogy. It comes with the panel, PWM charge controller, roof mounting brackets (for shed/flat roofs), 5m cable (from panel to charge controller) and battery cables. Just add a battery and you’re good to go.

Find out more at

Renogy 100 watt kit. With double the panel capacity over the 50 watt kit, this 100 watt kit comes with a PWM charge controller, roof mounting brackets (for shed/flat roofs), 5m cable (from panel to charge controller) and battery cables.

Find out more at

Renogy 100 watt kit with MPPT Charge Controller. 100 watt kit comes with an MPPT charge controller for improved performance, roof mounting brackets (for shed/flat roofs), 5m cable (from panel to charge controller) and battery cables.

Find out more at

Here’s some companies we know about (UK-based) that seem to have a good reputation.

Sunstore –
Range of products at sensible prices – personal advice online or over the phone. Good after-sales support for people new to solar.
Bimble Solar –
Very keen prices and often have deals on second hand panels. Great if you have an idea what you’re looking for. Website has online calculators to help work out your power requirements.

Some links may be affiliate links. This costs you nothing, but means we might earn a small commission for referring you. We only refer to companies that we know and who sell products we believe are of good quality, however always check suitability for your purpose before buying.


  1. Hi, maybe a daft question but, given that the toilet is only occupied for a tiny fraction of the day would it be possible to have the fan running on a timer/controller?
    That way it could be set to come on when the door is opened for say 30 minutes and also say ten minutes in every hour from 6 am until late evening.
    This would save a lot of battery capacity. Problem is, I don’t know what gadgets are needed to do this or how to incorporate them. But surely there must be simple electronic timers that could do this job?
    PS: Great info on your blog and your YouTube video. very clear and concise.

    • Thanks for your comments and message. It’s not a daft question at all! The answer is ‘it depends’… some of the fancier toilets are designed by the manufacturer to have the fan on all the time and because these toilets don’t use any cover material (sawdust etc), the fan is critical in both odour and moisture control. That said, I know of several people who operate these toilets with the fan on during the day only and have said they work OK with no adverse odours. You can buy 12v timers for £10 or less that can be programmed to do this, or just have a switch and remember to turn it off and then back on. One of the potential issues is flies – with an air flow, there won’t be a smell in the toilet to attract them, but without this, you might find them in the summer.

      You can also get a switch that when pressed, will operate a fan for 20 mins or 30 mins for example, but in these situations, I’d also use some sawdust to cover the solids too. In conclusion, if your loo entirely relies on the fan (ie no sawdust), then I’d probably not recommend turning the fan off, but if you use a fan and sawdust too, then it should be fine.

      Depending on the model, most toilet fans consume between 1.5 and 2.5 watts at 12v DC – that’s not a huge amount of power, but when the fan is running 24/7 it adds up, and in winter, with more limited scope to generate electricity, it can be an issue unless you have a decent solar & battery set up.

  2. I’ve noticed that some of the MPPT solar controllers have time switch functions built into their load output circuit. (Some PWM may have it too).
    My guess is they are intended to run something equivalent to street lighting, as often they can also incorporate sun rise and sun set into the switch timings. With suitable set up this could be useful for controlling the fan.

    • Most of my experience has been with PWM controllers, and the majority of the ones I’ve used, also have time switch functions for (street) lighting too. Most have variable modes so when the panel is in darkness, after a few minutes, it will switch the load on for a number of hours, or until dawn. Whilst useful for lighting, with a compost toilet, you probably want the fan on during the day (when it’s likely to be in use the most), and perhaps have it off at night (to conserve power and for silence at night), so in that case, the timer function isn’t ideal.

      You can buy relatively cheap electronic 12v timers that could be set to come on at, for example, 7am and run for four hours, then off for two and back on for a few hours etc. I also had an idea to run a fan at reduced power all the time, but when you have used the loo, you press a button to boost the fan for 15 or 20 minutes – something like this would be relatively straightforward for an electronics expert to put together (having said that, I’m not an expert, so maybe it isn’t!!!).

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