What size solar panel is best for a compost toilet?

Choosing the right solar panel for your installation

We're often asked about using solar panels to power the fan on a compost toilet (and perhaps some lights too), so here's what we'd recommend and why.

It's easy to under-specify both the panel size and the battery capacity. 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?

80 watt mono-crystalline solar panelThe 12 volt fans that are built into many of the compost toilets we sell draw between 1 and 2.5 watts at 12 volts depending on the make and model. Because they run 24 hours a day, you take the power (for example 2.5 watts) and multiply this by 24 (hours in the day) and you get a daily power requirement of 60 watt hours (for a 2.5 watt fan).

A 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 supply this power (or more specifically, put it into or top up a battery) in 3 hours of full sun (3 hours x 20 watts = 60 watt hours).

That's fine for summer (you hope), but winter brings with it the triple whammy of lower sun angles (ie the panel is unlikely to operating at 100% efficiency), shorter daylight hours, and greater chance of cloud cover. So, you need to factor this in, but the trouble is that it's impossible to know what the weather is going to be like, so you have to use 'rules of thumb' together with some good old guess work!

The rule of thumb we 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, but we're working on averages over several days, not specific conditions day to day). In other words, we'd recommend at least 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 them too.

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

If you don't intend or need to have the system running over winter, then you'll probably get away with a 30 or 40 watt solar panel, assuming it's in a good position etc.

Battery sizes and types

4-P1020800The other significant component in a solar system is the battery. This takes the electricity from the solar panel and stores it ready for use throughout the day and night (when the solar panel will not be producing any electricity). The best type of battery for a solar installation is a 'deep cycle' battery - so called because they are designed to be 'cycled' ie have power drained from them and then recharged day in, day out.

You might also come across 'leisure' batteries which are often used in boats, caravans and motorhomes - these are also good for solar installations, although they tend not to be as long lasting or durable as 'deep cycle' batteries, but they are much more reasonably priced.

Car batteries are not considered to be very good because they are not designed to be cycled in this way and generally won't last as long as leisure batteries, but if you have one going spare - use it!

Although we measure solar panels in watts, batteries work with 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 bare with us! Amps = Watts / Volts - so in our example, the Amps would be: 2.5 (watts) / 12 (volts) which is around 0.21 Amps.

If we look at a 'typical' deep cycle battery, you'll see that it has a capacity, and in this case our battery states 85 Ah (Amp hours). This means, assuming it's fully charged, it could supply 1 Amp for 85 hours, or 85 Amps for 1 hour (or indeed, anything in between those amounts). In reality, you would never run a battery completely flat as it will fail very quickly. We tend to suggest that no more than 50% of the battery be discharged, which gives you around 40 Ah of useable electricity.

Remember, we calculated that the fan consumes 0.21 Amps (or did we loose you in the maths?). So 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 - this gives you sufficient capacity to get over a few days without sun in the winter, on the basis that sooner or later, there will be some light falling on the solar panel and the battery will start recharging.

Charge Controllers & Wiring

2-P1020797The last component you'll have to consider is a 'charge controller'. This is a little 'black box' 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. They'll also monitor the battery charge levels, 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 as wiring that's too thin will result in significant power loss, and the last you want is to throw away your precious power. We'll typically supply 2.5mm diameter cable which is fine for short to medium cable runs. For longer runs, you might want thicker cable - let us know and we can advise.

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.

Video: How to connect a solar panel to a charge controller and battery.

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