Before I studied engineering at the ripe old age of 28, my pet hate was people who answered a question with ‘depends’. Now I find most questions about off grid power can only be answered with a depends. There are so many variables.
The most frequent question we get asked is “How many solar panels do I need to power my...” which is such an open ended question. So I’m gonna attempt to help you answer this for yourself.
Here’s a quick and fairly reliable tip to work out how many panels you need, with only a little maths!
Solar panels are rated in Watts, usually kiloWatts (kW). The energy we use is expressed as kiloWatt hours (kWh). Now let me introduce you to ‘sun hours’ which are the amount of hours that we get enough full sunshine for a kW of solar panels to produce a kW of power.
Start with what you want to power, for example say you want to power a 0.75kW pump for 12 hours a day. You will need 0.75kW x 12h = 9kWh per day.
Then you look at when you need to run the pump, if it’s all year on solar you need to size for the winter months where in December we only get one measly sun hour a day. The sun is it’s lowest in the sky, we’ve got winter solstice on the 21st, it’s the time of year we get the most phone calls saying that batteries are low, no power etc. Those of us who live off grid either grind to a halt during these dark days or have a back up generator to see us through. The ideal solution, obviously, is wind or water power which are both abundant in the winter. I will write more about hybrid systems another time but this post is about solar alone.
Back to the pump, let’s say we can top up the system in November, December and January with a generator (waste bio-diesel of course!) and use solar for the rest of the year. For these months we get 3 or more sun hours a day so to harness 9kWh in 3 sun hours we will need at least 3kW of solar panels. (3kW x 3h = 9kWh).
It’s important to remember that a solar power system has many components and you will need much more than solar panels but this easy calculation should help you visualise the size of array you will need.
Living on solar brings us in tune with the seasons. The difference between summer and winter sun hours is huge when you rely on it for basic needs such as light. Take some time to study the graph and visualise that many full hours of sun at each time of year. Think of extra loads you might need in the summer, like fridges, fans or louder music!
I made the graph below from official government statics found here and rounded them up to the nearest whole number.