What’s the “just right” size for your Solar Power system?

Southern Highland News, 15 May 2013. More articles from the CANWin column.
Goldilocks sat on Papa bear’s chair and it was too big. She sat on Mama bear’s chair and it was too small. Goldilocks sat on baby bear’s chair and it was just right.

The same applies for solar power systems. The “just right” system generates as much as possible for your household, but as little as possible for the electricity companies (because they pay next to nothing). So how do you know what’s just right for you?

Start with your last electricity bill. It has a graph showing your electricity usage over the last twelve months.

PowerBillIgnore off-peak electricity. Most solar power systems are only connected to one circuit, and that should be the biggest and most expensive circuit – the general household circuit.

You need to work out your average electricity usage per day over the whole year. In the illustration you can see the average use per day for the four quarters: 12kWh, 20kWh, 17kWh and 11kWh. Add them together and divide by 4 to get the average for the year: 15kWh. Pretty simple, but not quite the final answer yet.

Because a solar power system only generates power during daylight hours, for most households it will supply only about 40% of your daily electricity. (It’s less than half because most households use most of their electricity in the evening.)
40% of 15kWh is 6kWh. So your just right size solar power system will generate an average 6kWh per day.
One more step before you go shopping. You buy solar power systems in kilowatts (kW), not kilowatt hours (kWh). You need to convert your “just right” 6kWh into kW. In the Southern Highlands, that means you divide it by 3.5:

6kWh divided by 3.5 is about 1.7kW. So a 1.5kW to 2kW system will generate the right amount of power.
As electricity costs are rising, a 2kW system is “just right”.

Disclaimer: This is a simple way to calculate the approximate size for your solar power system. It neglects many factors such as roof orientation, pitch, future changes in electricity consumption, shading, solar panel efficiency etc. Please talk to a solar power professional for exact system sizing.

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