Tips From Our Experts

Steve asks: How do I establish my power needs so I can determine the KW levels for my solar panels? And, is it better to have gas-boosted or electric solar hot water?

KW Levels for Solar Panels

Steve says:

How do I go about establishing my power needs to be able to then look at what level KW I need my solar panels to be? Also, re solar hot water, is it better to have gas boosted or electric?

Our expert says:

Thanks for your question.

Probably the easiest method of estimating your power needs is to look at your energy bills. You should find your average kWh use for electricity and MJ use for gas per day. Converting MJ to kWh is easy – just divide the MJ by 3.6 (3.6MJ = 1kWh). If possible, check a full year’s bills and use the highest figures. Next you need to discover what the “peak sun hours” (PSH) for your location is. Check online or use this as a guide.

Divide your average daily use by the PSH (preferably in winter or spring) to get your average system size. This is not the end of the story though; if you want to be off-grid, you will need to factor in the dull days as well, together with conversion losses. Ideally, you would look to make all your appliances and lighting as energy efficient as possible from the outset; you would then organise your use of energy around the state of available energy. This would minimise the size of PV system required and, in the case of off grid, the size of the battery bank. Using energy as it is produced i.e. during daylight hours also assists as it reduces some of the conversion losses.

Personally, I prefer electric (heat pump) boosted solar hot water as you can get 100% renewable electricity but you can’t get GreenHouse Gas free gas. Heat pump hot water services also have a so-called co-efficiency of performance (CoP) greater than 1 i.e. for 1 kW of power in, you get more than 1 kW of heating out. Some get a CoP of greater than 3. You might find that bypassing the solar hot water collector and adding to your PV system may be a more cost effective option as well.

For more information about solar PV systems, see the 17 Things™.

John Knox

John Knox

The technical advice provided by the Alternative Technology Association as part of the ‘Ask an Expert’ forum/website, in relation to home and building sustainability improvements, is general in nature only. The impacts of specific energy or water saving or sustainability products will depend on how and where the are used and located, and particularly for larger investments, we would always suggest users seek a specific assessment from an appropriately qualified building designer or service / product supplier.

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