What size solar power system do I need?

One of the most common questions we get asked is “What size system would I need to wipe out my power bill?”

This is a very difficult question to answer because it depends on the percentage of the solar power you generate which goes into the grid and the percentage that is used internally.

The reason this makes a difference is that when you generate solar and use this power in your building, it is saving you the cost of buying the same kilowatt hour (kwh) of power from the grid. This cost is currently about 37 cents for the top rate at which you purchase most of the kwh’s that you use.

However, if the same kilowatt hour is generated and flows back into the grid you get the value of the feed in tariff, currently 25.2 cents until 2016, which means this power is worth a bit less.

So at the moment you probably need a system that produces about 115% of the kwh’s that you use each year to wipe out your bill.

After 30 September 2016 the feed in tariff will fall to simply be the Essential Service Commissioners determination of the fair value of solar power fed back into the grid (currently 11.2 cents but by then it may be 15 cents or so)

For this reason it is likely that after 30 September 2016 you will need a system that produces about 120-130 % of your annual power needs to wipe out your power bill. It is important to keep this in mind when sizing a system.

This is based on a residential customer that uses about 2/3 of the solar power they generate within their house during the day. This is only a best guess, rule of thumb figure and should not be relied upon exactly.

How do I know how much power I use per year?

This information is generally available as a daily or quarterly figure from your power bill.

You can then multiply this figure out to reach an annual figure.

Once you get this annual figure then you can easily work out 115% of this. Once you do, you will know how many kwh per year you want your system to generate.

You can then use the tool below to see how much power various systems produce. The details will appear in a table at the bottom of this page.
On average, systems installed facing true north in most parts of South Australia produce 1500 kwh per year per kilowatt of peak capacity solar.

If you divide the kwh you need to produce by 1500 you reach a figure that is the size of the system you require.



If you can’t be bothered with all this maths then here is a rough cheat table based on moderate day time electricity consumption, the current cost of power and the proposed feed in tariff. However please remember it is an estimate only and can not be relied on conclusively.

Quarterly Power Bill System Required to wipe out your bill
$200 2 kw
$300 3 kw
$400-$450 4 kw
$500-$550 5 kw
$600-$700 6 kw
$1000-1200 10 kw