How to work out how much your solar power system is saving you each quarter.

We get three or four calls a week from people who have forgotten or are unsure how to work out how much their solar power system is saving them each quarter and so we thought we would post a page for our customers to help you work it out.

It can be a bit tricky to work out the savings because there is not one screen that can tell you the answer. You have to use information from both your SAPN meter (or your power bill) and also from your inverter.

Simply looking on your power bill and seeing the credit for solar energy exported will not give you the whole picture of what your solar power system is saving you.


What is my solar power system saving me

The first thing to look at is the solar inverter and the setting called “Etotal”. This is the total amount of energy that has been produced by your system.

As we go through we will do an example. Lets say Etotal on the inverter of a 5kw system went up 2000 kwh over a quarter. i.e the reading on the inverter for Etotal was 6,000 kwh at the start of the quarter and 8,000 at the end of the quarter. (Obviously to know this you would have had to read your inverters Etotal number at the start of the period and at the end).

As such we know that in total the system has produced 2000 kwh for that period.

We also know that only two things could have happened to that power. It could have been used up by demand (loads) within your property or it could have been exported to the grid.

Your power bill only shows you how much of this power was exported to the grid. You can also read your SAPN meter to find out the amount of power exported.

(On the SAPN meter in your fuse box you can hit the scroll button several times until you bring up Tariff 9 which is the Solar feed in tariff. If you measure this figure at the start and end of a period you can see the amount of power exported for a period but it is much easier to get this figure under the Solar Bonus column of your power bill).

Lets say for the sake of the example above that 500kwh of the 2000 kwh produced in total was exported to the grid.

You can then work out how much of the solar power was consumed internally by the simple formula:

Solar Power Consumed Internally = Change in Etotal on inverter during the period -Power exported to the grid during the period

From our example above this means 1500 kwh were produced and used at the premises and 500 kwh were exported to the grid.

Once you know these numbers you can properly work out the amount of financial benefit your solar system has delivered you over a period by the following formula:

Total Return= (Solar Power consumed Internally x Top rate you pay for peak power) + (Solar power exported x Feed in Tariff for solar power)
so in our example:

Total Return = 1500 kwh x 34 cents(current rate for peak power) + 500 kwh x 5.3 cents (current rate for SA solar feed in tariff)
= $510 +$26.5
= $536.50

The common mistake people make with this calculation is to only look at the value of the exported power that shows up on the power bill and to forget to calculate the value of the remainder of the solar power that was generated but then used internally. If this power was not generated it would have been bought from the grid and so it is important to value the Solar Power Consumed Internally.


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