How Much Solar Power Do I Need?

My advice on solar power system sizing has changed over the years due to the cost of solar panels continuing to reduce. This video explains in a nutshell the best system size for most average Australian households that will provide the best bang for buck.

And here’s a video addressing some important misconceptions about the financial payback out of the way:

If you want to run through calculations yourself to determine the size of the solar power system you should get for your specific circumstances and how many solar panels you’ll need, the first points you’ll need to consider are:

1) How much money you are prepared to spend.

2) How much electricity your household uses.

The size of a solar power system is typically described by its solar PV “peak output” (in Watts or kW) i.e. how much electricity it will produce on a perfectly sunny day, when the sun is at its strongest. So, a 5kW solar system will produce 5kW peak or 5000W peak.

If your system was made up of 250W solar panels, it’s size (capacity) would be 20 x 250W = 5000W peak. (for example)

This value is measured in “kiloWatts” (kW). A Watt is a basic measure of electrical power, and the kilo means there are 1000 of them. i.e. 1 kW = 1000Watts

A word of caution: be careful to understand what you are being quoted by your installer and that it is the solar PV peak Watts and NOT the inverter size they are quoting. I’ve seen a few press adverts selling a “3kW system” that consists of a 3kW inverter with only 1.5kW of solar panels!

To give you a ‘feel’ for how much power a kiloWatt represents here is what you can run with 1kW of power:

what does 1kw of electricity power?

Wow, so does that mean you could power 60 globes with a 1kW solar power system?

Errrrr… Kind of..but not necessarily.

The thing is, a 1kW sized solar panel system will only give you 1kW of power for a few hours a day, and then only if it is a clear, sunny day. So your 60 globes will actually only be all on for a few hours a day.

This graph shows a what the electricity output of a 1kW solar panel system might look like over a summers day. You can see here that 1kW is only generated at midday when the sun is at its strongest:

typical power profile for a solar panel in one day

That is the reason that most solar systems are connected to the national electricity grid. The grid will absorb any electricity generated by the solar panels that you don’t use in your house. For example, when the sun is strong and you don’t have the air conditioning on. When you are using more electricity than can be produced by the sun (and at night of course), the grid is used to top-up your electricity needs.

How many solar panels will you need to offset your electricity usage?

It is dead easy to calculate an approximate size your solar power system should be and how many solar panels you’ll need, if you want to generate as much electricity as you consume.

But first a word of warning – I don’t recommend sizing solar systems this way! The calculation is too simplistic and may not result in the holy grail of a zero dollar electricity bill. The reason why is a long one, but I go through it in the 2nd video above (and also here).

OK here’s how to do it.

1) If you don’t totally understand the difference between a kW and a kWh, then click that link and learn! It matters.

2) Look at your most recent bill. I’ll assume it is quarterly. Look at how many kWh’s you have used in the previous 4 quarters. The information will be on your bill – look for the bar graph of usage over time and add up the last 4 quarters.  That is how many kWh’s of electricity you use per year. Let’s imagine I use 5840 kWh per year as that is the average Aussie usage. (In reality I use a hell of a lot less than that by the way!)

3) Divide your [annual kWh’s] by 365. That is your daily usage. In this example it comes out as 16kWh’s per day.

4) Check on this table how many kWh’s per installed kW of solar panels you can expect to get for your location.

Adelaide 4.2 kWh
Alice Springs 5.0 kWh
Brisbane 4.2 kWh
Cairns 4.2 kWh
Canberra 4.3 kWh
Darwin 4.4 kWh
Hobart 3.5 kWh
Melbourne 3.6 kWh
Perth 4.4 kWh
Sydney 3.9 kWh

Find the city nearest to you and divide your [daily kWhs] by the number.

In our example – I live in Adelaide, so I divide 16 by 4.2, which is 3.8.

5) I need a 3.8kW sized solar system in Adelaide to generate the same amount of electricity that I use.

But as I have said many times – this is not necessarily the optimum system size from an economic point of view for these reasons.

The simple answer is to get good advice from a qualified professional who can design a system sized to suit your needs. If all this talk of kilowatt hours is totally confusing you, then I suggest that you simply use my service to get 3 quotes for a solar power system from installers I’ve hand-picked. When you are asked what size you are looking for, simply select the ‘don’t know – please help’ option. This will allow you to talk directly to an expert who can explain all the ins and outs of system sizing with you and recommend how big a solar power system and how many solar panels you’ll require based on your needs and circumstances.


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