Sizing up a solar PV system – How many kW do you really need?

So you’ve seen a hot deal for a solar PV system that offers a 3kW, 5kW, 7kW, even a 10kW system for cheap, which looks good, right? But then, you think… well is that enough for my place? Is that many kW a lot, or not a lot? And what the heck is a kilowatt-hour; is that different?

We get it! Nobody but your solar installer or power supplier really thinks in terms of kW or kWh — most people just look at the amount at the top of their power bill, say a swear word or two, and then stick it on the fridge to pay later. But a kW measurement is really the only way to know if a solar PV system is fit for purpose. So…


Here’s how to work out what size solar PV system you need

First, let’s look at what you’re trying to achieve. If you’re just trying to reduce your bills, and stop having a conniption when the kids leave their lights and Playstation on all day, any size system is probably better than none. 

If you’re trying to get the smallest bill possible (or reach net-zero CO2 – good on ya!), then you’ll need to make some calculations — see below. These numbers will also help if you plan to go beyond your own needs and sell power back to the grid en masse or through a VPP (virtual power plant).

There will also be a difference in the size system you need depending on whether you need an on-grid or off-grid system. Most residential homes in Perth and the greater metro area will choose an on-grid system. They will use power from the panels, send excess electricity to the grid, and buy from the grid when the solar PV system isn’t generating enough power.

Off-grid systems are typically more complex and expensive, requiring backup generators, battery systems and power supply switching to prioritise essential services (such as water pumps or the refrigerator).


How to calculate your daily energy usage for a solar PV system

Head over to the fridge and grab that power bill. And a snack if you want; no judgement. You’ll likely have a daily energy consumption written right there in kilowatt-hours (or ‘units’ on some bills). If not, there’s likely a table that shows your usage year on year or for each quarter. Just divide your kWh usage by the billing frequency (divide by 30 if you’re billed monthly) for your daily use.

Here are some typical cases (averaged out over a year) for a Perth home’s energy usage:

  • A one-person apartment, full-time worker, typically uses about 8–9kWh per day
  • A typical two or three person household, on average, will use 15-20kWh daily
  • A household of five with a pool may use as much as 30-35kWh per day

So, does that mean even a 7kW system isn’t going to generate enough power for anyone?!

It’s actually heaps, because we need to translate that into kilowatt-hours.

The kW attributed to a solar PV system is what that system can potentially generate per hour in the ideal situation: the sun at noon, not too hot or cold, clear skies, ideal latitude. But it’s not noon all day (because, physics), so there’s a variance ineffectiveness through the day (less power generated in the late evening and early afternoon, and none at night).

So, we’re going to apply a very broad ‘rule of thumb’ here:

1kW of solar panels equates to 4kWh of electricity production per day (approx)

The amount of electricity generated per kW of solar panels will depend on many factors such as your location, the time of year, home orientation, panel orientation, age, and so on. That’s why a Koala Solar on-site quote is the best way to get a more accurate understanding of what’s going to be possible for your home or business.

If we opt for a 5kW solar PV system, that will generate about 20kWh on a clear day in Perth in summer. But, again, we’re looking at ballpark figures here.

So, how many solar panels will I need in Perth?

Most freestanding homes will have all the required roof space to install all the panels they will need for a system that meets the energy needs of its occupants.

Solar panels vary in quality and efficiency, which is reflected in the price. For example, if you were looking at 265W panels, you’d need 12 to supply a 3 kW system. A larger system would require more panels, but there are additional benefits to choosing a 5kW or even 10kW system if you have a larger household. Economies of scale dictate the cost curves off as the systems get larger, and a larger system makes having a battery more viable, meaning less need to rely on higher grid prices.

If you’re not home during the day, most of your power will be exported to the grid at a relatively low feed-in tariff, maybe 5 cents to 10 cents per kW. Having a SENEC battery system or Tesla Powerwall means you can store your power for your own use in the evenings when needs are higher, and grid prices are as well. Check out our full breakdown of WA solar rebates in 2021.


What else impacts the size solar PV system I need?

How and when you use electricity will affect the ROI on your solar PV system. You may want to consider power usage switching – a fancy way of saying to use high energy equipment in the day, such as the washing machine, dishwasher or pool cleaner. It’s also worthwhile setting timers on heating and cooling, mediating the temperature of your home during your power supplier’s off-peak periods and when your panels are generating at their peak.

Orientation of your panels is the other important consideration. Facing panels north is usually best to maximise the amount of sunlight they will be able to capture. However, if that’s not possible or agreeable, getting them on the northeast or northwest side of the home can be just as good. Even a south-facing installation can generate as much as 80% of the rated power for those panels. 


Still not sure what size solar PV system you need?

Never fear! You can always request a free quote from Koala Solar to help you work out what’s gonna work best for you. The guys can take you through the options on panels, inverters, batteries and installation setups at your place.



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