Quick answer: kW (kilowatts) measures power: how fast electricity is being produced or used at a moment in time. kWh (kilowatt-hours) measures energy: how much has been produced or used over time. Your solar system is named by its kW size, your electricity bill and your battery are measured in kWh, and a 1kW appliance running for one hour uses 1kWh.
Solar quotes are full of numbers, and kW and kWh sit side by side. They look almost identical and measure different things. Once the difference is clear, a quote becomes much easier to read.
What is the difference between kW and kWh?
Think of a car. Your speedometer shows how fast you are going right now. Your trip meter shows how far you have gone. kW is the speedometer: the rate at which power is flowing. kWh is the trip meter: the total energy that has flowed over a period of time.
| kW (kilowatt) | kWh (kilowatt-hour) |
Measures | Power, the rate | Energy, the total over time |
Car analogy | Speed | Distance travelled |
On your solar quote | System size and inverter size | Battery size, expected generation |
On your power bill | Not usually shown | Energy used, and exported if you have solar |
The arithmetic is simple. Power multiplied by time equals energy. A 1kW heater running for one hour uses 1kWh. A 2kW appliance running for 30 minutes also uses 1kWh. A 3kW heater running for four hours uses 12kWh.
What does the kW on a solar quote mean?
Solar systems are named by their size in kW, such as a 6.6kW solar system, a 10kW solar system or a 13kW solar system. But a quote can show two kW figures, and they are not the same thing.
Panel kW
This is the combined rating of the panels, the “system size”. It is the most the panels can produce under ideal conditions. It is a ceiling, not what you will see every hour, because sun angle, cloud, heat, shade and season all move the real output below it.
Inverter kW
This is the capacity of the inverter that converts the panels’ DC output to the AC your home uses. It can be smaller than the panel kW. Solar Victoria’s planning guide says kW refers to system size and inverter capacity, notes that residential single-phase connections are typically limited to 5kW, and says arrays can be oversized by up to 33 per cent above the inverter capacity.
So a quote for 6.6kW of panels with a 5kW inverter is normal, not a mistake. Network rules vary by distributor and state, so ask your installer what applies at your address. If you want to know more about the part that does the conversion, see our explainer on what a solar inverter is.
What does the kWh on a quote or bill mean?
kWh is the unit your electricity is billed in. It is also how much your solar panels produce, and how much your battery can store. Solar Victoria’s guide describes kWh as what appears on your household bill. When you generate more than you use, the extra goes to the grid, and you may be credited per kWh exported. Our guides on feed-in tariffs and net metering explain how that works.
Two systems with the same kW rating can produce different kWh. Roof direction, tilt, shade and location all change the output, which is why your installer should be able to give you an estimate of generation for your roof. Our blog on tilt angle for solar panels shows one of those factors.
To know more about kW vs kWh: What the Numbers on Your Solar Quote Actually Mean
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