kW vs kWh: the two units that matter
Almost every confusing thing about electricity bills, solar quotes, and tariff comparisons comes down to mixing up these two units. Understand the difference and the rest of this site will make much more sense.
The question that trips everyone up
“My solar installer says my system is 6.6kW, but my bill shows kWh. What is the difference?” This is the single most common point of confusion in household energy, and it is the one that matters most to sort out.
The simple answer: speed vs distance
kW (kilowatt) is a measure of power — how fast electricity is being used or produced right now. Think of it as the speedometer in your car.
kWh (kilowatt-hour) is a measure of energy — how much electricity was used or produced over time. Think of it as the odometer — the total distance you have driven.
A car travelling at 60 km/h (speed) for 2 hours covers 120 km (distance). Electricity works the same way: an appliance drawing 2 kW (power) for 3 hours (time) uses 6 kWh (energy).
See it in action
kW
kilowatt
How fast electricity is being used
Think:
Speed
kWh
kilowatt-hour
How much electricity was used over time
Think:
Distance travelled
2 kW
heater
3 hrs
running time
6 kWh
on your bill
The heater’s power rating (kW) stays the same — but the energy it consumes (kWh) keeps adding up the longer it runs.
Real household examples
The power rating of an appliance tells you how fast it uses electricity. But what really matters for your bill is the energy — how long it runs for. Here are some examples that show how this plays out:
Electric kettle
Power: about 2.2 kW (a hungry appliance while it is running)
Typical use: 5 minutes to boil
Energy per boil: 2.2 kW × 0.083 hours = about 0.18 kWh
Cost per boil at 30c/kWh: about 5.5 cents
Fridge
Power: about 0.15 kW (much gentler than a kettle)
Typical use: runs 24 hours a day, 365 days a year
Energy per day: 0.15 kW × 24 hours = 3.6 kWh
Cost per day at 30c/kWh: about $1.08 — roughly $395 per year
Reverse-cycle air conditioner
Power: about 2.5 kW (varies with temperature and size)
Typical use: 6 hours on a hot evening
Energy per session: 2.5 kW × 6 hours = 15 kWh
Cost per session at 30c/kWh: about $4.50
The fridge vs kettle lesson
The kettle uses electricity 15 times faster than the fridge — but the fridge costs far more per year because it never stops running. When it comes to your bill, low power × long time can easily beat high power × short time. This is why always-on appliances (fridges, pool pumps, hot water systems) often dominate your bill even though they do not feel like heavy users.
What this means for solar
When your solar installer says you have a 6.6kW system, they are telling you the system’s maximum power output — its top speed under ideal conditions. That is the kW rating.
But what you really care about is how much energy it produces over a day — the kWh. A 6.6kW system in a sunny part of Australia might produce 25–30 kWh on a good day in summer and 15–20 kWh on a shorter winter day.
6.6 kW
system size (max speed)
~4.5 hrs
peak sun hours
~30 kWh
daily output
Peak sun hours vary by location and season. This is a good summer day in most of eastern Australia.
Why this matters
Once you understand the kW vs kWh distinction, several things click into place:
- Your bill charges in kWh — it is measuring the total energy you consumed over the billing period. Running a high-powered appliance for a short time might use less energy than a low-powered one that is always on.
- Solar quotes use both units — system size (kW) tells you the maximum output; estimated annual production (kWh) tells you how much energy you will actually get.
- Demand charges use kW — some tariffs charge based on your peak rate of draw (kW), not just total energy (kWh). Understanding this distinction helps you compare tariff types.
- Battery sizes are in kWh — a 10 kWh battery stores that much energy. Whether it lasts all evening depends on how fast you draw from it (kW).
Quick reference
kW = speed (how fast you are using electricity right now).
kWh = distance (how much total electricity you have used).
kW × hours = kWh.