A kilowatt-hour (kWh) is a way of measuring how much electricity you use over a period of time. If you have a 1,000-watt appliance running for one hour, that’s one kilowatt-hour of energy. On your electric bill, you’re charged for each kilowatt-hour you use, so the total kWh number tells you how much electricity you consumed during the billing period.
A kilowatt (kW) is a measure of how much power something can supply or use at a particular instant. Think of it like the speedometer in your car: it shows how much energy is flowing at any given moment. For solar, we typically see arrays described in kilowatts because it shows how much power they can produce under ideal sunlight conditions.
Watch how speed affects energy use.
The Blue Car takes its time—traveling 100 miles over 10 hours. It uses 1,000 kWh of electricity, averaging just 100 kW of power per hour. Because it draws less power at any given moment, the cost to travel those 100 miles stays relatively low.
The Red Car, on the other hand, races through the same 100 miles in just 1 hour. It also uses 1,000 kWh, but it burns through that energy all at once—drawing 1,000 kW of power in a single hour. Even though both cars used the same amount of energy, the Red Car’s higher power demand makes the trip much more expensive.
Let’s take a closer look at the math for the Red Car:
Power (kW) = Energy (kWh) ÷ Time (hours) = 1,000 kWh ÷ 1 hr = 1,000 kW
– or –
1,000 kWh ÷ 1 hour = 1,000 kW
Although kW and kWh sound similar, they refer to related but different concepts. A kilowatt (kW) measures power output or usage right now, while a kilowatt-hour (kWh) measures the total energy used over time. In solar terms, you might install a 10 kW system (how much power it can produce in full sun), and then you track how many kWh of electricity it generates over hours, days, or months.
Curious how kW demand impacts your utility bill? See Reading Your Electric Bill to understand demand charges.Want to know how system size (kW) relates to overall cost? Jump to What Determines the Cost of a Commercial Solar Project? for factors influencing project pricing.