Kilowatt-hour (kWh)

kW is a rate, kWh is an amount

A kilowatt (kW) is how fast electricity is being used or produced at a given moment. A kilowatt-hour is how much was used or produced over time. On a hose, kW is the gallons per minute and kWh is the water in the bucket.

The math is one multiplication, kWh = kW × hours, and a kW is 1,000 watts.

Example: A 1,500-watt heater draws 1.5 kW. Running for 4 hours, it uses 1.5 × 4 = 6 kWh. A 500-watt work light left on for 12 hours also uses 6 kWh: a third of the power for three times as long. The wattages are round numbers for illustration.

Equipment that cycles rarely runs at full rating for hours, so spec-sheet watts times hours is a high-end estimate. Put numbers in writing only from the customer's bills or measured use.

kWh on a utility bill

The meter counts kWh, and the energy part of a typical home's bill is the kWh used times a price per kWh, sometimes split into supply and delivery lines. Other charges don't follow usage at all, such as a fixed monthly charge. Some rate plans also change the price by time of day, step it up in tiers as usage rises, or add a demand charge based on the highest kW drawn during the billing period.

That matters when you talk about savings. The total bill divided by the kWh used gives an average that includes fixed charges, which don't shrink when usage does.

Example: A bill shows 1,000 kWh at $0.18 per kWh ($180) plus a $20 fixed charge, $200 in all. The average works out to $0.20 per kWh, but using 100 fewer kWh saves $18, not $20, because the fixed charge stays. The rates are made up for illustration.

Where it shows up on the job

• Solar. System size is in kW of panel (DC) capacity. Production is in kWh per year and depends on location, tilt, direction and shade. Sizing starts from the customer's annual kWh on 12 months of bills, and a proposal should say whether each kW figure is DC (the panels) or AC (the inverter output), since the two differ. How exported kWh are credited depends on the utility; see net metering. The solar pricing guide walks through sizing.
• Batteries. Capacity in kWh is how much energy is stored. The power rating in kW is how much it can deliver at once, so a battery with plenty of kWh can still fall short of running several big loads together.
• EV chargers. The car's battery holds kWh and the charger delivers kW. Charging time is roughly the kWh needed divided by the charger's kW, before losses and any limit set by the car itself.
• HVAC. Tonnage is a rate too: how much heat the system can move per hour. The kWh it uses over a season depend on its electric draw and how many hours it runs, which is why a higher efficiency rating such as SEER2 means fewer kWh for the same cooling.

Example: A customer's daily driving takes about 20 kWh of charging. At 8 kW that's about 2.5 hours; at 2 kW, about 10 hours. Either way the car takes on about 20 kWh, so at $0.15 per kWh the energy costs about $3 a day. The bigger charger changes the time, not the kWh. The numbers are made up for illustration.

Common mistakes

• Writing kW when you mean kWh. "kW/h" is wrong too: energy is kW times hours, not kW divided by hours.
• Pricing savings at the average bill rate. It includes fixed charges that never go away, so it overstates every kWh saved.
• Giving a savings figure without its assumptions. Savings depend on the rate plan, export credits and when the customer uses power. Put the rate you used next to any dollar figure.