kW vs. kWh: the only solar unit lesson you need

Kilowatts are speed. Kilowatt-hours are distance. Everything else follows from that.

kW: how fast

A kilowatt is a rate of power. A 1,500-watt space heater draws 1.5 kW while it runs. A solar system rated 8 kW DC peaks well below 8 kW in practice: panels run hot and below their nameplate in summer, and the inverter that converts DC to AC is usually sized smaller than the array (PVWatts assumes a 1.2 DC-to-AC ratio, which caps an 8 kW DC system near 6.7 kW AC). The best hours tend to be cool, clear spring days. Most of the time it produces far less, and at night nothing. The kW rating is the size of the engine.

kWh: how far

A kilowatt-hour is energy: one kilowatt sustained for one hour. Your utility bills you per kWh. Run the 1.5 kW heater for two hours and you used 3 kWh. An 8 kW solar system running at full output for one hour makes 8 kWh. Over a whole year, with nights, clouds and seasons, it makes far more than 8 kWh and far less than 8 × 8,760.

The bridge: kWh per kW per year

This is the number every solar estimate turns on. For a given place, roof direction and tilt, a kilowatt of panels makes a fairly predictable number of kilowatt-hours a year. In Virginia, NREL's PVWatts puts a south-facing roof at about 1,389 kWh per kW per year (median across the towns on this site; see yours).

Sizing from your bill

1. Annual kWh. Add up twelve bills, or take one bill's dollars ÷ its ¢/kWh × 12. At Virginia's trailing-12-month average of 16.48¢ (to July 2026), a $180 bill is about 1,092 kWh a month, 13,107 a year.
2. Divide by production per kW. 13,107 ÷ 1,389 ≈ 9.4 kW. That is the system that offsets 100% of usage under net metering.

The calculator does exactly this, then adjusts for roof direction and shade and prices it out.

Three traps

• DC vs. AC. Panels are rated in DC watts; inverters convert to AC and lose a few percent (PVWatts assumes 96% inverter efficiency, counted separately from its "14% system losses", which cover soiling, shading, mismatch, wiring, connections, light-induced degradation, nameplate tolerance and availability). Quotes usually state the DC size.
• Panel count × wattage = kW. Twenty 400-watt panels is an 8 kW system. Two quotes for "8 kW" with different panel counts are the same size; compare on $/W.
• Battery sizes are kWh. A home battery is sold by the kilowatt-hours it stores. A battery also has a kW rating: how much it can deliver at once. A big kWh with a small kW will run a fridge for days but may not start a well pump.