Power vs. energy: the two numbers everything else builds on

Power (measured in watts, or kilowatts — kW) is how fast electricity is being used or produced right now. It’s a speedometer reading.

Energy (measured in watt-hours, or kilowatt-hours — kWh) is power sustained over time. It’s the odometer — the total distance travelled.

Your bill’s usage charge is priced in cents per kWh — it’s charging you for total energy consumed over the billing period, not for how fast you used it at any one moment. (The exception is a demand charge, covered on the bill page, which does charge based on your peak rate of draw.)

When someone says they have a “6.6kW solar system”, that’s the system’s maximum power output under ideal sun — the speedometer’s top speed. How much energy (kWh) it actually produces in a day depends on sunlight hours, weather, and panel orientation, which is why a 6.6kW system might produce anywhere from 15 to 30+ kWh on a given day.

Generation in the NEM comes from a mix of coal, gas, hydro, wind, and both large-scale and rooftop solar, and the mix shifts constantly through the day and across seasons. Two patterns matter most for understanding your bill and the market:

This shape — a midday dip and a sharp evening ramp — is nicknamed the duck curve, and it explains most of what feels counterintuitive about solar pricing and the NEM. It’s covered in detail on the NEM page.