Power vs. Energy
The terms Energy and Power are often used interchangeably when describing electricity. But despite their close relationship, the terms energy and power actually represent different measurements in the context of both solar power systems and electric consumption (loads).
Power (watt) is capacity to do work at any moment (i.e. instantaneous)
Energy (watt-hours) is capacity to do work over time (i.e. hours, days, months, etc.)
The kilowatt (one thousands watts) is the typical measure for power for a household. A kilowatt (kW) represents the level of power produced or consumed at any moment. For example, a water heater uses 4.5 kW of power, and a toaster uses 1.2 kW of power; running both at the same time equals 5.7 kW of power. Likewise, a solar panel produces power the moment it is exposed to sunlight.
Power use over a period of time is measured in kilowatt-hours (kWh), or a total amount of electricity (whether produced, consumed, or stored in a battery). Watt-hours are a measurement of energy. For example, powering a 1 kilowatt load (i.e. a coffee pot) continuously for one hour equals 1 kilowatt-hour of energy consumed. In reality, loads run at fluctuating power levels and are rarely constant, and the kWh represents average power use over the number of hours measured.
Kilowatt-hours are quantified over longer periods as well, such as day, month or year. For example, electricity bills are monthly, so energy charges are denoted in kWh/Month.
For an average South Dakota home...
Average day: 33 kWh
Average month: 1000 kWh
Annual: 12,000 kWh
Peak power consumption:
Average: 5 to 9 kW
High: 10 to 15 kW
Very high: typically 20 kW maximum (esp. for electric heat)
Your power bills distinguish between energy and power consumption as well, particularly for residential demand charges and commercial accounts. Most residential accounts are billed only for energy (kWh). Accounts with demand charges are billed for both energy and power (kW). The demand charge represents the peak power demand (during specified, "peak" times of the day) for electricity over the billing period.
For solar system design, we account for the daily and seasonal variations of both energy (kWh) consumption and solar generation to determine the best average match.
For back-up power system design, the battery capacity must be able to handle the peak load power (kW), which is when multiple high-power appliances operate simultaneously. Individual loads that exceed the battery power capacity cannot be backed-up, but running multiple large loads at once is usually avoidable. The battery capacity must also be sufficient for powering the back-up loads over a period of time, which is the energy (kWh) capacity.