Power and energy

The difference between power and electrical energy is very important when discussing PV systems and energy efficiency. Do not get them confused.

Power (P) is the rate at which energy is used by electrical equipment or produced by a generator:

• The unit of power is the watt (W)
• 1 kilowatt (kW) = 1,000 watts = 1 kW
• 1 megawatt (MW) = 1,000,000 watts = 1 MW = 1,000 kW
• 1 gigawatt (GW) = 1,000,000,000 watts = 1 GW = 1,000 MW = 1,000,000 kW

Energy (E) is the power (P) consumed by electrical equipment or produced by a generator over a period of time (t):

• The unit of energy is the watt-hour (Wh).
• Energy (watt-hours) = power (watts) × time (hours) or E = P × t
• 1 kilowatt-hour (kWh) = 1,000 watts-hours = 1 kWh

Example 1

A 300 W machine is on for 8 hours.

How much energy will the machine consume?

Energy consumed = 300 W × 8 h = 2,400 Wh = 2.4 kWh.

Example 2

A 100 W computer is on for 8 hours.

How much energy will the computer consume?

Energy consumed = 100 W × 8 h = 800 Wh = 0.8 kWh.