Power is measured in watts. A kilowatt (kW) is 1,000 watts of power. A megawatt (MW) is 1 million watts of power. Watts measure the rate of power at a moment in time.
Solar power systems are sold as a measure of their kilowatts, for example, a 5-kW system. This means that all the solar modules or panels together add up to 5 kW. This could look like a system of 20 solar modules, each with 250 watts of power. 250 W x 20 modules = 5,000 W or 5 kW.
Kilowatt-hours (kWh) measure the amount of energy used over a specific time period, in kilowatts per hour. Electricity used for powering home electronics and small or large appliances, whether for just a few seconds or for most of the day, is measured in kWh.
The size of the solar energy system you need depends on how many kWh of electricity your home uses and by how much you want to offset your usage from the grid. The greater your energy requirements, the bigger the solar power system needed to offset this usage.