There are two types of solar energy measurement, based on the type of energy: photovoltaic energy produces electricity, and solar thermal energy heats water. The energy output is expressed by the amount of solar radiation that reaches the absorbent surface – be it a solar panel or solar collector.
Electricity is measured in watts, with one thousand watts in a kilowatt. Using one thousand watts of electricity in one hour is a kilowatt-hour (kWh), the measurement on your utility bill.
For solar panels, the measurement of kWh refers to the amount of energy produced by the panel. This measurement is represented as kWh per square meter of panel surface.
An appliance rated at 1kW uses one kWh of energy when operating for an hour. The average electricity consumption for a household in the United States is 8900 kWh per year.
A 1kW system generates almost 1,000 kWh per year in cloudy regions, and almost double in sunny climates. The term peak refers to energy production in optimal conditions. Whether the system generates the stated amount depends primarily on the amount of solar radiation reaching the solar array.
Solar Thermal Energy
Solar thermal energy is measured in British thermal units (Btu) per square foot of collector space. A Btu is about the amount of energy it takes to heat a pound of water from 39°F to 40°F. For example, it takes 38,000 Btus to heat 80 gallons of cold water to 122 degrees. A solar hot water heating system with a rating of 30,500 Btu/day rating will be able to provide 80 percent of the daily hot water needs.