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What is the Difference Between Watts (W), kilo-Watts (kW) and kilo-Watt hours (kWh)?

This is a common question and a topic that easily confuses many of us. To keep it simple, they are all fruit, but they are apples and oranges.

A watt is defined as unit of power, equivalent to one joule per second, corresponding to the rate of energy in an electric circuit. A kilo-watt is simply 1,000 watts. The abbreviation for kilo-watt is kW. So when we say 10 kW, it means 10,000 watts.

A kilo-watt hour is a measure of 1,000 watts during one hour. The abbreviation for kilo-watt hour is kWh. So 1,000 watts during one hour is 1 kWh. The power company measures energy in kWh in order to calculate your monthly bill.

Here is a practical way to think about how watts, kilo-watts and kilo-watt hours are related in a solar power system. Let's start with a single solar panel capable of producing 100 watts. This is commonly referred to as a 100 watt solar panel. Now, during the period of the day when the sunshine is at it's peak, the solar panel is capable of generating up to 100 watts of power every second. Think about that; it's a lotta watts.

Next, let's measure that 100 watts of peak power for 60 minutes or 1 hour. Remember, that solar panel can produce 100 watts per second under peak conditions. So that's 100 watts X 60 seconds X 60 minutes. This calculates to 360,000 watts per hour, or 360 kW per hour. Therefore, that 100 watt solar panel operating at peak performance could generate 360 kWh of power.

Remember, this is a theoretical example using the definitions and a mathematical example. In a real world setting, a 100 watt solar panel will seldom produce 360 kWh of power. The sunshine is only at the peak for a very limited time. The location, temperature and other environmental factors like clouds could effect solar panel performance. Plus, solar panels are subject to the laws of physics so that wire line losses, distance and DC to AC conversion derating must all be factored to estimate actual power production.

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