Solar panel glossary: kW, kWh, and mWh defined
If you're getting quotes for home solar, the electrical terminology can get pretty confusing. Here's some simple explanations.
If you’re doing research into home solar, you may come across some terminology that you’re unfamiliar with.
A few common ones are kilowatts and kilowatt-hours. They’re confusing because of the similar names, but they refer to distinct things. Here’s an attempt at some simple explanations.
What is a kilowatt?
A watt is a measure of electrical power. A kilowatt is equal to 1,000 watts. The abbreviation for kilowatt is kW. (The W is capitalized because it is named after the Scottish inventor James Watt.)
Power, in this context, refers to the rate of flow of electricity. To give you a sense of scale, a hair dryer on its low setting is using roughly 1 kW of power. If you switch the hair dryer to high, the flow of electricity will increase to approximately 1.8 kW.
What is a kilowatt-hour?
A kilowatt-hour is a measure of energy. The abbreviation for kilowatt-hour is kWh.
“Energy” means a quantity of electricity. That hair dryer on low uses about 1 kW of power. If you leave it running for one hour, it will have consumed about 1 kWh.
Some more examples of kW and kWh to help make sense of it
Your monthly electricity bill measures the quantity of electricity you consumed during the previous month. This is why your bill is based on kilowatt-hours.
When talking about the power requirements of appliances like your hair dryer, we use watts or kilowatts. A refrigerator uses perhaps 100 or 200 watts (0.1 kW or 0.2 kW).
A large central air conditioner might use 3 kW. If that A/C runs for one hour, it will have used 3 kWh. After 2 hours it would be 6 kWh, and 9 kWh after three hours. Make sense?
What is a megawatt-hour?
A megawatt is a 1,000 kilowatts. This means that one megawatt-hour is equal to 1,000 kWh. Its abbreviation is MWh (with a capital M).
When it comes to household electricity usage, you usually don’t use megawatt-hours as the unit of measure. An exception is when you’re talking about annual usage or annual power generation of a solar array.
What does kW, kWh, and MWh mean with home solar?
When it comes to home solar, you’ll see kW and kWh frequently mentioned.
Kilowatts are used when discussing the size of your system. For example, my home solar installation is relatively small: 18 panels that are 260 watts each, for a total of 4.6 kW. Because of better panel efficiency these days, the average system is slowly increasing in size and is now close to 7 kW.
You’ll see kilowatt-hours mentioned when it comes to how much electricity your system generates. That’s a function of its size in kW, but also how much sunlight they receive during the year. No shading, a south-facing roof, and a sunny climate will help your system generate more kWh during a year.
When working with a solar installer to design your system, they’ll need to know how much electricity you use in an average year, which is measured in kilowatt-hours. To find that out, look at your most recent utility bill. Most bills will have a summary of your past 12 months of usage. If not, you’ll need to add up the total number of kWh from your previous 12 bills.
Give that info to your solar installer, and they’ll be able to design a system that generate enough electricity to meet your annual needs.
Once you have a system installed, you’ll have access to monitoring that will tell you how much electricity you generate. Daily generation is measured in kWh, but your annual generation will be multiple megawatt-hours. For example, in an average year my system generates around 5 MWh.