What is the difference between a watt and a watt-hour?

Understand solar energy terms such as watt and watt-hour

Knowing the difference between a watt (W) and a watt-hour (Wh) helps you understand the impact of your home energy use on your electric bill. You can also compare your home energy use to the energy generated with your Enphase microinverter system. Read on below or watch the video here.

While watts and watt-hours are related terms, they are not the same thing.

So what is a watt?

A watt (W) is a unit of power, and power is the rate at which energy is produced or consumed. Think of watts as a measure of electrical flow. Does an electrical device need a big flow or a small flow to work? For example, a 100 W light bulb uses energy at a higher rate than a 60 W bulb; this means that the 100 W light bulb needs a bigger “flow” to work. Likewise, the rate at which your solar energy system “flows” power into your home is measured in watts.

MyEnlighten displays your system’s latest and daily peak power generation in kW, which is equal to is 1,000 W.

So what is a watt-hour?

A watt-hour (Wh) is a unit of energy; it’s a way to measure the amount of work performed or generated. Household appliances and other electrical devices perform “work” and that requires energy in the form of electricity. Utilities typically charge you for electrical energy by the kilowatt-hour (kWh), which is equal to 1,000 watt-hours. Depending on your interconnection agreement, your utility may credit you for excess generation which will also be measured in kilowatt-hours.

MyEnlighten displays the amount of energy generated by your microinverter system in watt-hours. For example, an Enphase microinverter system might generate 400 kWh (amount) in the month of September (period of time). A kilowatt-hour (kWh) is 1,000 Wh, so 400 kWh is 400,000 Wh. In some cases, MyEnlighten will display energy as megawatt-hours (MWh), which is one million watt-hours.

So what is the difference?

In a nutshell, watt-hours measure amounts of energy for a specific period of time, and watts measure rates of power at a moment in time.

A common analogy for watts and watt-hours is speed and distance. Speed is a rate of how fast you drive at an instant in time (watts); distance is the length, or amount that you drive over a period of time (watt-hours). For example, if you drive at a constant rate of 60 miles per hour for one hour, then you will have traveled 60 miles.

Similarly, if a 60 W light bulb is on for one hour, then that light bulb will have used 60 Wh of energy. If left on for two hours, then the 60 W light bulb will have used 120Wh of energy.