What is a kilowatt-hour (kwh) vs. a megawatt-hour (mwh)?

If you've been considering your energy costs lately, you may be looking into energy efficiency upgrades for your home, such as installing solar panels, adding smart thermostats, or pursuing an EnergyStar home certification. And when comparing various energy solutions and the prices for new options, you'll hear one metric used incessantly: kilowatt-hour (kWh). So what exactly does a kWh mean, and how does it differ from a kilowatt (kW)? A megawatt-hour (MWh)?

A kilowatt is a metric that equals 1,000 watts of power. Wattage, in turn, indicates how much power a device can provide over a relative amount of time. Thus, a 1,000-watt (1 kW) microwave will warm up a meal much faster than a 600-watt microwave. Because of this relationship between capacity and time, we use the terms watt-hours (Wh) or kilowatt-hours (kWh) to describe energy use.

Watt-hours and kilowatt-hours define the amount of work performed or energy used in one hour. A simple analogy is that speed is a metric that defines distance traveled over time, while energy defines power consumption over time. Using that same 1,000-watt (1 kW) microwave for an hour would use up 1 kilowatt-hour (kWh) of energy.

Watts and kilowatts refer to different quantities of the same thing – energy. The next step up from a kilowatt is a megawatt. One megawatt equals 1,000 kilowatts or 1 million watts; the same conversion applies to megawatt-hours and kilowatt-hours. Thus, if a 1,000-watt (1 kW) microwave is left running for 41.6 days straight, it would use up one megawatt-hour (MWh) of energy (1,000 watts/24 hours per day = 41.6 days). Thus, any comparison between kilowatts and kilowatt-hours can be applied to megawatts and megawatt-hours, just 1,000 times as large. 1 MWh of energy is equivalent to 1,000 kWh of energy.

It's important to understand the context for these metrics to comprehend kWh and MWh. For example, the average U.S. household uses 10,972 kWh of energy each year (according to the latest data from the Energy Information Administration). Using that information, we can estimate that monthly energy use is roughly 914 kWh, and daily energy use is a little lower than 30 kWh for the average home in the United States.

When talking about residential energy use, it is most common to use kilowatt-hours. Your monthly energy bill will report your usage with this metric. When evaluating energy upgrades such as a solar installation, companies will discuss what size your system would need to be in kW to meet your kWh needs.

By comparison, MWh is typically used in reference to larger-scale electricity use, such as a new power plant being built or an entire town or city launching an energy upgrade. In one of these scenarios where energy use is being discussed at a great scale, the terminology of choice will be megawatt-hours or gigawatt-hours (GWh), which refers to one billion watts of power.

When you look at your solar options, you must determine whether the system you buy is powerful enough to offset your energy needs. This is where kilowatts and kilowatt-hours come in: paired with another data point known as the "solar panel production ratio," they make it possible to size your system accurately.

In the earlier example, we discussed how a 1,000-watt microwave would use one kilowatt-hour in one hour. When talking about solar, the process of converting power capacity (watts) into expected energy output (kWh) is not as straightforward due to several factors. One thing is sure: a microwave will produce power at a constant rate regardless of whether or not you live in California, if it's raining, or if you place it on a certain type of countertop. With solar panels, on the other hand, the environment and circumstances will significantly impact the panel's rate of output.

The solar panel production ratio (also referred to as the solar performance ratio) is a coefficient that determines how suitable your roof and location will be for solar, allowing you to size your solar system accurately. Factors such as the angle and orientation of your roof and the amount of sunlight hours in your town or state will significantly impact your solar array's performance. The solar panel production ratio is a metric that allows you to estimate your needed solar system size and the expected hours of energy that it will produce. So, for example, a solar panel on the north side of a roof will have a much lower production ratio than the same panel on the south side of the roof because south-facing roofs typically receive higher sunlight exposure. The kilowatt-hour formula to use for conversion is the following:

Solar array system size (kW) = Annual energy needs (kWh) / solar panel production ratio

Thus, if your household uses 10,972 kWh per year (the national average) and your location and roof type offer a 1.57 solar production ratio, you'll need about a 7 kW solar system to offset your energy needs.

For those looking for a ballpark estimate of what a solar installation would cost for their home, try our Solar Calculator for instant insight. If you want customized system designs for your home, the best place to get more information is through a reputable solar installer. They can help you estimate your home's potential for solar, how much a solar panel system will produce at your home, and how much it will cost. If you want quotes from vetted solar installers in your area, check out the EnergySage Marketplace.