kW vs. kWh: Reading a Solar Quote

Kilowatts and kilowatt-hours are on every electric bill and every solar quote, and they're easy to mix up. They sound alike, but they measure different things. Once you know which is which, a solar quote gets a lot easier to read.

This article explains both, then shows where each one appears on your bill, your quote and a battery spec sheet. For how the whole system fits together, see our guide to how home solar works.

What is the difference between kW and kWh?

A kilowatt (kW) measures power: how fast electricity is being made or used at a given moment. One kilowatt is 1,000 watts.

A kilowatt-hour (kWh) measures energy: how much electricity is made or used over a stretch of time.

A car trip makes the difference easy to picture. Kilowatts are like your speed, the miles per hour on the speedometer. Kilowatt-hours are like the miles you actually drive. Speed tells you the rate; distance tells you how far you went. You can drive fast for a few minutes or slowly all day, and the miles add up differently.

The same goes for solar. A system's size in kW is its rate at full sun. The kWh it makes over a year is the amount of electricity you actually get to use.

That's also why the two units turn up in different places. Installers describe equipment in kilowatts, because that's how panels, inverters and systems are rated. Utilities bill in kilowatt-hours, because that's how much electricity you used. A good solar quote connects the two.

What is a kilowatt-hour?

A kilowatt-hour is one kilowatt of power used, or made, for one hour. The U.S. Energy Information Administration notes that utilities and power plants generally measure electricity this way.

A few examples show how the math works:

• A 1,000-watt appliance running for one hour uses 1 kWh.
• A 100-watt device running for ten hours also uses 1 kWh.
• A 40-watt bulb left on for five hours uses 200 watt-hours, or 0.2 kWh.

In other words, kilowatt-hours are watts multiplied by hours, divided by 1,000. A small device left on all the time can use as much energy as a big one that runs for a few minutes.

Appliances and devices usually list their power in watts on a label or in the manual. Your home's total use, in kilowatt-hours, is on your electric bill. Many bills also include a chart of your use month by month over the past year, which is a quick way to see your summer and winter peaks.

Where do kW and kWh show up on your electric bill?

On a home electric bill, kilowatt-hours do most of the work. Your bill shows how many kWh your home used during the billing period, and most charges are priced per kWh:

• Supply. The price of the electricity itself. Eversource's Massachusetts basic service rate rose from 15.6 to 17.3 cents per kWh on August 1, 2026.
• Delivery. Charges for carrying power to your home over the utility's lines, such as local delivery and transmission.
• Customer charge. A fixed monthly fee you pay no matter how much you use. National Grid's Massachusetts R-1 customer charge is $10.00 a month.

On a sample Massachusetts bill, supply was only 43% of the total. Charges and fees made up 57%, with local delivery at 27%. That's why the kWh line matters so much: most of what you pay rises and falls with the kilowatt-hours you use. Our article on your electric bill with solar panels shows how the bill changes once your system is running.

With solar, kilowatt-hours show up on the credit side too. On an Eversource Massachusetts bill, net metering credits for the power your panels send to the grid appear under "Other Charges or Credits," and unused credits carry forward to the next bill. So the kWh your system makes lowers your bill in two ways: by covering power you use as it's made, and through credits for what you send to the grid.

Where do kW and kWh show up on a solar quote?

A solar quote uses both units, and each one answers a different question. Here are the lines to look for:

The system size describes the equipment. The production estimate describes the result, and it's the number that lowers your bill. It accounts for the things that affect your roof in particular: its direction and tilt, any shade, and the weather where you live. Two systems of the same size can produce different amounts on different roofs.

Evergreen Solar's engineering team estimates production in our own design software, using historical weather data for your location, so the estimate averages out over sunny summers and short winter days.

Don't expect production to match your use month by month. Long summer days often produce more than a home uses, and short winter days produce less. With net metering, the extra kilowatt-hours your panels send to the grid in summer earn credits that help cover your bills in winter. That's why the yearly kWh total is the number to compare.

What do kW DC and kW AC mean on a solar quote?

Solar panels make direct current (DC) electricity. An inverter converts it to the alternating current (AC) your home and the grid use. Because of that, a system's size can be measured two ways:

• kW DC is the total of all the panels' ratings.
• kW AC is the total of all the inverters' ratings.

Panel capacity is often designed to be somewhat larger than inverter capacity, so the DC number is usually the larger one. Neither is wrong. They just measure different points in the system.

How do kW and kWh apply to solar batteries?

Battery spec sheets use both units, and the difference matters just as much:

• Capacity, in kWh, is how much energy the battery stores. It decides how long your essentials can run during an outage.
• Power, in kW, is how much the battery can deliver at once. It decides what can run at the same time.

A battery with plenty of storage but too little power may not start a well pump. One with lots of power but little storage runs out sooner. Our article on sizing a solar battery goes further, and the solar batteries and backup power guide covers backup power and battery programs.

How do you use kW and kWh to compare solar quotes?

With both units straight, comparing quotes comes down to a few checks:

1. Add up the kWh on your last 12 electric bills to get your yearly use.
2. Compare each quote's yearly production estimate, in kWh, with that total.
3. Check that system sizes use the same measure, DC or AC, before comparing them.
4. Ask how each company estimated production, and what shade and weather data it used.

A quote that covers far less than your use leaves more of your bill behind. One that covers far more can mean paying for credits you may never use. Our guide to comparing solar quotes walks through the rest of the offer, from equipment to warranties.

Your Evergreen Solar proposal shows where the panels go on your roof and your expected savings, and every package includes a 25-year power production guarantee. Our article on what happens at a consultation walks through the next step, or call us at (508) 507-2908.

Frequently asked questions