# How much energy does a solar panel produce, city by city

URL: https://solarrealcost.com/does-solar-pay/how-much-energy-does-a-solar-panel-produce
Updated: 2026-10-26
Review: Reviewed by SolarRealCost Editorial Team, editorial review: primary sources checked and independently fact-checked

How much energy does a solar panel produce? From national-lab PVWatts runs, a 400-watt panel makes about 435 kWh a year in Seattle and 702 in Phoenix.

How much energy does a solar panel produce? A 400-watt panel on a south-facing roof makes roughly 435 kWh a year in Seattle and about 702 kWh in Phoenix, according to our runs of the national lab's PVWatts model. That's 1.2 to 1.9 kWh a day on average. The watt rating on the label tells you the panel's size, not what it makes, and location changes the answer by more than half.

- A panel's **watt rating is power at one moment** under lab conditions. What you use and pay for is **energy in kilowatt-hours**.
- In our PVWatts runs, each kilowatt of panels made about **1,088 kWh a year in Seattle, 1,191 in Pittsburgh and 1,755 in Phoenix**.
- That makes a **400-watt panel worth about 435, 476 and 702 kWh a year** in those three cities.
- Even in Phoenix, a panel produced about **20% of what it would if it ran at full rating around the clock**. In Seattle it was about 12%.
- One panel covers **4% to 6.5% of an average home's yearly use**, based on EIA's 10,791 kWh figure.

## Watts vs. kilowatt-hours: what the label tells you

The U.S. Energy Information Administration's [guide to measuring electricity](https://www.eia.gov/energyexplained/electricity/measuring-electricity.php) says "Watts measure power at a specific moment," while watthours measure electricity used over a period of time. "One kWh is one kilowatt generated for one hour." Your utility bill is in kWh, so that's the number that matters.

The watt rating on a solar panel comes from a lab test. The Department of Energy's [performance basics](https://www.energy.gov/cmei/systems/solar-photovoltaic-performance-and-efficiency-basics) explain that panels are tested under "a constant, standard level of light" while keeping a constant cell temperature. A roof gets nothing close to that. The sun sets, sits low in winter and goes behind clouds. Heat matters too: higher temperatures cause a small increase in current "but a much larger decrease in voltage," the DOE says, and the panel makes less power.

So a 400-watt panel is a 400-watt panel only in the test. To learn how much energy a solar panel will produce on your roof, you need a model that knows your weather.

## How much energy does a solar panel produce where you live?

The National Laboratory of the Rockies (NLR, formerly NREL) runs [PVWatts](https://pvwatts.nlr.gov/), a public online calculator that estimates a system's output for a given address. Version 8 uses [2020 typical-year weather data](https://developer.nlr.gov/docs/solar/pvwatts/v8/) from the lab's National Solar Radiation Database.

We ran PVWatts V8 on 2026-10-14 for three cities with the same inputs: a 1 kW system, standard modules, fixed roof mount, 20° tilt, facing due south, 14% system losses. We then scaled the results to a single 400-watt panel (0.4 kW) by multiplying by 0.4. Your panel's rating is on its datasheet; scale the same way for a different size.

| City | Per kW, a year | 400 W panel, a year | 400 W panel, a day (average) | 400 W panel, July | 400 W panel, December |
| --- | --- | --- | --- | --- | --- |
| Seattle, WA | 1,088 kWh | 435 kWh | 1.19 kWh | 57 kWh | 13 kWh |
| Pittsburgh, PA | 1,191 kWh | 476 kWh | 1.30 kWh | 54 kWh | 22 kWh |
| Phoenix, AZ | 1,755 kWh | 702 kWh | 1.92 kWh | 63 kWh | 47 kWh |

The daily figures are yearly averages, and no single day looks like them. In Seattle, a July day produces several times what a December day does.

Another way to read the table: a 1 kW panel running at full rating every hour of the year would make 8,760 kWh. The Phoenix result is about 20% of that, and Seattle's about 12%. That gap between rating and output is normal. It's also why a sales pitch built on the watt figure alone can mislead you.

These are model estimates for one roof setup. Actual production varies with weather, soiling, shading and equipment. The DOE's [homeowner guide](https://www.energy.gov/cmei/systems/homeowners-guide-solar) says panels typically perform best on south-facing roofs with a slope between 15 and 40 degrees, so an east- or west-facing or shaded roof will usually come in below these numbers. PVWatts takes your address, tilt, direction and losses, so run it for your own roof.

Tool: [How many panels do you need?](https://solarrealcost.com/does-solar-pay/tools/how-many-panels). From what you actually use, the panel wattage available, and the production factor where you live.

## How one panel compares with a home's use

The EIA's [household electricity figures](https://www.eia.gov/tools/faqs/faq.php?id=97&t=3) put the 2022 average at 10,791 kWh a year per U.S. residential customer, about 899 kWh a month. Louisiana customers bought the most, 14,774 kWh, and Hawaii customers the least, 6,178.

Against that average, one 400-watt panel in our runs covers about 4% of a year's use in Seattle, 4.4% in Pittsburgh and 6.5% in Phoenix. That's why home systems use many panels, and why the count changes so much by city. Turning your own bills into a system size is a separate step, covered in [how many solar panels you need](https://solarrealcost.com/does-solar-pay/how-many-solar-panels-do-i-need).

**Compare a quote's production figure, not its panel count**
A proposal should state expected yearly production in kWh. Check it against a PVWatts run for your address with the same size, tilt and direction. If the installer's figure is much higher, ask what weather data and losses it assumed.

## Bottom line

How much energy does a solar panel produce? Less than its label suggests, and it depends on where you live. In our PVWatts runs, a 400-watt panel made about 435 kWh a year in Seattle, 476 in Pittsburgh and 702 in Phoenix. Get yearly kWh for your own roof, check it against any quote, and only then look at [what solar costs to install](https://solarrealcost.com/does-solar-pay/install-solar-panels-cost) and whether [solar panels are worth it](https://solarrealcost.com/does-solar-pay/are-solar-panels-worth-it-now) at your electricity price.

Start the Solar Check: https://solarrealcost.com/check

## Common questions

### How much energy does a 400-watt solar panel produce in a day?

It depends on where it is. In our PVWatts runs for a south-facing roof at a 20° tilt, a 400-watt panel averaged about 1.19 kWh a day over a year in Seattle, 1.30 in Pittsburgh and 1.92 in Phoenix.

### Why doesn't a 400-watt panel make 400 watts all day?

The watt rating is measured in the lab under a constant, standard level of light and a constant cell temperature. On a roof, the sun is down at night, low in winter and sometimes behind clouds, and heat lowers a cell's voltage.

### What is the difference between watts and kilowatt-hours?

Watts measure power at one moment. Kilowatt-hours measure energy over time: one kWh is one kilowatt generated for one hour, according to the U.S. Energy Information Administration.

### How many solar panels would it take to cover an average home?

The EIA puts average 2022 use at 10,791 kWh per U.S. residential customer. In our runs, one 400-watt panel covered about 4% of that in Seattle and about 6.5% in Phoenix. Sizing a whole system is its own step; start from your own bills.

## Sources

- [U.S. Energy Information Administration: Measuring electricity](https://www.eia.gov/energyexplained/electricity/measuring-electricity.php)
- [U.S. Energy Information Administration: How much electricity does an American home use?](https://www.eia.gov/tools/faqs/faq.php?id=97&t=3)
- [National Laboratory of the Rockies: Our history (formerly NREL)](https://www.nlr.gov/about/history)
- [National Laboratory of the Rockies: PVWatts calculator](https://pvwatts.nlr.gov/)
- [National Laboratory of the Rockies: PVWatts V8 API documentation](https://developer.nlr.gov/docs/solar/pvwatts/v8/)
- [U.S. Department of Energy: Solar photovoltaic performance and efficiency basics](https://www.energy.gov/cmei/systems/solar-photovoltaic-performance-and-efficiency-basics)
- [U.S. Department of Energy: Homeowner's guide to going solar](https://www.energy.gov/cmei/systems/homeowners-guide-solar)