Divide your yearly electricity use by the yearly output of one kilowatt of panels at your address, then by one panel’s rated kilowatts, and round up. A Maryland home using 11,334 kWh a year needs 21 panels of 410 watts in the model below.1,2,3 Most U.S. systems installed in 2025 ran 5 to 12 kW, or 13 to 30 panels.4,5
That 21-panel answer is one worked case. Your bills, your roof and the panel you pick can each move it, and the sections below show by how much.
How do you calculate the number of panels?
Panels equal yearly use in kilowatt-hours, divided by the yearly output of one kilowatt of panels at your address, divided by one panel’s rated kilowatts, rounded up. A designer later swaps that first guess for a model built from your actual roof.
Two units trip people up. A kilowatt (kW) measures size: how fast a system can produce. A kilowatt-hour (kWh) measures energy: how much gets produced or used over time. Your electric bill is in kWh. Panels are rated in watts, and 1,000 watts make one kilowatt.
A panel’s rating is a direct-current (DC) lab number. Real roofs run hotter and dirtier than the lab, so real output comes in lower. The model below uses the default 14 percent for system losses (soiling, shading, snow, wiring, connections, mismatch and similar) plus a 96 percent inverter efficiency, both from PVWatts.2
- Add up 12 months of kWh from your electric bills.
- Find the yearly output of one kilowatt of panels near you. PVWatts, a free model from the National Laboratory of the Rockies (formerly NREL), gives a first estimate. Shading, roof direction, equipment and weather change the real number.
- Pick a panel wattage. Higher wattage means fewer panels for the same system size.
- Divide yearly kWh by the yearly output per kilowatt, divide again by the panel’s kilowatts, and round up to whole panels.
- Check what the roof can hold and what your utility allows. The section on caps below covers both.
How much electricity does the average home use here?
The U.S. Energy Information Administration (EIA) reports that the average home used 944 kWh a month in Maryland, 639 in the District of Columbia (DC) and 1,067 in Virginia in 2025.1,6,7 Virginia’s figure covers the whole state, not only Northern Virginia.
Treat these as a starting point, not a target. An electric vehicle or a heat pump can push a household far above them. Your own bills beat any average.
How many panels do three average homes need?
Under the assumptions below, a Maryland-average home in the Silver Spring model needs 21 panels of 410 watts, a DC-average home needs 14, and a Virginia-average home in the Arlington model needs 23. Later tables change use, panel wattage and roof yield.
Illustrative example, not a quote and not a Soleil project. Assumptions:
- Yearly use is the EIA monthly average for each place multiplied by 12, standing in for your own 12 months of bills. Table 2 uses the yearly figures shown instead.
- Panel rating is 410 watts, changed only in Table 3. Table 4 cuts the yield by 10 and 20 percent as what-if scenarios.
- Panels are rounded up to whole panels. The model has no shading study, no degradation and no check that the panels fit the roof. Its only shading is the small allowance inside the 14 percent loss figure.
- Silver Spring (Montgomery County), Washington, DC and Arlington are weather-file locations for the model, not a statement about where any customer lives.
How to reproduce the yields. The yearly output per kilowatt comes from PVWatts version 8.5.0, run October 21, 2026 (and repeated October 24, 2026 with identical results) with these inputs for every location:2
- System size: 1 kW DC. Module type: standard. Array type: fixed roof mount.
- Tilt: 20 degrees. Azimuth: 180 degrees (due south).
- System losses: 14 percent (the PVWatts default). Inverter efficiency: 96 percent. Ratio of direct-current to alternating-current (AC) capacity: 1.2.
- Locations: Silver Spring, Maryland (latitude 38.9907, longitude -77.0261) gives 1,377.9 kWh per kW per year. Washington, DC (38.9072, -77.0369) gives 1,391.5. Arlington, Virginia (38.8816, -77.0910) gives 1,379.9.
Enter the same settings at pvwatts.nlr.gov and you should see the same yields. A different tilt, direction or loss figure will change them.
Table 1. Average homes in three places (410 watt panels)
| Model home | Yearly use, kWh | Output per kW DC, kWh a year | Panels | System size, kW DC | Modeled year-one output, kWh |
|---|---|---|---|---|---|
| Maryland average, Silver Spring model | 11,334 | 1,377.9 | 21 | 8.61 | 11,864 |
| District of Columbia average, Washington, DC model | 7,667 | 1,391.5 | 14 | 5.74 | 7,987 |
| Virginia average, Arlington model | 12,803 | 1,379.9 | 23 | 9.43 | 13,012 |
Table 2. How yearly use moves the count (Silver Spring model, 410 watt panels)
| Yearly use, kWh | Panels | System size, kW DC | Modeled year-one output, kWh |
|---|---|---|---|
| 6,000 | 11 | 4.51 | 6,215 |
| 9,000 | 16 | 6.56 | 9,039 |
| 11,334 | 21 | 8.61 | 11,864 |
| 14,000 | 25 | 10.25 | 14,124 |
| 18,000 | 32 | 13.12 | 18,079 |
Table 3. How panel wattage moves the count (Silver Spring model, 11,334 kWh a year)
| Panel rating, watts | Panels | System size, kW DC | Modeled year-one output, kWh |
|---|---|---|---|
| 370 | 23 | 8.51 | 11,726 |
| 410 | 21 | 8.61 | 11,864 |
| 440 | 19 | 8.36 | 11,520 |
Table 4. What if the roof yields less? (Silver Spring model, 11,334 kWh a year, 410 watt panels)
| Scenario (what-if, not a measurement) | Yearly output per kW DC, kWh | Panels | System size, kW DC |
|---|---|---|---|
| Yield as modeled | 1,377.9 | 21 | 8.61 |
| Yield 10 percent lower | 1,240 | 23 | 9.43 |
| Yield 20 percent lower | 1,102 | 26 | 10.66 |
Each result is a modeled estimate, not a quote.3
Read the tables as ranges. The same Maryland home needs 19 to 23 panels depending on the wattage you pick, yet system size stays between 8.36 and 8.61 kW DC, so kW DC is the better number for comparing quotes.3 Every case lands slightly above the home’s use, up to 105 percent, because whole panels round up.3
Two inputs move the count most. Yearly use is the big one: Table 2 runs from 11 panels at 6,000 kWh to 32 at 18,000 kWh, and it is the one you can check today with your own bills.3 Roof yield comes next. A roof that produces 20 percent less than modeled needs 26 panels instead of 21.8 Panel wattage changes the piece count but barely changes system size.
For a reality check, Lawrence Berkeley National Laboratory reports that the median U.S. residential system installed in 2025 was 7.7 kW, with most between roughly 5 and 12 kW (the 20th to 80th percentile).4 At 410 watts, that band is roughly 13 to 30 panels (5.33 to 12.30 kW once rounded to whole panels).5 A 7.7 kW system would make about 10,610 kWh a year in the Silver Spring model, which sits inside the range in Table 2.9 The laboratory’s sample is national, so don’t read its median as a Maryland number.
Does square footage set the panel count?
No. Floor area is a rough stand-in for electricity use, and a poor one. Two houses of the same size can use very different amounts, depending on heating and cooling, appliances and electric vehicle charging. Start from your kWh bills, not your floor plan.
Picture two 2,000 square foot homes on the same street (an example, not real customers). One uses 6,000 kWh a year, the other 14,000.3 In the Silver Spring model the first needs 11 panels and the second 25.3 A panels-per-square-foot table would hide that spread, along with the other two inputs that matter: local output and panel wattage.
Planning an electric vehicle or a heat pump? Add the expected extra use to your 12-month total before you run the formula. A system sized to last year’s bills can fall short the first winter after a heat pump goes in. Our guide to what happens at a solar consultation and site survey shows how a designer turns these inputs into a layout.
What caps the panel count: the roof or the utility?
Both can. Roof space, shade, direction and obstructions decide how many panels fit. In some places, net-metering rules also cap system size against your past use. A designer checks both before any count is final. The caps below are as of October 25, 2026.
The tables above assume a south-facing roof with no shade. A north-facing roof, heavy shade or an aging roof changes the answer, and Table 4 shows what a lower yield costs you in panels. How many panels fit comes from the datasheet of the panel being quoted and a measured roof. If the roof needs work first, that comes first; see replacing a roof before installing solar.
Covering all of your yearly use is a choice, not a rule. A smaller system covers less of it. Where a rule caps system size, a bigger one may not be allowed.
Permits are the other gate, and counties and cities run their own. In Montgomery County, Maryland, the Department of Permitting Services says rooftop solar needs a building permit and a separate electrical permit. Its eSolar process can issue the building permit online for approved installers.10
Net metering is the billing arrangement that credits you for power your system sends to the grid. The size rules differ by place and change over time. This table shows the caps that apply to a typical home.
| Place | Rule that can cap system size |
|---|---|
| Washington, DC | The DC Public Service Commission’s rule lets a net-metered customer filing a new or amended interconnection application size generation above 100 percent of historical 12-month usage, on a yearly schedule that reached 200 percent in 2024. The rule lets the electric company request suspension of a yearly increase.11 |
| Dominion Energy Virginia territory | Dominion says the system must be sized so it does not reasonably produce more than 150 percent of expected annual electrical consumption.12 On April 30, 2026, the State Corporation Commission approved a net energy metering 2.0 (NEM 2.0) tariff, applicable to new customers and to existing customers who choose to switch, and ordered Dominion to file revised tariffs within 90 days of the order’s publication.13 The Dominion page cited here doesn’t mention NEM 2.0, so check the current tariff before you rely on either. |
| Virginia, by statute | A residential net-metered facility is limited to 25 kilowatts of capacity.14 |
| Maryland | The net-metering statute limits a system’s generating capacity to 2 megawatts, with exceptions for certain local-government and community projects, far above any house.15 The state’s net-metering regulation adds a usage test: to initially qualify, a system may not exceed 200 percent of the customer’s baseline annual usage.16 Your electric company sets its own interconnection rules. Pepco serves most of Montgomery and Prince George’s counties, and Baltimore Gas and Electric (BGE) serves most of Howard County.17 |
In Maryland, the bigger question is timing. The 2026 Utility RELIEF Act says the standard net-metering contract or tariff stays available until the earlier of 3,000 megawatts statewide or July 1, 2027.18 The Public Service Commission’s proposed rule, still open in its RM56 docket as of October 25, 2026, lists three competing versions of what a customer must have done by July 1, 2027.19 See the net-metering deadline and what replaces Maryland net metering. Whatever the table says, confirm the current limit with your utility before you rely on it.
Do more panels give you backup power?
No. Matching panels to a year of electricity use is one job, and backup power is another. Most grid-connected panels shut off automatically during a grid outage to protect utility workers, so without a battery or other added equipment they won’t keep the lights on.20
Backup depends on a battery, how the system is wired and which circuits it covers. A bigger array adds none by itself. For the details, read whether solar panels work in a power outage.
Soleil Energy adds home battery backup to existing solar, including systems it did not install.
What should you bring to a solar consultation?
A designer can replace the illustrative numbers above with a real count once they know a few facts about your home. Gather these before the first call, because your roof and your bills drive almost everything else in the design:
- 12 months of electric bills, or your utility’s yearly usage report.
- Any planned change in use: an electric vehicle, a heat pump, a home addition.
- Your roof’s approximate age and any known leaks.
- Trees or buildings that shade the roof, and which way the main roof faces.
- Your electric company’s name.
Your bills set the target and your roof sets the ceiling. Panel wattage just changes how many pieces it takes to get there. When quotes arrive, compare them by system size in kW DC and modeled yearly kWh, not by panel count.
Soleil Energy serves the counties on its service area page and Washington, DC. The residential solar consultation starts with a review of your roof, panels and electric bill. Book a free consultation or call (301) 953-4140.
Sizing is separate from taxes. Have a tax professional check any credit before you build one into a budget.
Solar savings and incentive eligibility depend on your roof, your usage, your utility, and your tax situation. Incentive programs change and some have limited funding. We are not tax advisors — we will review your bill with you and tell you what you may qualify for. Confirm any tax credit with a qualified tax professional.
Sources (20)
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U.S. Energy Information Administration, Form EIA-861 final 2025 annual data, file Average_Bill_2025.xlsx (file dated October 7, 2026 in the download), sheet Residential (Maryland, 944.49 kWh a month), https://www.eia.gov/electricity/data/eia861/zip/f8612025.zip, read October 24, 2026. ↩ ↩2 ↩3
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National Laboratory of the Rockies (formerly NREL), PVWatts Calculator version 8.5.0, https://pvwatts.nlr.gov/pvwatts.php, model run October 21, 2026 and repeated October 24, 2026 with identical results, for 1 kW DC, standard module, fixed roof mount, 20 degree tilt, 180 degree azimuth, 14 percent system losses, 96 percent inverter efficiency, DC to AC ratio 1.2 (Silver Spring 1,377.9; Washington, DC 1,391.5; Arlington 1,379.9 kWh per kW DC per year). ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7
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Illustrative sizing calculation, run October 21, 2026 (PVWatts yields rechecked October 24, 2026), using PVWatts yields (https://pvwatts.nlr.gov/pvwatts.php, settings at 2); module ratings and yearly use are stated assumptions. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7
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Lawrence Berkeley National Laboratory, U.S. Distributed Solar and Storage 2026 Data Update, published August 2026, https://eta-publications.lbl.gov/sites/default/files/2026-08/distributed_solar_storage-2026_data_update_final.pdf, read October 24, 2026. ↩ ↩2
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Illustrative sizing calculation, run October 21, 2026, for 5 kW and 12 kW systems at 410 watts, using PVWatts yields (https://pvwatts.nlr.gov/pvwatts.php, settings at 2); the 410 watt rating is an assumption. ↩ ↩2
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U.S. Energy Information Administration, same file (District of Columbia, 638.90 kWh a month), https://www.eia.gov/electricity/data/eia861/zip/f8612025.zip, file dated October 7, 2026, read October 24, 2026. ↩ ↩2
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U.S. Energy Information Administration, same file (Virginia, 1,066.92 kWh a month, statewide), https://www.eia.gov/electricity/data/eia861/zip/f8612025.zip, file dated October 7, 2026, read October 24, 2026. ↩ ↩2
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Illustrative sizing calculation, run October 21, 2026, with the modeled yield cut by 10 and 20 percent as what-if scenarios, using PVWatts yields (https://pvwatts.nlr.gov/pvwatts.php, settings at 2); the 410 watt rating is an assumption. ↩
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Illustrative sizing calculation, run October 21, 2026, for a 7.7 kW system, using PVWatts yields (https://pvwatts.nlr.gov/pvwatts.php, settings at 2); the 410 watt rating is an assumption. ↩
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Montgomery County Department of Permitting Services, eSolar Residential Rooftop Solar Panels, https://www.montgomerycountymd.gov/department-permitting-services/all-permits/residential-permits/esolar-residential-rooftop-solar-panels, current page (undated), read October 25, 2026. ↩
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DC Public Service Commission, 15 DCMR Chapter 9 compiled copy, section 901.2, compiled copy updated April 7, 2025 (later amendments, if any, are not reflected), https://dcpsc.org/getattachment/Orders-and-Regulations/Orders-Rules-and-Regulations/District-of-Columbia-Municipal-Regulations-Title-1/15-9-Compiled-(Updated-4-7-25-)-(1).pdf.aspx?lang=en-US, read October 25, 2026. ↩
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Dominion Energy Virginia, Net metering, https://www.dominionenergy.com/virginia/renewable-energy-programs/net-metering, current page (undated), read October 25, 2026. ↩
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Virginia State Corporation Commission, Final Order, Case No. PUR-2025-00079, dated April 30, 2026, https://www.scc.virginia.gov/docketsearch/DOCS/8bxb01!.PDF, read October 25, 2026. ↩
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Code of Virginia, section 56-594, https://law.lis.virginia.gov/vacode/title56/chapter23/section56-594/, current statute page (the 25 kilowatt residential limit for an eligible customer-generator is the same in the version effective January 1, 2027), read October 25, 2026. ↩
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Maryland General Assembly, Public Utilities Article section 7-306(g)(1), https://mgaleg.maryland.gov/mgawebsite/Laws/StatuteText?article=gpu§ion=7-306&enactments=false, statute text as served, read October 25, 2026. ↩
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Maryland Division of State Documents, COMAR 20.50.10.01D(1)(b) (qualifying for net energy metering), https://dsd.maryland.gov/regulations/Pages/20.50.10.01.aspx, current regulation text, read October 25, 2026. ↩
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Maryland Office of People’s Counsel, Electricity (utility service areas), https://opc.maryland.gov/Consumer-Learning/Electricity, current page (undated), read October 24, 2026. ↩
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Maryland General Assembly, Chapter 353 of the 2026 Laws of Maryland (House Bill 1532, Utility RELIEF Act of 2026), Public Utilities Article section 7-306(d)(1), approved by the Governor May 12, 2026, https://mgaleg.maryland.gov/2026RS/chapters_noln/Ch_353_hb1532E.pdf; read October 25, 2026. ↩
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Maryland Public Service Commission, Proposed Regulations RM56, COMAR 20.50.10.01D (versions 1 to 3), notice dated June 25, 2026, https://psc.maryland.gov/wp-content/uploads/2026/06/Proposed-Regulations-RM-56.pdf; PSC docket RM56, latest entry dated October 13, 2026, https://webpscxb.pscmaryland.com/DMS/rm/RM56; both read October 25, 2026. ↩
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National Laboratory of the Rockies (formerly NREL), “Will Solar Panels Help When the Power Goes Out?”, TP-7A40-75704, published March 2020, https://docs.nlr.gov/docs/fy20osti/75704.pdf, read October 25, 2026. ↩
