How to Size a Home Solar System

Size a home solar system from your own electricity use, then from the roof and the utility rules that limit what that roof can do. A neighbor’s kilowatt number is not a template. Homes on the same street can use very different amounts of power, and two roofs with the same square footage can produce different energy because of direction, tilt, and shade.

You can do the first step with paperwork you already have. The later steps need a site look: shade, structure, and the tariff. This page is that first step, plus the questions that keep a proposal honest. When you have two quotes, check them against the solar quote FAQ.

Step 1: Add up a year of kilowatt-hours

Pull 12 months of electric bills. You want kilowatt-hours, not dollars. Dollars mix season, rate changes, taxes, and fees. kWh is the energy.

• Add the kWh for all 12 months. That total is your baseline annual use.
• Note the highest months and why they were high: air conditioning, electric heat, a well, a workshop, guests.
• If you only have a few months, do not multiply a July bill by 12. Ask the utility for a usage history. Most will provide it.
• If you recently electrified (a heat pump, an EV, a new tenant), say so. Last year’s total will understate next year’s use. If you are about to buy an EV or convert heat, decide whether the array should leave room for that load or whether you will expand later. Expanding later is not always simple; panel space and interconnection limits can stop a second phase.

Ignore “average home” charts. Your total is the number.

Step 2: Decide what share of that use you are trying to cover

“Offset” means how much of your annual kWh the array’s annual production might match. One hundred percent offset is not a goal for every house.

Cover less than your full annual use when:

• The utility credits exports for much less than you pay for imports. Extra midday kWh you do not use on site are then the least valuable kWh the system makes. How net metering works shows the difference between retail credit and a lower export rate.
• The sunny, unshaded roof area runs out before the math hits 100 percent.
• A larger system would bump you into a different rate or a program that is closed above a certain size. Ask the installer whether your utility caps system size as a percent of historical use.
• You expect to use less power, not more (kids moving out, a finished renovation that was inflating the bills).

Design for more than last year’s use only when you have a specific new load and the roof and the tariff allow it. Do not add a cushion “just in case” that you will export for a tiny credit.

A battery does not, by itself, make a too-small roof into a larger one. It moves energy in time. Read do I need a solar battery after the array size is clear, not before.

Step 3: Separate system size from yearly production

Installers quote size in kilowatts. That is the array’s capacity under standard test conditions, usually the DC rating of the modules added together. A 6 kW system is not a promise of 6 kW every hour, and it is not a promise of any particular yearly kWh.

Yearly production depends on:

• Where you live (how much sun the site gets).
• Tilt and the direction the roof faces. Equator-facing roofs produce more than roofs that face away from the sun; east and west roofs can still be workable.
• Shade from trees, chimneys, and neighboring buildings, including shade that arrives in winter when the sun is lower.
• Equipment efficiency and whether the inverter limits output below the module total (clipping) during the brightest hours. A little clipping is common. A design that clips away a large share of the year needs an explanation.
• Soiling, snow, and heat. Modules produce less when they are very hot than the nameplate implies.

Ask for the annual kWh, the tool that produced it, and the shade assumption. NREL PVWatts is a public estimator you can run yourself. Enter your address, tilt, azimuth, and a system size, and compare the yearly kWh to the quote. If the quote is far higher, ask what loss factors they set to zero. PVWatts is not a substitute for a shade study on a tree-covered lot. It is a reasonableness check.

There is no honest universal “kWh per kW” for the whole country. A desert roof and a cloudy, north-facing roof are different machines. Use your site’s estimate. A Chicago bungalow or flat-roofed two-flat also loses area to city setbacks, winter shade from the next building, and snow on a low slope. Those limits are local. If the house is in Chicago, read Chicago solar installation and solar panel installers before you treat a national offset percentage as the design.

Step 4: Fit the array to the roof you will still have

The bill can justify a system the roof cannot hold. Walk the constraints before you fall in love with an offset percentage:

• Usable planes, minus setbacks the fire code requires at ridges, eaves, and valleys. The installer should show a layout, not only a kW number.
• Shade. A module in heavy shade can pull down more than itself on older string designs. Module-level electronics limit that, but they do not make a shaded module produce like a clear one.
• Obstacles: vents, skylights, HVAC equipment. Moving a vent is a roofing job.
• Roof condition. If you will replace the covering soon, size the system for the roof you will have after that work. See should I replace my roof before solar.
• Structure. Weight and wind uplift are an engineering question. A layout that the framer will not sign is not a real option.
• Main electrical panel. Some homes need a panel upgrade or a different interconnection method before a larger inverter is allowed. That cost belongs in the comparison.

Price per watt (the system price divided by the DC watts) is a way to compare two arrays of similar scope. It is not a way to compare a roof layout that includes a panel upgrade with one that ignores it. Scope has to match.

Step 5: Make the two quotes describe the same house

When you ask companies for proposals, give each of them the same 12-month kWh total, the same note about future loads, and the same roof findings. Then check:

• DC system size and the module model.
• Estimated annual kWh and the loss assumptions.
• Inverter model and whether it is sized for a later battery, if you care about that.
• Price, price per watt, and what is excluded (roofing, panel upgrade, monitoring, critter guard).
• The utility tariff and export credit used in any savings page. A bigger kWh number is not a bigger bill credit if exports are cheap.
• Warranties: product, performance, and workmanship.

How you pay does not change the physics, but it changes whether you should chase a larger system. A loan, lease, or PPA can make a bigger array look like a small monthly change. Read the term, the escalator, and who owns the system in solar financing options. Ownership also changes a future sale. Selling a home with solar is the short version of that paperwork.

Commercial meters are sized differently because demand charges and daytime business use do not behave like a house that is empty at noon. Use residential vs commercial solar if the meter is a business.

How many solar panels does my house need?

Count energy first, panels second. Add up a year of kWh, choose how much of that energy you are trying to match, and convert that target into a system size using a production estimate for your roof — not a national rule of thumb. The panel count is the system size divided by the wattage of the module someone specifies. A design that uses higher-watt modules needs fewer panels for the same kilowatts, but the roof layout and the shade still decide what fits. Ask for the module wattage, the count, and the annual kWh together.

Should I size solar for 100 percent of my electric bill?

Not always. One hundred percent annual offset is a reasonable target when exports are credited near the retail rate, the roof can hold the array, and you expect similar use next year. It is a weak target when exports are worth much less than the power you buy, or when the unshaded roof runs out. In those cases a smaller system that covers daytime use can be the better match. Confirm the export rule before you pay for kilowatt-hours you will sell cheaply.

What is the difference between kW and kWh on a solar quote?

A kilowatt (kW) is the system’s capacity: how much power it can produce under standard test conditions. A kilowatt-hour (kWh) is energy over time: what you buy from the utility, and what the array might produce in a year. Your bill’s important number for sizing is kWh. The quote’s size is kW. They are related through sun, tilt, direction, and shade, which is why the same kW produces different yearly kWh in different places. Compare both numbers, and ask for the assumptions behind the kWh.

Can I size a solar system from one summer electric bill?

No. A single summer bill overstates annual use in a cooling climate and understates it in a heating climate. It also hides the months when solar production and your use line up poorly. Ask the utility for 12 months of kWh if you do not have the bills. If your use just changed because of a new appliance or a new work-from-home schedule, tell the installer what changed instead of pretending last July is a full year.

This page is general information, not an engineering design and not tax or utility advice. A layout on your roof, a shade reading, and your utility’s current tariff are what make a size real.

To have someone look at a specific property, request a free energy analysis. City-level notes live in the solar guides, and published locations are in the directory.