How Many Solar Panels Do I Need? Size a Home System

The short version

Solar sizing is three divisions in a row. You start with energy (kWh a year), turn it into power (kW of panels), and turn that into a count of panels. If kW and kWh still blur together, the kW vs kWh guide explains the difference in a few minutes; everything below depends on it.

The middle number is the one people get wrong. It depends on where you live, which way your roof faces, how steep it is and what shades it. No national figure can stand in for it, so this guide shows the formula with example values and then sends you to the free PVWatts calculator from the National Laboratory of the Rockies (NLR, formerly NREL) for the real figure at your address.

Step 1: find your yearly kWh

The University of Illinois Extension starts the same way: gather the last 12 months of electric bills. Each bill shows the kWh you used in that billing period. Add up all twelve. One month is not enough, because use changes with the seasons, for example with air conditioning in summer or electric heat in winter.

If you do not have a year of bills, ask your utility for your usage history. The guide to lowering your electric bill shows where the kWh figure sits on a typical bill.

For a sense of scale, the US Energy Information Administration (EIA) reports that the average residential customer bought 10,791 kWh in 2022, about 899 kWh a month. The spread between states is large: Louisiana had the highest use per customer at 14,774 kWh, Hawaii the lowest at 6,178 kWh. Your own bills beat any average, but these three numbers make useful examples, and the tables below use them.

Adjust for what will change

A solar system lasts many years, so size it for the home you expect to have, not only last year's bills:

• New loads. An electric car, a heat pump or an electric water heater adds kWh. Estimate each one in kWh a year and add it. The electricity cost calculator turns an appliance's watts and hours into kWh per year.
• Efficiency first. The Department of Energy notes that energy efficiency upgrades complement solar economically: the less you use, the less solar you need. Every kWh you save before you size is a kWh you do not have to buy panels for.
• Changes in the household. Someone moving in or out, or working from home, shifts the total too.

Step 2: what 1 kW of panels makes where you live

A panel's watt rating is measured under standard test conditions. PVWatts' help pages define those as 1,000 W (1 kW) of sunlight per square meter and a cell temperature of 25°C (77°F). Daily sunlight is published in kWh per square meter per day, so 5 kWh/m² per day is the same energy as 5 hours of that full test sun. That is what "sun hours" means in solar sizing.

Real systems never deliver the full rating. PVWatts uses a default of 14% for system losses, built from these categories:

The categories do not simply add up. PVWatts' help page combines them by multiplying: each one takes its share of what is left after the others, which works out to about 14.1%, rounded to 14%. On top of that, the inverter that turns the panels' direct current into household alternating current has a default efficiency of 96% in PVWatts. Together they give the derate used on this page:

The rough output of 1 kW of panels in a year is then sun hours × 365 × 0.83. The sun values in this table are example values only, picked to show the math. They are not averages for any place:

Get the real number from PVWatts

The rough formula leaves out the effect of panel temperature, the exact tilt and direction of your roof, and how sunlight changes through the year. PVWatts models all of that from long-term weather data. To get your number:

1. Open pvwatts.nlr.gov and enter your address.
2. Set the DC system size to 1 kW. Enter your roof's tilt and the direction it faces, or keep the defaults if you do not know them yet.
3. If trees or buildings shade the roof, or you expect more than 5% loss from snow or dirt, raise the losses. PVWatts' own help warns that the default is likely too optimistic in those cases and points to its loss calculator.
4. Read the kWh per year at the top of the results page. That is your kWh per kW per year.

The results page also lists the monthly solar radiation in kWh/m² per day, if you want to see your sun hours month by month. PVWatts is careful about what its numbers mean: sunlight can vary from the long-term average by ±30% for a month and ±10% for a year. Treat the figure as a long-term average, not a promise for any given year.

Step 3: turn yearly kWh into system kW

Divide your yearly use by the kWh one kW makes in a year:

Here are the three EIA use figures at the three example sun values:

For the worked example on the rest of this page we take the US average home, 10,791 kWh, at an example 5 sun hours. That gives 10,791 ÷ (5 × 365 × 0.83) = 7.16 kW. If PVWatts had instead reported an example 1,400 kWh per kW for your roof, the same home would need 10,791 ÷ 1,400 = 7.71 kW. The PVWatts route is the one to trust, because it is built on your address.

As a sanity check, the Department of Energy writes that NREL uses an average home system size of 7.15 kW DC in its analyses, with a range of 3 to 11 kW. If your result lands far outside that range, check your kWh total and the units you entered before anything else. The power converter helps if a quote or spec sheet gives power in a unit you need to translate.

Step 4: count the panels

Multiply the system kW by 1,000 to get watts, then divide by the rated watts of one panel. The rating is the nameplate figure on the panel's spec sheet; PVWatts notes that nameplate ratings are given at standard test conditions (1,000 W/m² of sunlight and a cell temperature of 25°C). You cannot buy part of a panel, so round up.

For the worked example of 7.16 kW (7,162 W), with panel ratings that are example inputs:

Rounding up makes the system slightly bigger than the target, which is why the last column lands a little above 10,791 kWh. Higher-watt panels mean fewer of them for the same kW; they do not change how many kWh each kW makes. What a kW of panels makes depends on the sun and the losses, not on how the kW is split into panels.

An installer may also round down to fit the roof, or pick a count that suits the inverter. Use your count to check a quote, not to replace the installer's design.

Step 5: check the roof area

PVWatts gives the link between size and area: size in kW = array area in m² × 1 kW/m² × module efficiency. Turned around, area = kW ÷ efficiency. Module efficiency is on the spec sheet. PVWatts' own example uses 16% efficient modules, which put its 4 kW default system at about 25 m². For the 18-panel, 7.20 kW example:

That is the area of the panels themselves. The roof needs room around them as well, and the shape of each roof section decides how many panels really fit. The Department of Energy notes that panels typically perform best on south-facing roofs with a slope between 15 and 40 degrees, though other roofs may be suitable too, and warns about trees that shade the roof. If the best roof face is too small, a system split over two faces makes less per kW on the weaker side; run each face through PVWatts separately with its own direction and tilt.

How much of your use to cover: net metering

Solar panels produce most around midday and little or nothing at night, so almost every home with solar both sends power to the grid and draws power from it. What you get for the power you send out shapes the right size.

The EIA defines net metering as utility tariffs or billing programs that let certain customers reduce the electricity they are billed for within a billing period, taking into account the power they sent back when their own generation exceeded their use. The Department of Energy adds that the name comes from the 1990s, when the meter simply ran backwards, but that it is rarely that simple today.

The rules are set state by state and utility by utility. The EIA, citing the DSIRE database, counted 44 states and the District of Columbia with some form of state net metering policy as of December 27, 2022, and notes that the programs vary. The Department of Energy suggests asking your utility about:

• whether net metering is available in your service area,
• which system sizes and customer types are eligible,
• the rates, and how the bill credits are designed.

Why it matters for sizing: if exported power earns roughly what you pay for power, covering 100% of your yearly use is a reasonable target. If it earns less, extra panels that mostly feed the grid pay back more slowly, and a smaller system may suit you better. The University of Illinois Extension, writing for Illinois, where systems enrolled since 2025 are credited only for the supply portion of the bill for power sent to the grid, suggests a generation goal of 50 to 75% of yearly use as a starting point. Your state may be different. With the example 5 sun hours and 400 W panels, the US average home would need:

A home battery changes the picture too: it lets you store midday output for the evening instead of exporting it. Sizing a large home battery is beyond this guide; for a small backup setup, the portable power station guide walks through Wh, watts and solar recharge.

Costs, credits and payback

Once you have a size, the money question is how long the system takes to earn back its cost. You need two numbers: the installed price from a quote, and the yearly savings. The savings are the kWh the system makes, valued at what each kWh is worth to you: your retail rate for power you use yourself, and your utility's credit rate for power you export. The electricity cost calculator handles the kWh × rate part. Then put the price and the yearly savings into the payback period calculator, which covers simple and discounted payback and what each leaves out.

Some points to check before you trust a payback figure:

• Federal tax credit. The IRS page for the residential clean energy credit says the credit is not available for any property placed in service after December 31, 2025. Do not count on it for a system installed now, and check the IRS page yourself, because tax rules change.
• State and local incentives. The Department of Energy points to DSIRE, the Database of State Incentives for Renewables and Efficiency, as the most comprehensive source on incentives and policies. Look up your state there.
• The production estimate in the quote. Compare it with your own PVWatts run for the same size, direction and tilt. A quote that promises much more than PVWatts deserves a question.
• Year-to-year swings. PVWatts' ±10% annual variation in sunlight means some years save less than the average.

This page is general information, not financial or tax advice. Whether solar is worth it for you depends on your prices, your utility's rules and your plans for the house.

Mistakes that throw the count off

• Sizing from one month's bill. Add up twelve months; seasons change use a lot.
• Using someone else's sun hours. A figure from another state or a national average can be far from your roof. Run PVWatts for your address.
• Forgetting the losses. Dividing by sun hours × 365 alone overstates output. Apply the derate, or let PVWatts do it.
• Keeping default losses on a shaded roof. PVWatts itself says the default is likely too optimistic with shade or heavy snow and dirt.
• Mixing kW and kWh. System size is kW; use and output are kWh. A 7 kW system does not make 7 kWh a day.
• Sizing past what your utility credits. Read your net metering rules before you add panels that mostly export.
• Ignoring planned changes. An electric car or heat pump bought two years later can outgrow a system sized only for today.

Frequently asked questions

Sources

• US Energy Information Administration, How much electricity does an American home use? (2022: average residential customer 10,791 kWh a year, about 899 kWh a month; Louisiana highest at 14,774 kWh, Hawaii lowest at 6,178 kWh)
• NLR PVWatts Calculator
• NLR PVWatts Calculator help: System Losses (default 14% and its loss categories)
• NLR PVWatts Calculator help: DC System Size (DC rating at 1 kW/m², default 4 kW, size = array area × 1 kW/m² × module efficiency, 16% example of about 25 m²)
• NLR PVWatts Calculator help: Inverter Efficiency (default 96%)
• NLR PVWatts Calculator help: Results (monthly and annual solar radiation, kWh per year at the top of the results page)
• NLR PVWatts Calculator help: Cautions for Interpreting the Results (±30% monthly and ±10% annual variation in sunlight; default losses may be optimistic with shading or more than 5% soiling or snow)
• US Department of Energy, Homeowner's Guide to Solar (net metering depends on state and utility; NREL average system 7.15 kW DC with a 3 to 11 kW range; south-facing roofs with a 15 to 40 degree slope; efficiency upgrades complement solar; DSIRE for incentives)
• US Energy Information Administration, Glossary: Net metering
• US Energy Information Administration, Renewable energy explained: incentives (net metering bills the net use; 44 states and DC with some form of net metering policy as of 12/27/2022, per DSIRE; programs vary)
• University of Illinois Extension, Homeowner's Guide to Solar Energy (gather 12 months of bills; generation goal of 50 to 75% of annual use as a starting point under Illinois export credits)
• Internal Revenue Service, Residential Clean Energy Credit (not available for property placed in service after December 31, 2025)
• DSIRE, Database of State Incentives for Renewables and Efficiency
• NIST Guide to the SI, Appendix B.8: conversion factors (square foot to square meter)

Written by Tobias. Updated October 22, 2026.