How Many Solar Panels Do I Need? Sizing a System the Right Way

To find how many solar panels you need, divide your yearly electricity use (kWh) by how much one kW of panels produces where you live, then divide by the panel’s wattage. For example, a home using 10,800 kWh a year in an area that produces 1,300 kWh per kW needs about 8.3 kW, or about 20 panels of 430 watts. But the right size is not always the size that covers 100% of your usage. If your utility pays little for exported power, a smaller system often saves more per dollar. Here is the full method.

Step 1: Get 12 months of usage

Log into your utility account and find your monthly kWh for the last 12 months. Add them up. Many bills include a 12-month usage chart.

Why 12 months matters:

• Usage swings with the seasons. Air conditioning in summer or electric heat in winter can double a month’s use.
• A single bill times 12 can be far off.
• Square footage and “average home” numbers are guesses.

If you recently moved, changed heating, or added an EV, adjust for that. An energy monitor can show where your power goes and what you can cut first. See efficiency before solar and our cheaper first steps. Cutting usage before sizing means buying fewer panels.

Step 2: Find your production per kW

One kW of panels produces different amounts in different places, and orientation and shade matter as much as region. These are rough figures for a well-placed, unshaded, south-facing array. Use them only as a starting point.

Region (rough)	Approximate yearly production per kW DC
Pacific Northwest	About 1,000 to 1,200 kWh
Northeast and Great Lakes	About 1,150 to 1,350 kWh
Southeast and mid-Atlantic	About 1,300 to 1,450 kWh
Southwest, California, Mountain West	About 1,450 to 1,750 kWh

East- or west-facing roofs typically produce less than south-facing ones, and shade can cut output sharply. For your actual address, run PVWatts, the free model from the national lab formerly known as NREL, with our production checker.

Step 3: Calculate system size and panel count

System size (kW) = yearly kWh you want to offset ÷ yearly kWh per kW

Panel count = system size in watts ÷ panel wattage

Typical residential panels in 2026 are about 400 to 460 watts. We use 430 W as a common example. The median residential panel efficiency in 2025 was 21.3%, according to LBNL.

Example:

• Yearly use: 10,800 kWh
• Production: 1,300 kWh per kW
• System size: 10,800 ÷ 1,300 = 8.3 kW
• Panels: 8,300 ÷ 430 = 19.3, so 20 panels (8.6 kW)

Yearly use	At 1,150 kWh/kW	At 1,300 kWh/kW	At 1,600 kWh/kW
6,000 kWh	5.2 kW / 13 panels	4.6 kW / 11 panels	3.8 kW / 9 panels
9,000 kWh	7.8 kW / 19 panels	6.9 kW / 17 panels	5.6 kW / 14 panels
12,000 kWh	10.4 kW / 25 panels	9.2 kW / 22 panels	7.5 kW / 18 panels
15,000 kWh	13.0 kW / 31 panels	11.5 kW / 27 panels	9.4 kW / 22 panels

Panel counts are rounded up at 430 W per panel. The median U.S. residential system in 2025 was 7.7 kW, and most were between 5 and 12 kW. Our system size calculator does this math with your data.

Step 4: Check your roof space

A 430 W panel is roughly 21 square feet. So:

• 20 panels × 21 sq ft = about 420 square feet of panel area for an 8.6 kW system.
• 25 panels need about 525 square feet.

Usable roof area is less than total roof area. You lose space to:

• Fire code setbacks from ridges and edges, which vary by jurisdiction.
• Vents, chimneys, skylights and dormers.
• North-facing sections, which usually are not worth covering.
• Shaded areas from trees or neighboring buildings.

If your best roof faces only fit 14 panels, that is your practical limit unless you add a ground mount or use higher-wattage panels. Premium high-efficiency panels cost more per watt but can make sense when roof space is tight. See best solar panels.

Why 100% offset is not always the best target

Installers often aim to cover 100% of your yearly usage. That can be right, but it depends on how your utility pays for exports.

Under full retail net metering

Every kWh is worth about the same, whether you use it or export it. Sizing close to 100% of yearly usage usually makes sense. Going over 100% often does not, because many utilities pay little or nothing for a yearly surplus.

Under net billing or reduced export credits

Solar produces most at midday, when many homes use little. Under net billing, exported kWh earn far less than you pay for power. In California’s Net Billing Tariff, exports average roughly 4 to 5 cents per kWh. Other states and utilities with reduced export credits include Arizona, Illinois, North Carolina (Duke), Idaho Power and others. See net metering explained and California net billing (NEM 3.0).

Here is what that does to the value of the last kW you add:

	Full retail net metering, 30¢ rate	Net billing, mostly exported at 5¢
Cost of one more kW at $3.00/W	$3,000	$3,000
Yearly output of that kW	1,300 kWh	1,300 kWh
Yearly value	About $390	About $65
Simple payback of that kW	About 7.7 years	About 46 years

Under net billing, the first few kW, which offset power you use as it is made, are valuable. The extra kW that mostly export can be a poor investment. A smaller system sized to daytime use, a shift of flexible loads like dishwashers, laundry or EV charging to midday, or a battery may beat a bigger array. See time-of-use rates and solar and do solar batteries pay off.

Oversizing is also a common sales tactic, because a bigger system means a bigger commission. See oversized system. Compare sizes yourself with compare options.

Planning for future loads

Adding panels later is usually more expensive per watt than including them up front. But paying today for loads you may never add is also a waste.

Electric vehicle. An EV driven 12,000 miles a year at about 3.5 miles per kWh uses roughly 3,400 kWh a year. At 1,300 kWh per kW, that is about 2.6 kW more, or 6 or 7 extra panels. See solar and EV charging.

Heat pump. Switching from gas, oil or propane heat to a heat pump can add thousands of kWh a year, especially in cold climates. Most of that use comes in winter, when solar produces least, so under net billing the extra panels help less than you might hope. Get a heating load estimate from an HVAC contractor before sizing for it.

Heat pump water heater or induction cooking. Smaller increases, but they add up.

A sensible rule: size for loads you expect within the next two or three years, and check that your utility allows the larger system. Some utilities cap system size based on past usage, so ask your installer to confirm before signing.

Common sizing mistakes

• Using one bill instead of 12 months.
• Trusting an installer’s production estimate without checking PVWatts.
• Sizing to 100% or more under net billing without checking the value of exports.
• Ignoring shade and roof direction.
• Paying for future loads that are many years away.
• Skipping cheap efficiency fixes that would shrink the system you need.

Bottom line: Size from your real 12-month usage and your real production, then let your export rules decide how close to 100% you should go. Under net billing, smaller often wins.

Run your numbers in the system size calculator, then see whether that size is worth it in the main calculator. For the full decision, read is solar worth it in 2026.