What is the system efficiency factor used in the calculation?

Solar calculators typically apply a 0.75–0.85 derate factor to account for inverter losses, wiring losses, soiling, and temperature effects. A derate of 0.80 (80%) is a common industry standard for residential systems.

How to Calculate How Many Solar Panels You Need

Q: How many solar panels does the average house need?

Most homes need between 15 and 25 panels, depending on electricity usage and location. A home using 1,000 kWh per month in a state with 4.5 peak sun hours typically needs about 18–20 panels at 400W each.

Q: Should I size the system to cover 100% of my electricity use?

Not necessarily. Many homeowners target 80–90% offset to avoid overbuilding. Producing more than you consume at certain times sends excess energy to the grid, and net metering rates vary by utility. Use our system size calculator to model different offset percentages.

Q: Does roof size limit how many panels I can install?

Yes. A standard 400W residential panel is roughly 18 square feet. If your south-facing roof section is 400 square feet, you have room for approximately 20–22 panels, accounting for spacing and obstructions.

Before you talk to a single installer, you should know roughly how many panels your home needs. Installers who quote without asking about your electricity bill are guessing. You don't have to guess — the calculation takes three inputs and five minutes.

The Three Inputs You Need

Every solar sizing calculation starts with three numbers:

Your monthly electricity usage — in kilowatt-hours (kWh). Find this on your utility bill.
Your location's peak sun hours — the daily average solar irradiance where you live, measured in hours at full intensity.
The panel wattage — how much power each panel produces at peak conditions, typically between 350W and 450W for modern residential panels.

The Formula

System Size Formula

System kW = (Monthly kWh × 12 × Target Offset) ÷ (Peak Sun Hours × 365 × 0.80)

Panel Count Formula

Number of Panels = (System kW × 1,000) ÷ Panel Wattage

The 0.80 derate factor accounts for real-world losses — inverter inefficiency, wiring resistance, dust on panels, and high-temperature performance degradation. Most installers use 0.75 to 0.85; 0.80 is a solid middle estimate.

Step-by-Step Worked Example

Example: Phoenix, AZ homeowner

Monthly usage: 1,200 kWh
Peak sun hours: 5.5 hrs/day (Phoenix average)
Panel wattage: 400W
Target offset: 100%

System kW = (1,200 × 12 × 1.0) ÷ (5.5 × 365 × 0.80)
= 14,400 ÷ 1,606 = 8.97 kW ≈ 9 kW

Panel count = (9,000W) ÷ 400W = 22–23 panels

Example: Boston, MA homeowner

Monthly usage: 900 kWh
Peak sun hours: 4.0 hrs/day (New England average)
Panel wattage: 400W
Target offset: 90%

System kW = (900 × 12 × 0.90) ÷ (4.0 × 365 × 0.80)
= 9,720 ÷ 1,168 = 8.32 kW ≈ 8.5 kW

Panel count = (8,500W) ÷ 400W = 21–22 panels

How Peak Sun Hours Vary by State

Peak sun hours are the biggest variable in this calculation — they swing the panel count by 30% or more between the sunniest and cloudiest states. General ranges:

Southwest (AZ, NV, NM, CA desert): 5.5–6.5 hrs/day
Southeast & Midwest (TX, FL, CO, GA): 4.5–5.5 hrs/day
Mid-Atlantic & Mountain (NC, VA, UT): 4.0–5.0 hrs/day
Northwest & Northeast (WA, OR, ME, VT): 3.5–4.5 hrs/day

Our System Size Calculator has state-level peak sun hour averages built in — you don't need to look them up manually.

What About Roof Space?

A standard 400W panel measures roughly 68" × 40" — about 18.9 square feet. If a calculation calls for 20 panels, you need at least 380 square feet of suitable roof space. Suitable means south- or southwest-facing, minimal shading from trees or chimneys, and a pitch between 15° and 40°.

If your roof can't fit the full system, you have two options: use higher-wattage panels to get more power from fewer panels, or size down the system and plan for a partial offset.

Should You Size for 80% or 100% Offset?

Sizing for exactly 100% of your annual usage sounds ideal, but it's often not the most cost-effective choice. Here's why:

A 100% offset system produces more electricity than you need in summer, and the excess energy credit rates from utilities vary widely.
An 80–90% offset system is typically smaller, cheaper, and still eliminates the vast majority of your electricity bill.
If you're adding an EV or a heat pump in the near future, size up to account for that new load now rather than adding panels later.

Panel Wattage Comparison

Modern residential panels range from about 350W to 450W. Higher wattage means fewer panels to reach the same system size:

350W panels: 26 panels for a 9 kW system
400W panels: 23 panels for a 9 kW system
440W panels: 21 panels for a 9 kW system

Higher-wattage panels typically cost more per panel but save on installation labor, racking hardware, and roof penetrations — the per-watt cost can end up similar or better.

Use the Calculator for Your Exact Numbers

The formulas above give you a reliable estimate. For your exact panel count with your specific monthly usage and your state's precise sun hours, use the free calculator below.

Ready to calculate your exact system size?

Use the Free System Size Calculator →

Frequently Asked Questions

How many solar panels does the average house need?

Most homes need between 15 and 25 panels. A home using 1,000 kWh per month in a state with 4.5 peak sun hours typically needs about 18–20 panels at 400W each. Homes with above-average usage or in less sunny states will need more.

What is a peak sun hour?

A peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square meter — the standardized test condition for solar panels. It is not the same as an hour of daylight. Arizona gets 5.5–6 peak sun hours per day; New England gets about 4. More peak sun hours means each panel produces more electricity per day.

Does panel wattage affect how many panels I need?

Yes. A 400W panel produces more electricity per panel than a 300W panel, so you need fewer panels for the same total output. Higher-wattage panels often cost more per panel but may reduce overall installation cost by requiring fewer panels, less racking, and fewer roof penetrations.

Should I size the system to cover 100% of my electricity use?

Not necessarily. Many homeowners target 80–90% offset to keep system cost lower. Producing more than you consume sends excess power to the grid, and net metering rates from utilities vary widely. If you're planning to add an EV or a heat pump soon, factor that future load into your sizing now.

Does roof size limit how many panels I can install?

Yes. A standard 400W residential panel takes up about 19 square feet. A 400-square-foot south-facing roof section can fit approximately 20–22 panels, accounting for spacing and obstructions. If your roof can't fit the full calculated system, use higher-wattage panels or accept a partial offset.

What is the 0.80 derate factor?

The derate factor accounts for real-world energy losses between what panels produce at rated conditions and what actually reaches your home. These losses come from inverter conversion inefficiency (~4–6%), wiring resistance (~2%), soiling (~2%), and temperature — hot panels produce less power than cool ones. A derate of 0.80 means you get 80% of the nameplate output on an annual average basis.