green calculator

Solar Panel Sizing Calculator

Calculate how many solar panels you need from your monthly usage, peak sun hours, panel wattage, and system derate factor.

Required system size in kW is daily electricity usage ÷ (peak sun hours × derate factor, typically 0.80). Panel count is that required kW × 1000 ÷ panel wattage, rounded up. A US-average home (864 kWh/month, 4.98 peak sun hours) needs about 18 400W panels for full offset.

Formula Verified by Dr. Olivia Martinez, PhD

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Usage & Location

Avg monthly usagekWh

Peak sun hourshr/day

System Assumptions

Panel wattageW

Derate factor%

Offset target%

Results

Panels Needed

18 panels

System Size7.20 kW

Required Size (exact)7.13 kW

Est. Annual Production10,470 kWh

Est. Roof Area353 sq ft

Tip

Re-run it whenever your usage changes or you're comparing quotes — installers sometimes size to their inventory rather than your actual bill.

Formula

Solar panel sizing formula

Required kW = (Daily usage × Offset%) ÷ (Peak sun hours × Derate factor); Panels = ROUND UP(Required kW × 1000 ÷ Panel wattage)

Daily usage is monthly usage × 12 ÷ 365. Derate factor bundles inverter, wiring, soiling, and temperature losses.

Step by step

How the calculation works

1Enter your average monthly electricity usage in kWh, from your utility bill.
2Enter your location’s peak sun hours per day (national US average is 4.98).
3Set panel wattage — 400W is a common residential panel in 2026.
4Adjust the derate factor and offset target to match your goals.

Related guides

Learn more about this calculation

Guide

Solar Panel Payback: ROI, Incentives, and Bill Savings

Estimate solar payback by connecting installation cost, incentives, bill offset, utility-rate growth, 20-year savings, and the live DTC solar payback calculator.

Site Guide

How to Compare Calculator Scenarios Step by Step

Build baseline, upside, and downside calculator scenarios, compare changes fairly, and find the assumptions that have the greatest effect on a decision.

Financial Tool

Solar Payback Calculator

Estimate net system cost, incentives, and break-even timeline for the system size calculated here.

Solar Panel Sizing Calculator

Calculate how many solar panels you need from your monthly usage, peak sun hours, panel wattage, and system derate factor.

Formula Verified by Dr. Olivia Martinez, PhD

How to understand and use this calculator

Quick Answer — How Many Solar Panels Do I Need?

The core formula is (Monthly usage ÷ Peak sun hours ÷ 30.4) ÷ (Panel wattage × Derate factor), rearranged here as a daily-energy-target approach: convert your monthly kWh to a daily figure, divide by your location's peak sun hours to get the required system size in kW, then divide by panel wattage to get a panel count.

Quick reference (400W panels, 80% derate factor, 100% offset):

600 kWh/month, 5.0 peak sun hours (mild-sun state) → 13 panels (5.2 kW system)
864 kWh/month, 4.98 peak sun hours (US average) → 18 panels (7.2 kW system)
1,000 kWh/month, 6.0 peak sun hours (sunny state) → 17 panels (6.8 kW system)
1,200 kWh/month, 6.5 peak sun hours (desert Southwest) → 19 panels (7.6 kW system)

Notice the desert-Southwest example needs *more* usage covered with *only one extra panel* versus the US-average example — that's peak sun hours doing the heavy lifting. Two homes with identical electric bills can need meaningfully different system sizes purely because of where the sun hits their roof.

How to Use This Calculator

Enter your average monthly electricity usage in kWh (check your utility bill — most bills report this directly), your location's peak sun hours per day, your target panel wattage (400W is a common residential panel size in 2026), a system derate factor (80% is a reasonable default covering inverter losses, wiring losses, soiling, and temperature effects), and what percentage of your usage you want the system to offset.

Worked example: 864 kWh/month usage (the 2026 US household average), 4.98 peak sun hours/day (the 2026 US average), 400W panels, 80% derate, 100% offset target. Daily usage = 864 × 12 ÷ 365 = 28.41 kWh/day. Target daily production (100% offset) = 28.41 kWh. Required system size = 28.41 ÷ (4.98 × 0.80) = 7.13 kW. Panel count = 7.13 kW × 1000 ÷ 400W = 17.8, rounded up to 18 panels (you can't install a fraction of a panel). Actual system size with 18 panels = 18 × 400W = 7.2 kW, producing about 10,470 kWh/year — slightly above your 10,368 kWh/year usage, which is expected since panel counts round up.

A second example, sunnier location: 1,200 kWh/month usage, 6.5 peak sun hours/day (Arizona-level sun), same 400W panels and 80% derate, 100% offset. Daily usage = 39.45 kWh/day. Required system size = 39.45 ÷ (6.5 × 0.80) = 7.59 kW. Panel count = 7.59 × 1000 ÷ 400 = 18.97, rounded up to 19 panels — a 7.6 kW system producing about 14,425 kWh/year. Despite using 39% more electricity than the first example, this home needs only 1 additional panel, because its peak sun hours are 30% higher.

To see whether that system size makes financial sense given installation cost, incentives, and utility rate escalation, run the resulting system cost through the solar payback calculator.

The Formula This Calculator Uses

Daily usage (kWh) = Monthly usage × 12 ÷ 365.

Target daily production = Daily usage × Offset % ÷ 100 (set offset above 100% to intentionally oversize for future EV charging or electrification).

Required system size (kW) = Target daily production ÷ (Peak sun hours × Derate factor).

Panel count = ROUND UP(Required system size × 1000 ÷ Panel wattage) — always rounded up, since partial panels aren't installable.

Actual system size = Panel count × Panel wattage ÷ 1000.

Annual production estimate = Actual system size × Peak sun hours × 365 × Derate factor.

Estimated roof area = Panel count × ~19.6 sq ft, based on a typical 400W residential panel footprint of roughly 17.5-21 sq ft — treat this as a rough planning number, not a substitute for a site survey.

Why the Derate Factor Matters More Than People Expect

A solar panel's nameplate wattage is measured under lab-standard test conditions that real rooftops rarely match. The derate factor bundles together every real-world loss: inverter conversion loss (typically 2-4%), wiring and connection losses (1-3%), soiling from dust and debris (2-5% depending on climate and cleaning frequency), and — the biggest single factor — temperature. Panels lose efficiency as they heat up, and a hot summer rooftop can run panels 25-40°C above the standard test temperature, cutting output by 10-15% during peak sun hours.

Stacked together, these losses commonly total 15-25%, which is why this calculator defaults to an 80% derate factor (a 20% total loss) rather than assuming panels perform at their full nameplate rating. Using 100% (no derate) instead of 80% would understate your required system size by roughly 20% — exactly the kind of gap that leaves a homeowner short of their target offset after installation.

If you're also sizing battery backup rather than just grid-tied panels, the solar battery sizing calculator and the solar inverter sizing calculator handle those adjacent calculations using the same system-size output from this page.

Peak Sun Hours: The Number That Changes Everything

Peak sun hours is not the same as daylight hours — it's a measure of total daily solar energy compressed into an equivalent number of hours at maximum intensity (1,000 W/m²). The continental US ranges from about 3.0-4.2 peak sun hours in the Pacific Northwest and northern Great Lakes up to 6.5-7.5 in the desert Southwest, with a national average of 4.98.

Because peak sun hours sits in the denominator of the system-sizing formula, a location with low sun needs a proportionally larger system to produce the same energy — the 2.2x panel-count difference between a 900 kWh/month home in Arizona (13 panels) versus an identical home in Seattle (29 panels) illustrates just how much this single input moves the result. Always use your actual location's figure rather than a national average if precision matters for your budget.

What This Calculator Doesn't Account For

This is a planning-stage sizing tool, not a substitute for a professional site survey. It doesn't account for roof orientation and tilt (a south-facing roof at your latitude's optimal tilt angle outperforms a north-facing or flat roof by 20% or more), shading from trees or nearby structures, roof structural capacity, or local permitting and setback requirements that can reduce usable roof area.

It also assumes a flat, year-round average usage pattern. Homes with strong seasonal swings — heavy summer AC load or winter electric heating — may want to size against their peak month rather than the annual average used here, to avoid a system that undershoots during the highest-usage season.

Finally, panel wattage and derate factors continue to improve year over year; check your specific panel model's real-world specifications rather than relying solely on the 400W/80% defaults if you're finalizing a purchase decision.

Related concepts

Peak sun hours (PSH)

A measure of total daily solar energy expressed as an equivalent number of hours at maximum intensity (1,000 W/m²), used to convert panel wattage into daily kWh output.

System derate factor

The combined real-world efficiency loss from inverter conversion, wiring, soiling, and temperature effects, typically 15-25% below a panel's nameplate rating.

Nameplate wattage

A solar panel's rated output measured under standardized lab test conditions, which real rooftop conditions rarely match exactly.

Frequently Asked Questions

Why does my panel count round up instead of showing a decimal?+

Because you can't install a fraction of a physical panel. This calculator always rounds the raw panel-count calculation up to the next whole number, which means your actual system will be very slightly oversized relative to your offset target.

What's a reasonable derate factor to use?+

80% (a 20% total loss from inverter, wiring, soiling, and temperature effects) is a common industry planning default. Hot, dusty climates may see slightly more loss; cool, clean climates slightly less.

Should I use my location's peak sun hours or the national average?+

Your actual location's figure, if you know it — the national average of 4.98 is a fallback only. Peak sun hours can range from about 3.0 in parts of the Pacific Northwest to 6.5+ in the desert Southwest, and it directly scales your required system size.

Why would I set the offset percentage above 100%?+

To intentionally oversize the system for planned future electricity use, such as adding an EV, a heat pump, or electrifying a gas appliance, so the system doesn't need to be expanded again shortly after installation.

Does this calculator size battery storage too?+

No — this tool sizes grid-tied solar panel capacity only. Use the solar battery sizing calculator to size backup storage capacity separately, and the solar inverter sizing calculator to size the inverter and charge controller.

How accurate is the roof area estimate?+

It's a rough planning figure (roughly 19.6 sq ft per 400W panel), not a substitute for a site survey. Actual usable roof area also depends on orientation, shading, setbacks, and roof shape, none of which this calculator models.

Why do panel counts differ so much between similar-usage homes in different states?+

Peak sun hours varies significantly by location and sits in the denominator of the sizing formula. A home in a lower-sun region needs a proportionally larger system to produce the same annual energy as an identical home in a sunnier region.

How to understand and use this calculator

Quick Answer — How Many Solar Panels Do I Need?

Quick reference (400W panels, 80% derate factor, 100% offset):

600 kWh/month, 5.0 peak sun hours (mild-sun state) → 13 panels (5.2 kW system)
864 kWh/month, 4.98 peak sun hours (US average) → 18 panels (7.2 kW system)
1,000 kWh/month, 6.0 peak sun hours (sunny state) → 17 panels (6.8 kW system)
1,200 kWh/month, 6.5 peak sun hours (desert Southwest) → 19 panels (7.6 kW system)

How to Use This Calculator

To see whether that system size makes financial sense given installation cost, incentives, and utility rate escalation, run the resulting system cost through the solar payback calculator.

The Formula This Calculator Uses

Daily usage (kWh) = Monthly usage × 12 ÷ 365.

Target daily production = Daily usage × Offset % ÷ 100 (set offset above 100% to intentionally oversize for future EV charging or electrification).

Required system size (kW) = Target daily production ÷ (Peak sun hours × Derate factor).

Panel count = ROUND UP(Required system size × 1000 ÷ Panel wattage) — always rounded up, since partial panels aren't installable.

Actual system size = Panel count × Panel wattage ÷ 1000.

Annual production estimate = Actual system size × Peak sun hours × 365 × Derate factor.

Why the Derate Factor Matters More Than People Expect

Peak Sun Hours: The Number That Changes Everything

What This Calculator Doesn't Account For

Related concepts

Peak sun hours (PSH)

A measure of total daily solar energy expressed as an equivalent number of hours at maximum intensity (1,000 W/m²), used to convert panel wattage into daily kWh output.

System derate factor

The combined real-world efficiency loss from inverter conversion, wiring, soiling, and temperature effects, typically 15-25% below a panel's nameplate rating.

Nameplate wattage

A solar panel's rated output measured under standardized lab test conditions, which real rooftop conditions rarely match exactly.

Frequently Asked Questions

Why does my panel count round up instead of showing a decimal?+

What's a reasonable derate factor to use?+

Should I use my location's peak sun hours or the national average?+

Why would I set the offset percentage above 100%?+

Does this calculator size battery storage too?+

How accurate is the roof area estimate?+

Why do panel counts differ so much between similar-usage homes in different states?+

Solar Panel Sizing Calculator

Inputs

Usage & Location

System Assumptions

Results

Solar panel sizing formula

How the calculation works

Learn more about this calculation

Solar Panel Payback: ROI, Incentives, and Bill Savings

How to Compare Calculator Scenarios Step by Step

Solar Payback Calculator

Solar Panel Sizing Calculator

Inputs

Usage & Location

System Assumptions

Results

Key output metrics

Daily usage vs required production

How to understand and use this calculator

Quick Answer — How Many Solar Panels Do I Need?

How to Use This Calculator

The Formula This Calculator Uses

Why the Derate Factor Matters More Than People Expect

Peak Sun Hours: The Number That Changes Everything

What This Calculator Doesn't Account For

Related concepts

Frequently Asked Questions

Solar panel sizing formula

How the calculation works

Learn more about this calculation

Solar Panel Payback: ROI, Incentives, and Bill Savings

How to Compare Calculator Scenarios Step by Step

Solar Payback Calculator

Key output metrics

Daily usage vs required production

How to understand and use this calculator

Quick Answer — How Many Solar Panels Do I Need?

How to Use This Calculator

The Formula This Calculator Uses

Why the Derate Factor Matters More Than People Expect

Peak Sun Hours: The Number That Changes Everything

What This Calculator Doesn't Account For

Related concepts

Frequently Asked Questions