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Panel Count Calculator: how many panels fit — and how many do you need?

Two different questions with two different answers. Work from your roof (what physically fits) or from your usage (what you’d actually use), pick your panel wattage and direction, and see both the array size and the bill coverage it buys.

Count my panels

live results


sq ft

After setbacks & obstructions



kWh

For coverage %

Panels that fit your roof
28 panels
11.5 kW system · Arizona sunshine · covers ≈182% of 12,000 kWh/yr

Panel wattage410 W
Production per panel / yr778 kWh
Total system size11.5 kW
Estimated annual production21,789 kWh
Roof footprint required588 sq ft

A modern panel is ≈17.5 sq ft; we reserve ≈21 sq ft per panel including code setbacks and walkways. Estimates only — a site survey confirms what’s truly usable.

Data basis: ≈21 sq ft per panel including setbacks, your state’s average peak sun hours and the direction factor you select. Runs entirely in your browser — no sign-up, no email needed.

Array summary

11.5 kW

Total system size the panel count makes — the figure quotes should be priced on, per watt.

Panels 28
Est. cost at $2.85/W $32,718
Bill coverage 182%
Direction factor applied 100%
The 21 sq ft rule: fire codes require clear pathways and ridge/eave setbacks, dormers and vents eat space, and panels need small gaps. Planning at ≈21 sq ft per panel (instead of the panel’s bare 17.5) is the difference between a layout that passes inspection and a drawing that doesn’t.

2026 rules, stated plainly: the federal residential solar tax credit (Section 25D) ended on 31 December 2025, so no figure on this page subtracts it. Only state and local incentives count now — and only ones you add yourself. See what still applies →

What this calculator is

Panel counts are where quotes get slippery: “28 panels!” sounds bigger than “20 panels” until you learn the 28 are low-wattage. This tool works in both directions — from your roof’s usable area to a maximum panel count, or from your yearly usage to the count you genuinely need — and always translates the count into the two numbers that matter: total kilowatts and expected annual production for your state and roof direction.

How it works

From roof: panels = ⌊usable sq ft ÷ 21⌋ · From usage: panels = ⌈annual kWh ÷ (panel kW × sun hours × 365 × 0.80 × direction factor)⌉

In roof mode the tool divides your usable area by the realistic per-panel footprint including setbacks. In usage mode it divides your yearly consumption by one panel’s realistic yearly output in your state, adjusted for your roof’s direction — a west-facing panel produces about 15% less than a south-facing one, a north-facing one about 35% less, and the count rises to compensate. Either way, total kW = panels × panel wattage ÷ 1,000.

Worked example (real numbers)

600 usable sq ft, 410 W panels, Arizona. 600 ÷ 21 ≈ 28 panels = 11.5 kW. At Arizona’s 6.5 peak sun hours with the 0.80 factor, that array makes ≈21,800 kWh a year facing south — enough to cover a 12,000 kWh household with room to spare (or an EV). Point the same roof east and production falls ≈15% to ≈18,500 kWh. Now shrink the roof to 350 sq ft: 16 panels, 6.6 kW, ≈12,500 kWh south-facing — a useful reminder that panel count follows the roof, not the wish.

Limitations

  • “Usable” is doing heavy lifting. Chimneys, vents, skylights, shade from trees and neighbouring houses, and split roof planes all cut usable area. Measure each unobstructed rectangle, not the whole roof.
  • Direction factor is an average. Pitch interacts with direction: a steep north roof is worse than a shallow one. Ground mounts and trackers break these rules entirely (in your favour).
  • Electrical limits apply. Many homes’ main panels cap practical system size (the “120% rule”); a service upgrade can add $1,500–$3,500.
  • More isn’t always better. A count beyond your usage earns export rates, not retail savings, in net-billing states.

Common mistakes

  • Counting the whole roof. Plans drawn on gross roof area fail permitting and shrink on install day — after you’ve signed.
  • Comparing quotes by panel count. Always convert to kW and $/W first; wattage differences make raw counts meaningless.
  • Ignoring the north roof discount. Installers keen to fill a big order sometimes load north planes at south-facing production estimates.
  • Forgetting roof age. Panels on a roof with <10 years of life left means paying for removal and re-installation mid-life.

Questions about panel counts

How big is one solar panel, physically?
A typical 2025 residential panel is about 68 × 45 inches (≈17.5 sq ft) and 40–50 lbs, rated 370–460 W. Commercial-format panels are larger but rarely used on homes.
My roof faces east-west. Is solar still worth it?
Often yes. An east+west split produces ≈80–85% of a south array but spreads generation across morning and afternoon, which suits households using power before and after work — and under time-of-use rates that timing can be worth more than raw volume.
Can panels go on multiple roof planes?
Yes, with modern inverters handling each plane (or microinverters per panel). Expect slightly higher cost per watt for split arrays, reflected in quotes.

Rates, prices and incentive rules on this page checked November 2026.

Disclaimer: planning estimates only — not an engineering layout, structural assessment or financial advice. Usable area and final counts require a site survey.

Illustration of a roof plane filled with a neat grid of fifteen solar panels under a sunny sky
Panel count follows the roof: at roughly 21 sq ft per panel including setbacks, usable area — not wishful thinking — sets the maximum.

Keep reading: is your roof actually good for solar?

Direction, pitch, shade, age and material — the full roof audit, with the production losses each flaw costs you.

Read: What Makes a Roof Good or Bad for Solar? →
 ·  Cross-check with the System Size Estimator →