Skip to main content
Tool Belt Calc

Electrical

Solar Panel Calculator

How many panels your electricity use actually needs.

Your measurementsEvery assumption editable01

From your bill. Use an annual average rather than one month.

Not daylight hours — the equivalent hours at full rated irradiance. Varies by region and season.

Losses from temperature, soiling, wiring, inverter and mismatch. 80% is the usual planning figure.

Modern residential modules run 380–450 W.

A typical 400 W residential module is about 21 sq ft.

How much of your usage the array should cover. 100% is a full offset.

Array sizeCalculated live02

Result

21panels

8.4 kW array — about 441 sq ft of roof

Daily energy target100% of 900 kWh a month
29.59 kWh
Array size requiredat 4.5 sun hours and 80% derate
8.219 kW
Panels
21 × 400 W
Installed capacity
8.4 kW DC
Estimated annual production
11038 kWh
Actual offsetafter rounding up to whole panels
102.2%
Roof area neededmodules only, before setbacks and access paths
441 sq ft

Peak sun hours is not daylight hours. It is the number of hours per day equivalent to full rated irradiance — a location averaging 4.5 peak sun hours might see twelve hours of daylight, most of it well below test conditions.

The derate factor is where nameplate meets reality. Panel temperature alone commonly costs ten per cent on a hot roof, and soiling, wiring, inverter efficiency and module mismatch account for the rest. Eighty per cent is a planning figure; a shaded or poorly oriented array does worse.

Roof area here is modules only. Fire setbacks, access pathways, obstructions and the fact that only some roof planes face usefully mean the roof you actually need is considerably larger — often half again.

Enter your monthly usage and pick your region's peak sun hours. The derate factor is exposed rather than hidden, because a system sized on nameplate output underperforms from the day it is switched on.

Why use this tool?What it does differently03

Why use this tool?

Derate exposed

Temperature, soiling, wiring and inverter losses, as an editable field rather than a hidden constant.

Regional sun hours

From Pacific Northwest to desert Southwest — nearly double the output for the same array.

Roof area shown

Module footprint, with an honest note that the roof you need is larger still.

Actual offset

After rounding up to whole panels, which usually overshoots your target slightly.

How this worksThe method04

How this solar panel calculator works

Daily energy need comes from monthly usage divided by the average days in a month, scaled by your target offset. Array size is that daily figure divided by peak sun hours times the derate factor.

Peak sun hours is not daylight hours — it is the equivalent number of hours at full rated irradiance. A location averaging 4.5 peak sun hours might see twelve hours of daylight, most of it well below test conditions.

The derate is where nameplate meets reality. Panel temperature alone commonly costs ten per cent on a hot roof; soiling, wiring, inverter efficiency and module mismatch account for the rest. Eighty per cent is a planning figure, and a shaded or badly oriented array does worse.

How to use itStep by step05

How to use it

  1. Step 1: Find your usage

    Monthly kWh from your bill. Use an annual average rather than one month.

  2. Step 2: Pick your sun hours

    Regional presets are a starting point; your installer will have a site-specific figure.

  3. Step 3: Keep the derate honest

    80% is standard. Lower it for shading or a poor roof orientation.

  4. Step 4: Check the roof

    The module area is a floor. Setbacks, access paths and unusable roof planes all add to it.

Example usageWorked figures06

Example usage

An average home
900 kWh a month at 4.5 sun hours and 80% derate needs an 8.22 kW array — 21 panels at 400 W, giving 8.4 kW and about 11,038 kWh a year across 441 sq ft of modules.
The same house in Arizona
At 6.2 sun hours the array drops to 5.97 kW — 15 panels rather than 21. Location moves this more than any other input.
A partial offset
Targeting 50% rather than 100% halves the array to 11 panels, which is a common approach where roof space or budget is limited.
Frequently asked questionsCommon questions07

Frequently asked questions

How many solar panels do I need?

For a typical home using 900 kWh a month at average sun, around 20 to 22 panels at 400 W — an 8 kW array. Usage and location move that substantially.

What are peak sun hours?

The equivalent hours per day at full rated irradiance, not hours of daylight. A place with twelve hours of daylight might average only four or five peak sun hours.

Why is the derate factor necessary?

Because panels are rated under laboratory conditions a real roof never meets. Heat, dirt, wiring losses and inverter efficiency mean about 80% of nameplate reaches your meter.

How much roof space do solar panels need?

About 21 square feet per 400 W module, so an 8 kW array is around 440 square feet of panel. Allow considerably more actual roof for setbacks and access paths.

Should I size for 100% offset?

Not always. Net-metering rules, roof space and budget all argue for less in some cases, and exports credited below retail make a full offset less attractive than it looks.

Related toolsElsewhere on the site08

Browse every tool in Electrical Calculators.