How many peak sun hours do I get?

3–6 per day in most of the US; check NREL PVWatts.

How many panels for a house?

The average US home (≈ 900 kWh/month) needs about 18–22 panels of 400–450 W.

South-facing at roughly your latitude in tilt.

How many panels for 900 kWh a month?

With 4.5 sun hours, about 20 panels of 400 W.

How much does a 400 W panel produce?

Roughly 450–650 kWh a year in most of the US.

Not with net metering; they add backup and night-time self-use.

Solar panel calculator

Monthly usage, peak sun hours, losses and panel wattage: you get system size in kW, number of panels, annual production and roof area.

Purchase quantity

7 panel

Net measurement: 2.74 kWp

Solar panels7 panel
Peak power needed2.74 kWp
Installed peak power3.15 kWp
Estimated annual production4139.1 kWh
Roof area (≈ 2 m² per panel)150.695 ft²

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How it's calculated (visible formula)

kW = daily kWh ÷ (sun hours × (1 − losses)); panels = ceil(kW × 1000 ÷ panel watts)

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Add up monthly usage from your bills.
Look up peak sun hours (PVWatts or PVGIS).
Estimate system losses (15–25%).
Work out the kW needed.
Divide by panel wattage and check roof space.

Worked example: 300 kWh/month, 4.5 sun hours, 20% losses, 450 W panels

300 ÷ 30.4 = 9.87 kWh/day
9.87 ÷ (4.5 × 0.8) = 2.74 kW
2,740 ÷ 450 = 6.1 → 7

→ 7 panels (3.15 kW) and about 150 sq ft of roof.

Common mistakes

Using daylight hours instead of peak sun hours.
Ignoring shading from chimneys and trees.
Ignoring your utility's net metering rules.

Pro tips

Size with a full year of usage, not one month.
Check shading from chimneys, trees and buildings all day.
South-facing at roughly latitude tilt maximizes yearly output.
Check your utility's net metering rules.

From calculation to quote

Add labor, overhead and your margin, and generate the professional PDF.

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FAQs

How many peak sun hours do I get?: 3–6 per day in most of the US; check NREL PVWatts.
How many panels for a house?: The average US home (≈ 900 kWh/month) needs about 18–22 panels of 400–450 W.
Best orientation?: South-facing at roughly your latitude in tilt.
How many panels for 900 kWh a month?: With 4.5 sun hours, about 20 panels of 400 W.
How much does a 400 W panel produce?: Roughly 450–650 kWh a year in most of the US.
Do I need batteries?: Not with net metering; they add backup and night-time self-use.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.