Solar Panel Size Estimator

Find out how many solar panels you need to cover your electricity use. Enter your monthly kWh, local sun hours, and panel wattage — we'll calculate the system size and estimated costs.

Last verified: August 23, 2026

Monthly electricity usage

kWh/month

Find this on your electricity bill. US average: 886 kWh/month (EIA 2023).

Peak sun hours per day

Or enter custom value:

hrs

Panel wattage

System efficiency factor

80% is standard (accounts for inverter, heat, soiling, wiring).

Cost per watt installed

$ /W

US average ~$2.50–$3.50/W installed (2025). Before incentives.

Electricity rate (for savings estimate)

$ /kWh

Default: EIA US average $0.182/kWh.

Panels needed

System size

6.8 kW

Est. system cost

$20,400

Annual savings

$1,855

Daily generation: 29.6 kWh
Monthly generation: 886 kWh
Roof space needed: ~306 sq ft

⚠️ Federal 30% ITC has ended: The federal Residential Clean Energy Credit (30% solar ITC) ended on 31 December 2025 and no longer applies to a 2026 purchase, so the cost above is before incentives. Many state and utility programmes still cut the price — see our US solar incentives guide.

Estimates only. Get quotes from 3+ local installers for accurate pricing. Savings assume 100% self-consumption of generated power.

How this is calculated

The calculator determines how many panels are needed to generate your monthly electricity usage, then estimates system cost and annual savings.

Daily kWh needed   = Monthly kWh ÷ 30
System kW needed   = Daily kWh ÷ (Sun hrs × Efficiency)
Panels needed      = System kW × 1000 ÷ Panel watts (rounded up)
Actual system kW   = Panels × Panel watts ÷ 1000
Daily generation   = Actual kW × Sun hrs × Efficiency
Annual savings     = Monthly kWh × 12 × Electricity rate
System cost        = Actual system kW × 1000 × Cost per watt
Roof space         = Panels × 18 sq ft (standard panel)

Example (886 kWh/mo, 4 sun hrs, 400W panel, 80% eff, $3/W, $0.1820/kWh):
  Daily need         = 886 ÷ 30           = 29.5 kWh
  System kW          = 29.5 ÷ (4 × 0.80) = 9.2 kW
  Panels             = 9,200 ÷ 400        = 23 panels
  Annual savings     = 886 × 12 × $0.1820 = $1,935

Assumptions

System efficiency of 80% accounts for inverter losses (~4%), temperature derating (~8%), soiling (~2%), and wiring (~2%)
Roof space estimate: 18 sq ft per standard 400W panel (approximately 3.3 ft × 5.4 ft)
Annual savings assume 100% of generated power offsets grid purchases (net metering)
Cost per watt is for fully installed systems including panels, inverter, racking, wiring, and labour
The federal 30% ITC ended 31 Dec 2025; the "before incentives" cost shown does not deduct any federal credit — check current state/utility incentives
Does not include battery storage costs

System size by monthly usage (400W panels, 4 sun hrs, 80% eff.)

Monthly usage	Panels	System size	Est. installed cost	Annual savings
400 kWh/mo	11 panels	4.4 kW	$13,200.00	$873.60/yr
600 kWh/mo	16 panels	6.4 kW	$19,200.00	$1,310.40/yr
886 kWh/mo	24 panels	9.6 kW	$28,800.00	$1,935.02/yr
1,200 kWh/mo	32 panels	12.8 kW	$38,400.00	$2,620.80/yr
1,600 kWh/mo	42 panels	16.8 kW	$50,400.00	$3,494.40/yr

Assumptions: 400W panels, 4 peak sun hours, 80% system efficiency, $3.00/W installed, $0.182/kWh electricity rate.

Frequently asked questions

How many solar panels does an average home need? ▾

A typical US home uses about 886 kWh/month (EIA 2023). With 5 peak sun hours/day and 400W panels, that's roughly 15–20 panels. Homes in sunnier states (Arizona, California) need fewer; northern states need more. This calculator gives you a personalised estimate.

What are peak sun hours? ▾

Peak sun hours measure how many hours per day your location receives enough sunlight intensity (1,000 W/m²) to generate rated panel output. It's not the same as daylight hours. Phoenix, AZ averages ~6.5 peak sun hours; Seattle, WA averages ~3.5. The UK averages 2.5–4. Your solar installer can provide local values from PVWatts data.

What wattage are modern solar panels? ▾

Residential panels in 2024–2026 typically range from 370W to 440W per panel. Premium monocrystalline panels (SunPower, REC) reach 420–450W. The most common residential size installed is 400W. Commercial panels can exceed 500W but are larger and heavier.

Does this account for system losses? ▾

Yes — this calculator applies a 20% system loss factor (80% efficiency) to account for inverter losses, wiring resistance, soiling, temperature derating, and shading. NREL's PVWatts uses 14% as a default; we use 20% for a conservative, realistic estimate.

Can I go off-grid with this many panels? ▾

Covering your monthly usage with solar doesn't mean off-grid readiness. Off-grid requires battery storage sized for several days of use, plus generator backup. For grid-tied systems (most residential installs), excess power is exported to the grid and you draw power at night.

Sources

U.S. EIA — Residential Energy Consumption Survey — US average monthly electricity consumption (886 kWh) (last checked 2026-08-23)
NREL PVWatts Calculator — Peak sun hours by location, system loss methodology (last checked 2026-08-23)
IRS — Residential Clean Energy Credit — Federal 30% solar ITC status (ended 31 Dec 2025 under OBBBA) (last checked 2026-08-23)
Lawrence Berkeley National Lab — Tracking the Sun — Installed cost per watt benchmarks ($2.50–$3.50/W) (last checked 2026-08-23)