Solar Panel Estimate Calculator

How to use this calculator

The estimator turns a single bill column — monthly kWh — into a complete project view: system size, annual production, gross cost, net cost after the 30% federal ITC, year-1 savings, simple payback, and a 25-year cumulative savings figure with tariff escalation built in. Use it to sanity-check the first quote you receive before paying for a site visit.

Inputs:

1. Monthly electricity use (kWh) — average the last 12 bills. The 2026 U.S. residential average per the EIA is 875 kWh/month, but ranges from 565 kWh in Vermont to 1,210 kWh in Tennessee. Add 250-350 kWh/month if you’ve added an EV in the last year, 200-400 kWh/month if you’re in the South with central A/C.
2. Target offset (%) — 100% sizes the system to match your annual consumption. Net-metering customers (NEM-1, NEM-2, full-retail states) should target 100%; NEM-3 customers in California should target 70-90% with paired battery; off-grid sizing requires a separate calculation that includes battery autonomy and DOD.
3. Peak sun hours/day — your location’s annual average. Use NREL PVWatts (pvwatts.nrel.gov) for an exact number. Reference: Phoenix 6.5, Albuquerque 6.3, Denver 5.5, Atlanta 4.8, Chicago 4.4, Boston 4.2, Portland 3.9, Seattle 3.4.
4. Installed cost per watt — the all-in turnkey price. The 2026 EnergySage H2 2025 marketplace median is $2.85/W; NREL Tracking the Sun 2025 reports a similar $2.86/W national median. Leave the default until you have at least one quote.
5. Tax credit / rebate (%) — 30% for the federal ITC alone. Add state and utility incentives as a higher percentage if applicable.
6. Electricity rate ($/kWh) — the all-in rate at the bottom of your bill (energy + delivery + taxes). The 2026 U.S. residential average is $0.165/kWh per EIA, but ranges from $0.105 in Washington to $0.46 in Hawaii.
7. Annual rate escalation (%) — EIA Annual Energy Outlook 2025 projects 2.7-3.2% nominal residential price growth through 2050, so 3% is a reasonable national default. California, Massachusetts, and New York have averaged 5-7% since 2020.

How the math works

annual_target_kWh   = monthly_kWh × 12 × (offset% / 100)
system_kW           = annual_target_kWh / (peak_sun_hours × 365 × 0.78)
annual_production   = system_kW × peak_sun_hours × 365 × 0.78
gross_cost          = system_kW × 1000 × $/W
net_cost            = gross_cost × (1 - incentive% / 100)
year_1_savings      = annual_production × $/kWh
simple_payback      = net_cost / year_1_savings
lifetime_savings    = sum over 25 years of (production × (1-0.005)^year × rate × (1+esc%)^year)

The 0.78 derate factor follows NREL PVWatts default — inverter efficiency (96-98%), DC and AC wiring losses (2-3%), soiling and shading (4-6%), and ambient-temperature derating (3-5%) — validated against thousands of metered residential systems by Lawrence Berkeley National Laboratory.

Worked example for an Atlanta home using 1,000 kWh/month at $0.135/kWh:

• Annual target: 1,000 × 12 × 1.0 = 12,000 kWh
• System size: 12,000 / (4.8 × 365 × 0.78) = 8.79 kW
• Annual production: 8.79 × 4.8 × 365 × 0.78 = 12,001 kWh
• Gross cost at $2.80/W: 8,790 × $2.80 = $24,612
• Net after 30% ITC: $24,612 × 0.70 = $17,228
• Year-1 savings: 12,001 × $0.135 = $1,620
• Simple payback: $17,228 / $1,620 = 10.6 years
• 25-year lifetime savings (3% esc): ~$58,200

U.S. payback ranges by region (2026)

EnergySage H2 2025 marketplace + EIA residential rate data:

Sources: EnergySage Solar Marketplace Report H2 2025, EIA Form 826 monthly residential rates 2025, DSIRE database (NC State).

When the estimator gets it wrong

The model assumes a south-facing roof at near-optimal tilt, no shading, and a single-rate retail tariff. Knock off 10-25% of annual production and savings if any of these apply:

• East- or west-facing roof: -12% production at the same tilt
• North-facing (Northern Hemisphere): -25 to -35% — the system rarely makes financial sense
• Heavy shading 2+ hours/day: -8 to -25% depending on bypass diode behaviour and inverter type (microinverters salvage about 6-8% more than string in shaded conditions)
• Time-of-use tariff with afternoon peak: production overlaps peak well, often +5 to +10% effective savings
• Time-of-use tariff with evening peak: production misses peak, -10 to -20% effective savings unless paired with battery
• NEM-3 net billing (California): -40 to -50% on exported kWh value unless 70-90% self-consumption is achieved with battery

Pair this with the cost calculator, payback calculator, and ROI calculator

The estimator gives you the headline numbers in one pass; the cost calculator drills into installed-price-per-watt and adders; the payback calculator separates simple from discounted payback (with cost of capital); the ROI calculator brings it together as an internal rate of return so you can compare solar against an S&P 500 index fund or a bond ladder. Run all four with the same inputs before paying any deposits.

Sources

• EnergySage Solar Marketplace Report H2 2025 — installed cost-per-watt benchmarks by state
• NREL PVWatts Calculator — peak sun hours and modeled production by ZIP code
• NREL Tracking the Sun 2025 — residential PV system pricing trends
• EIA Annual Energy Outlook 2025 — long-run residential rate forecasts
• EIA Form 826 monthly retail electric data — current state rates
• DSIRE database (NC State) — state and local incentive details
• DOE SETO solar cost benchmark — itemized cost components