Solar Panels kWh Calculator

How to use this calculator

This tool sizes a solar system from the kWh number on your utility bill — the opposite direction of a production calculator. Enter six values and the calculator returns required system size in kW, panel count, daily/monthly/annual production, offset achieved, and year-1 bill savings:

1. Monthly electricity use (kWh) — the kWh column from your last 12 months of bills, averaged. EIA’s 2026 US residential average is 877 kWh/month.
2. Target offset (%) — how much of your bill you want to eliminate. 100% is full bill offset; 80% is partial offset for budget builds.
3. Peak sun hours per day — local average. Continental US ranges 3.5 (Seattle) to 6.5 (Phoenix). NREL PVWatts shows the exact value for any ZIP code.
4. System efficiency (%) — leave at 78% unless you know better. NREL PVWatts v6 default for residential rooftops.
5. Panel wattage (W) — the STC nameplate of each panel. Most 2026 residential panels are 400–440 W (Q CELLS Q.PEAK DUO, Silfab Prime, REC Alpha Pure-R). Premium high-efficiency modules reach 455–500 W (Maxeon, REC).
6. Electricity rate ($/kWh) — your blended residential retail rate. EIA’s 2026 average is $0.165/kWh; Hawaii is $0.41, Washington State is $0.105.

The formula

annual_need_kWh   = monthly_kWh × 12
target_kWh        = annual_need_kWh × (offset / 100)
required_array_W  = target_kWh × 1000 / (PSH × 365 × derate)
panel_count       = ceil(required_array_W / panel_W)
actual_array_W    = panel_count × panel_W
daily_production  = actual_array_W × PSH × derate / 1000
year1_savings     = min(annual_production, annual_need) × rate

The panel count uses ceil() because you cannot install half a panel. This means actual array size always slightly exceeds the bare minimum — typical overshoot is 0–5%.

A worked example for the US average household:

• Need: 877 × 12 = 10,524 kWh per year
• Required array: 10,524 × 1000 / (4.8 × 365 × 0.78) = 7,705 W
• Panel count: ceil(7705 / 400) = 20 panels
• Actual array: 20 × 400 = 8,000 W
• Daily production: 8000 × 4.8 × 0.78 / 1000 = 30.0 kWh
• Annual production: 30.0 × 365 = 10,931 kWh (104% offset)
• Year-1 savings at $0.165/kWh: $1,737

System size by household consumption

Using 4.8 peak sun hours, 78% derate, 400 W panels, 100% target offset:

*At $0.165/kWh blended residential rate.

What changes the result

Peak sun hours

Peak sun hours (PSH) is the single biggest variable. The same 877 kWh/month household needs:

• 5.4 kW in Phoenix at 6.5 PSH (14 panels)
• 7.7 kW in Denver at 4.8 PSH (20 panels)
• 10.5 kW in Seattle at 3.5 PSH (27 panels)

Always pull your exact PSH from NREL’s PVWatts tool — using a regional average can underbuild a system by 15%+.

Panel orientation and tilt

A south-facing array at latitude tilt is the reference case. Off-axis penalties:

• South at 0–10° off latitude: 0–2% loss
• East or west: 10–20% loss
• North (US): 25–35% loss
• Flat (0° tilt): 5–10% loss versus optimal

The solar panel orientation calculator and tilt angle calculator quantify this for your specific roof.

Net metering policy

The savings figure assumes 1:1 net metering — every kWh exported credits at the retail rate. This holds in roughly 38 US states as of 2026. California’s NEM 3.0, Hawaii’s grid-supply tariff, and Indiana’s 2022 reform all credit exports at avoided-cost rates (typically 30–50% of retail), which can drop year-1 savings by 20–35% on systems that overproduce. If your state uses non-net-metered tariffs, target 80–90% offset with the calculator and add battery storage to time-shift exports.

System derate

The default 78% comes from NREL PVWatts v6 and accounts for inverter losses (3%), wiring (2%), soiling (2%), mismatch (2%), light-induced degradation (1.5%), and temperature derating. Adjust:

• Bump to 82%: ground mount, oversized inverter, no shading, cool climate
• Drop to 72%: hot rooftop attic-mounted micro-inverters, central inverter without optimisers, partial shading

Why size from kWh, not from roof space

Roof-area sizing (panels × area) is a planning-stage approach. kWh sizing is the bill-paying approach. Two different households with identical roofs can have 3× different consumption — a retiree in a 1,500 sq ft home uses 400 kWh/month; a family with EV, heat pump, and pool pump in the same neighbourhood uses 1,800 kWh/month. Sizing from roof area would give them the same system; sizing from kWh gives them appropriately different systems.

The right workflow for a serious quote:

1. Sum 12 months of utility kWh. If you have an EV or heat pump arriving soon, add projected load (4 kWh/100 mi for an EV; 6,000 kWh/yr for a cold-climate heat pump replacing a gas furnace).
2. Decide your offset target based on net metering policy in your state.
3. Run this calculator with local PSH from PVWatts.
4. Sanity-check roof area: each 400 W panel needs ~22 sq ft installed, including racking gaps. A 7.7 kW system needs ~440 sq ft of unshaded south-facing roof.
5. Run the cost calculator for $ figures with the 30% federal ITC and your state incentive.

Common mistakes

• Using national-average kWh: 877 kWh/month is the EIA mean, but variance is huge. A Texas all-electric home with central AC can easily hit 1,400 kWh/month; a Pacific Northwest home with gas heat runs 600 kWh.
• Sizing for peak month: Sizing for July’s 1,400 kWh AC bill in a 700 kWh annual-average home overbuilds by 100%. Solar production is annualised — you bank summer surplus to cover winter.
• Ignoring future loads: An EV adds 3,000–5,000 kWh/yr. A cold-climate heat pump adds 4,000–8,000 kWh/yr. If you are buying either in the next 3 years, size for them now — adding panels later usually triggers a permit re-do.
• Forgetting to ratchet offset for non-1:1 net metering: If exports credit at 6¢/kWh and retail is 16¢, every overproduced kWh earns you 38% of what it costs you. Aim for 90–95% offset, not 105%.

Sources

• NREL PVWatts v6 — system loss conventions and irradiance data by ZIP
• EIA Annual Energy Outlook 2026 — US residential kWh averages and electricity rate forecasts
• EnergySage 2026 Solar Marketplace Report — actual sizing data for 30,000+ residential systems
• SEIA Annual Industry Report 2026 — sizing benchmarks, panel mix, net metering policy
• DSIRE — state-by-state net metering and incentive database
• NREL National Solar Radiation Database — peak sun hours for any US location