Off-Grid Solar System Calculator

How to use this calculator

The off-grid solar calculator above takes your daily energy use, peak sun hours, and a few system parameters and returns the three numbers that matter for an off-grid quote: PV array size in kW DC, battery bank capacity in kWh nameplate, and inverter continuous kW rating. Adjust the inputs and the outputs recalculate instantly.

1. Daily energy use (kWh) — your average daily AC load. Pull it from a recent grid bill (monthly kWh ÷ 30) or sum nameplate watts × hours-on for each appliance you intend to run off-grid. The U.S. EIA residential average is 28.8 kWh/day; rural off-grid cabins typically run 4-12 kWh/day because high-draw items like central HVAC, electric ovens, and electric water heaters get swapped for propane.
2. Peak sun hours (h/day) — the daily annual-average kWh/m² of solar radiation hitting a tilted panel at your latitude. NREL’s National Solar Radiation Database and PVWatts give 4.0-5.5 PSH for most of the lower 48: 5.0-6.0 across the Southwest (Arizona, Nevada, New Mexico, west Texas), 4.5-5.0 across the Sun Belt and Great Plains, 4.0-4.5 across the Midwest and Mid-Atlantic, and 3.0-4.0 across the Pacific Northwest, Northeast, and Great Lakes.
3. Days of autonomy — 2 days for general residential, 3-5 for critical-load and remote sites where a generator is the only backup.
4. Battery chemistry — lithium iron phosphate (LiFePO₄) is now standard for new U.S. off-grid builds: 90% usable depth-of-discharge, 6,000-8,000 cycles, 95% round-trip efficiency. Sealed AGM lead-acid is a third the upfront cost but only 50% DoD, 500-1,200 cycles, and 85% efficiency — usually a worse 10-year cost-per-kWh-cycled.
5. Peak instantaneous load (W) — the largest AC load you’ll run at one moment. Common big draws: well pump 1,500-3,000 W, microwave 1,200 W, central AC 3,000-5,000 W, table saw 1,800 W. Add a 1.25× surge factor (built into the calculator) to size the inverter.

How the math works

The calculator chains three first-principles formulas, each derivable from energy balance and standard PV system efficiency factors documented in NREL’s Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices (Sandia Report SAND87-7023):

PV array (kW DC):

kW = daily_kWh / (peak_sun_hours × derate)

The system derate (0.77 default) bundles inverter losses, charge-controller losses, wiring losses, soiling, and module mismatch — Sandia’s recommended off-grid PR. Using the en-us defaults of 8 kWh/day at 4.5 PSH: kW = 8 / (4.5 × 0.77) = 2.31 kW DC.

Battery bank (kWh nameplate):

usable_kWh    = daily_kWh × autonomy_days
nameplate_kWh = usable_kWh / (DoD × battery_round_trip_eff)

With 8 kWh/day × 2 days = 16 kWh usable; nameplate = 16 / (0.90 × 0.95) = 18.7 kWh of LiFePO₄. The same 16 kWh of usable energy from AGM lead-acid would need 16 / (0.50 × 0.85) = 37.6 kWh nameplate — roughly twice the bank for the same job.

Inverter (kW continuous):

kW = peak_load_W × 1.25 / 1000

A 3,000 W peak load needs a 3.75 kW continuous inverter. Stack two 3 kW Schneider XW Pro or Outback Radian units in parallel if you exceed 6 kW.

What an off-grid system actually costs in the U.S. (Q1 2026)

Pulling installed-cost ranges from EnergySage Q1 2026, Wholesale Solar (Mosier, OR), Backwoods Solar (Sandpoint, ID), and Mr. Solar (Atlanta, GA):

Generator backup adds $4,000-9,000 (Generac, Kohler 8-14 kW propane) and is essentially mandatory for full-time off-grid in any climate north of about 36° N latitude.

Where most U.S. off-grid systems get under-sized

Three common mistakes from the Backwoods Solar and Wholesale Solar customer-support post-mortems:

1. Loads measured at nameplate, not actual. A 1.5 kW well pump nameplate runs at maybe 800-1,000 W average; the inverter still needs to start it, but the daily kWh isn’t 1,500 × runtime. Use a Kill-A-Watt for plug loads and your utility bill (monthly kWh ÷ 30) for the whole-house number.
2. Single-day autonomy on a stick-built home. One-day autonomy works for an RV. A house with refrigeration loads needs 2 days minimum and 3 in the Pacific Northwest, New England, or upper Midwest.
3. Lead-acid bank sized to lithium DoD. A 20 kWh “lithium-equivalent” lead-acid bank is actually 40 kWh nameplate. Many builders forget the factor of two and watch their flooded bank fail within 2-3 years from chronic deep-cycling.

Pair this with the battery bank calculator, charge-time calculator, and wire-size calculator

This off-grid sizing calculator gives the headline three numbers (PV kW, bank kWh, inverter kW). The battery bank calculator drills into amp-hour capacity at your chosen system voltage. The charge-time calculator sanity-checks how many sun-hours you need to recharge the bank from a deep discharge. The wire-size calculator picks the AWG between PV array and charge controller, charge controller and bank, and bank and inverter — undersized DC wiring is the most common cause of off-grid voltage-drop complaints.

Sources

• NREL — Stand-Alone Photovoltaic Systems Handbook (Sandia SAND87-7023) — PSH design methodology and PR derate factors
• NREL National Solar Radiation Database (NSRDB) and PVWatts — peak-sun-hour data by ZIP
• IRS Form 5695 instructions — Residential Clean Energy Credit (30% ITC) including stand-alone storage ≥3 kWh
• DSIRE database (NC State) — state and utility off-grid incentives
• NEC 690 (Solar PV) and NEC 706 (Energy Storage) — code requirements for off-grid system design
• EnergySage Solar Marketplace Report — installed-cost data including off-grid quotes