Off-Grid Solar System Calculator

How to use this calculator

The calculator returns screening estimates for PV array kW DC, battery-bank kWh nameplate, and inverter continuous kW. It does not perform an hourly reliability simulation, equipment compatibility check, conductor/protection design, structural review, or code-compliance assessment.

1. Daily energy use (kWh/day) — the AC load energy you intend to serve. Use measured interval data where possible; monthly grid purchases ÷ billing days are only an average and may omit on-site generation. EIA reported 865 kWh/month per U.S. residential customer in 2024, about 28.8 kWh/day, but that grid-purchase average is not an off-grid design load (EIA, 2024 residential electricity).
2. Peak sun hours (h/day) — a simplified energy-equivalent solar input. Obtain a location-, tilt-, azimuth-, and month-appropriate value. PVWatts V8 uses 2020 typical-meteorological-year NSRDB weather where available and accepts explicit array and loss inputs (NREL PVWatts V8 documentation).
3. Days of autonomy — the number of full daily-load equivalents stored. This is not a forecast that every load will run normally through that many calendar days.
4. Depth of discharge and battery round-trip efficiency — chemistry selection loads presets of 90%/95% for LiFePO₄, 50%/85% for AGM, and 50%/80% for flooded lead-acid. These are editable modeling assumptions, not universal product specifications or warranties.
5. PV system derate — an aggregate multiplier, default 0.77, applied once to panel production. NREL’s older PVWatts V5 manual associated 0.77 with an earlier collection of default derates, while the current model separates inverter behavior and system losses; 0.77 is therefore a conservative calculator assumption, not an NREL off-grid requirement (NREL PVWatts V5 manual, pp. 10–11).
6. Inverter efficiency — the interface exposes this input, but the current calculation does not apply it to any output. Changing it alone will not change the result; account for conversion loss in the measured load/derate assumptions or a separate engineering model.
7. Peak instantaneous load (W) — the simultaneous running load entered by the user. The calculator multiplies it by 1.25 for planning headroom, not motor-start verification.

How the math works

The calculator chains three transparent arithmetic formulas. They are useful for a first pass but are simpler than NREL PVWatts or an hourly loss-of-load simulation.

PV array (kW DC):

kW = daily_kWh / (peak_sun_hours × derate)

daily_kWh is the load-energy input, peak_sun_hours is the selected solar input, and derate is an aggregate dimensionless multiplier. Using the en-US defaults: 8 ÷ (4.5 × 0.77) = 2.3088 kW DC, displayed as 2.3 kW DC. The formula does not model hourly clipping, shading, snow, temperature, battery state of charge, or generator dispatch.

Battery bank (kWh nameplate):

load_energy_target_kWh = daily_kWh × autonomy_days
reported_nameplate_kWh = load_energy_target_kWh / (DoD × battery_round_trip_eff)

With 8 kWh/day × 2 days, the load-energy target is 16 kWh; the reported nameplate value is 16 ÷ (0.90 × 0.95) = 18.7 kWh. The interface labels 16 kWh as usable battery energy, but it is more precisely the AC load-energy target before the calculator’s DoD/RTE derating. Treating full round-trip efficiency as a capacity derate is a conservative simplification; the model does not separately trace charging and discharging losses, inverter loss, aging, temperature, standby load, or manufacturer reserve.

Inverter (kW continuous):

kW = peak_load_W × 1.25 / 1000

A 3,000 W running-load input gives 3,000 × 1.25 ÷ 1,000 = 3.75 kW, displayed as 3.8 kW. This 25% margin does not establish that the inverter can start a motor; compare each candidate’s duration-specific surge rating with measured or manufacturer-specified starting demand.

Reproducible verification with the en-US defaults

Enter 8 kWh/day, 4.5 peak sun hours, two days of autonomy, LiFePO₄, 90% DoD, 95% battery round-trip efficiency, a 0.77 derate, and a 3,000 W peak load:

• PV: 8 ÷ (4.5 × 0.77) = 2.3088 → 2.3 kW DC after interface rounding.
• Load-energy target: 8 × 2 = 16.0 kWh.
• Reported bank: 16 ÷ (0.90 × 0.95) = 18.7135 → 18.7 kWh nameplate.
• Inverter: 3,000 × 1.25 ÷ 1,000 = 3.75 → 3.8 kW continuous after display rounding.

These values verify the interface arithmetic only. They do not demonstrate that the system meets a reliability target or equipment/code requirements.

What the calculator does not price

Installed cost depends on equipment, labor, permitting, site access, structural work, wiring distances, generator integration, taxes, and incentives. This calculator has no cost inputs and should not be used to quote a project. DOE likewise identifies consumption, system size, solar resource, and ownership/financing structure as material economic variables (DOE Homeowner’s Guide to Solar). Check current local programs independently; for homeowner Section 25D, the IRS says the credit is not available for property installed after December 31, 2025 (IRS Residential Clean Energy Credit).

Limitations and common U.S. under-sizing errors

Three common modeling mistakes:

1. Confusing power with energy. Inverter kW must cover simultaneous and starting power; daily kWh must come from measured power over time.
2. Using annual-average sun for the reliability case. Final design should test seasonal/hourly resource, load variability, battery state of charge, and backup dispatch. PVWatts itself is a production estimator, not a stand-alone reliability guarantee (NREL PVWatts).
3. Treating chemistry presets as specifications. Replace the default DoD, efficiency, temperature, charge/discharge power, and reserve assumptions with the selected battery’s datasheet values.

Related calculator checks

Use how many solar panels do I need to translate array kW into a panel count, the solar inverter size calculator for a separate load-based inverter check, and the RV solar calculator for nominal-voltage battery Ah estimates in mobile systems.

Sources

• NREL PVWatts V8 API documentation — weather dataset and production-model inputs
• NREL PVWatts V5 manual — historical 0.77 derate context and system-loss definitions
• U.S. EIA 2024 residential electricity analysis — U.S. grid-purchase average
• IRS energy-credit termination FAQs — Section 25D termination
• DOE Homeowner’s Guide to Solar — system and economic variables
• NFPA 70, National Electrical Code — source standard; verify the locally adopted edition and amendments