Solar Generator Sizing Calculator

What the result means

A packaged solar generator combines a battery, inverter, charge controller and outlets. This tool screens three ratings:

• Battery capacity (Wh): nominal energy required for the entered AC load and autonomy
• Inverter continuous (W): this site’s 1.25-times-peak screening output
• Solar input (W): array nameplate power for a one-average-sun-day refill of the recommended battery

A product must pass all three checks independently. It also must accept the array’s voltage and current, supply the required outlet power, and support the surge duration of the actual load.

Inputs

1. Average load (W): average simultaneous AC power while the scenario is operating.
2. Runtime per day (h): hours per day at that average power.
3. Days of autonomy: stored-energy days before recharge.
4. Peak surge load (W): highest short-duration input you want the screen to cover.
5. Depth of discharge: usable share of nominal battery capacity.
6. Inverter efficiency: battery-to-AC efficiency used in the energy calculation.
7. Peak sun hours: daily solar irradiation expressed as equivalent full-sun hours.
8. Charge efficiency: panel-to-battery efficiency used for the refill calculation.

For solar resource, use location and array-orientation data from NREL PVWatts Version 8. A yearly average can overstate winter recharge and says nothing about a particular cloudy outage.

Exact formulas

daily_Wh = average_load_W × runtime_hours battery_Wh = daily_Wh × autonomy_days / (depth_of_discharge × inverter_efficiency) inverter_W = peak_surge_W × 1.25 solar_input_W = battery_Wh / (peak_sun_hours × charge_efficiency)

Percentages are converted to decimals before use. The solar formula uses the full autonomy-sized battery, so increasing autonomy also increases the array required to refill it in one modeled solar day. If your design may recharge over several days, this result is deliberately conservative; if essential loads must run while recharging, a full energy-flow model is needed.

Worked verification using the U.S. defaults

The displayed defaults are 60 W, 6 h/day, 1 autonomy day, 1,200 W peak, 90% depth of discharge, 90% inverter efficiency, 4.5 peak sun hours and 85% charge efficiency:

• Daily energy = 60 × 6 = 360 Wh/day
• Battery = 360 / (0.90 × 0.90) = 444 Wh
• Inverter screen = 1,200 × 1.25 = 1,500 W
• Solar input = 444 / (4.5 × 0.85) = 116 W

These values verify the equations. They do not identify a suitable product: a nominal 500 Wh unit could still fail on usable capacity, outlet rating, surge duration, PV input limits or low-temperature operation.

Build a better load profile

For cycling equipment, daily energy is more reliable than multiplying maximum nameplate watts by 24 hours. Use a plug-in energy meter where practical, record at least a representative day, and keep startup power separate from daily Wh. For medical equipment, use the manufacturer’s specified power supply and backup guidance; do not infer a critical runtime from generic internet wattage tables.

Then run at least three cases:

• Expected daily load and typical solar resource
• Essential-load-only outage case
• Low-sun case using the poorest relevant month, not the annual average

The off-grid solar system calculator is better suited to fixed systems with broader energy balance, the RV solar calculator covers vehicle use, and the hybrid solar calculator provides another grid-plus-storage screen.

Limitations: what this model does not check

• Battery reserve required by a battery-management system
• Capacity loss with temperature, age, high discharge rate or standby use
• AC versus DC load-path efficiency
• Surge duration, waveform quality or motor-start compatibility
• Simultaneous charging and discharging
• Panel temperature, tilt, shade, weather variability or cable loss
• PV open-circuit voltage, operating voltage, current, polarity or connectors
• Product warranty, recall status, installation rules or tax eligibility

For fixed solar-plus-storage design context, see DOE’s Solar-Plus-Storage 101 overview.

Gas-generator safety comparison

A battery power station has different capabilities from a fuel generator; this calculator does not decide which backup type is appropriate. If a fuel generator is used, the U.S. Consumer Product Safety Commission says it must be operated outdoors only, at least 20 feet from a house, with exhaust directed away from buildings and openings. See CPSC portable-generator safety guidance.

Tax date

IRS guidance states Section 25D is not available for property installed after December 31, 2025. Portable-equipment facts can be product- and use-specific, but the historical storage rules do not create a post-2025 homeowner credit. See the IRS Residential Clean Energy Credit.

Sources

• NREL PVWatts Version 8 API documentation — location, weather and array-orientation inputs for a detailed solar-resource estimate
• DOE: Solar-Plus-Storage 101 — fixed solar-plus-storage context outside this portable model
• CPSC portable-generator safety guidance — carbon-monoxide safety for the separate fuel-generator comparison
• IRS Residential Clean Energy Credit — current Section 25D installation-date guidance