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Solar + Battery System Sizing Calculator: Array, Storage & Inverter

Estimate planning sizes for solar array wattage, battery-bank capacity, and inverter continuous/surge ratings from daily consumption, peak sun hours, autonomy, and explicit derating assumptions.

Core relationship Solar Array (W) = Daily Wh ÷ [Peak Sun Hours × System Efficiency × Derate Factor] Planning estimate. Verify equipment limits and site-specific requirements before final design.
Enter your assumptions
Average daily consumption (e.g. 4.0 kWh = 4,000 Wh)
Highest expected concurrent appliance draw
Winter baseline for your geography (typically 3.0 - 5.5 PSH)
Days of continuous power without any solar generation
Adjusts planning allowance for motor inrush startup surges
Higher voltage reduces current, wire size, and heat loss
80% planning default for LiFePO4; verify the battery manufacturer's usable-capacity and cycle-life guidance
Recommended Solar Array 1600 Watts ~4.0 x 400W modern monocrystalline panels
Recommended Battery Bank 208 Ah at 48V (10000 Wh nominal / 8000 Wh usable)
Inverter Continuous Rating 2500 W
Inverter Surge Rating 5000 W
Daily Expected Generation 4096 Wh/day
Autonomy Storage 8000 Usable Wh

Note — Inverter surge rating is an estimated planning allowance. Always verify equipment nameplate startup current (LRA) and inverter surge-duration ratings.

Default generation model: 80% electrical/system planning factor × 80% PV/environment factor = 64% combined. These are conservative planning assumptions, not measured efficiencies for a specific installation.

How to use the result

Start with the estimate, then compare it with actual equipment specifications, environmental conditions and applicable electrical requirements.

Off-Grid Solar Dimensioning Methodology

Sizing an autonomous off-grid electrical system requires balancing three independent sub-systems: Generation (Photovoltaics), Energy Storage (Battery Bank), and Power Conversion (Inverter / Charger).

1. Battery Bank Sizing (Storage & Autonomy)

Storage capacity must buffer your daily consumption across consecutive days of poor weather without sunshine: 

Usable Battery Wh = Daily Consumption (Wh) × Days of Autonomy
Nominal Battery Wh = Usable Wh ÷ Allowed Depth of Discharge (e.g. 0.80)
Battery Bank Capacity (Ah) = Nominal Wh ÷ System DC Voltage (e.g. 48V)

2. Photovoltaic Array Dimensioning

A solar array's nameplate rating is measured under Standard Test Conditions (STC: 1,000 W/m² irradiance and 25°C cell temperature). Real outdoor output varies with irradiance, module temperature, orientation, shading, soiling, wiring, mismatch, controller behavior, and battery charging conditions. The default model deliberately uses two conservative planning factors that multiply to 64%:

  • Electrical / system planning factor (80%): A conservative allowance for charge-controller, battery-charging, wiring, conversion, and other unmodeled system losses. It is not the mathematical product of the nominal component examples; for example, 98% × 97% × 98% is about 93%, not 80%. The extra margin is intentional.
  • PV / environmental planning factor (80%): A generic allowance for real operating temperature, soiling, orientation, mismatch, and other site effects. It is not a substitute for a location-specific irradiance and shading model or a module-specific temperature coefficient.

With the defaults, the calculator uses 0.80 × 0.80 = 0.64. These factors are planning assumptions rather than measured efficiencies for a particular installation.

Worked Engineering Example

Scenario: Sizing a four-season off-grid cabin consuming 4.0 kWh/day (4,000 Wh), with 2 days of autonomy and 4.0 peak sun hours in winter on a 48V LiFePO4 bus:

  1. Usable Storage: 4,000 Wh × 2 = 8,000 Usable Wh
  2. Nominal Bank: 8,000 Wh ÷ 0.80 DoD = 10,000 Wh (10 kWh)
  3. Bank Amp-Hours: 10,000 Wh ÷ 48V = 208 Ah @ 48V (approx. two 48V 100Ah server-rack batteries)
  4. Required Daily Generation: 4,000 Wh ÷ 0.64 Combined Eff = 6,250 Wh
  5. Solar Array Size: 6,250 Wh ÷ 4.0 PSH = 1,562.5W → 1,600 Watts (four 400W panels)

What this calculator does not model

  • Hour-by-hour or month-by-month irradiance, terrain/horizon shading, snow cover, or detailed array orientation.
  • Module-specific temperature coefficients, MPPT clipping/operating-window limits, charge-controller current limits, or equipment-specific efficiency curves.
  • Battery aging, temperature-dependent capacity, BMS limits, charge/discharge power limits, or generator contribution.
  • Site-specific electrical-code compliance, conductor/OCPD sizing, grounding, disconnects, or equipment listing requirements.

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