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

Compute recommended solar array wattage, LiFePO4 battery bank amp-hours, and inverter continuous/surge ratings based on daily consumption, peak sun hours, and storm autonomy requirements.

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% recommended for LiFePO4 to maximize longevity
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.

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 rated under Standard Test Conditions (STC: 1,000 W/m², 25°C cell temp) never operates at 100% rated nameplate capacity in real outdoor conditions. This calculator applies a conservative 64% combined derating factor:

  • Coulombic / MPPT Efficiency (80%): Covers MPPT charge controller tracking losses (98%), battery internal chemical charging losses (97%), and DC cable resistive losses (98%).
  • Environmental Derate Factor (80%): Covers elevated panel operating temperatures (-0.35%/°C above 25°C), atmospheric haze, dust accumulation, and non-perpendicular sun angles.

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)

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