Solar Generator Sizing: Battery (Wh), Inverter (W), Surge & Real-World Runtime

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How to Size a Solar Generator Correctly

Sizing a solar generator (battery power station) is different from sizing a gas generator. You must match:

  • Battery capacity (Wh) → how long it will run
  • Inverter power (W) → what it can run at once
  • Surge rating (W) → whether it can start motors and compressors

Ignoring any one of these leads to disappointment.

Solar Cluster (Navigation)

1) Battery Capacity (Wh) = Runtime

Battery capacity is measured in watt-hours (Wh).

Simple runtime formula:

Runtime (hours) ≈ Battery Wh × 0.85 ÷ Load (watts)

The 0.85 factor accounts for inverter losses and system inefficiencies.

Example

  • Battery: 2,000 Wh
  • Fridge average draw: 150 W (cycling)

2,000 × 0.85 ÷ 150 ≈ 11 hours (continuous equivalent)

Because fridges cycle on/off, real-world runtime may be longer — but plan conservatively.

2) Inverter Power (W) = What You Can Run

The inverter rating (e.g., 1,800 W, 2,400 W) determines how many devices you can power simultaneously.

If your combined loads exceed the inverter rating, the unit will shut down.

Before calculating inverter size, review: Generator Power Needs →

Example

  • Microwave: 1,200 W
  • Fridge (running): 150 W
  • Lights + router: 200 W

Total ≈ 1,550 W → requires at least a 1,800–2,000 W inverter.

3) Surge (Starting Watts) = Motor Compatibility

Motors and compressors (fridges, freezers, pumps) may require 2–4× their running watts for startup.

If the surge exceeds the inverter’s surge rating, the appliance will not start.

Typical Motor Examples

Appliance Run W Start W (approx.)
Refrigerator 150–700 1,200–2,800
Chest freezer 150–500 1,200–2,500
Sump pump (½ hp) 500 1,200–1,500
Well pump (1 hp) 1,000 2,500+

Always check manufacturer specs when possible.

4) Typical Solar Generator Setups

Light Emergency Kit (500–1,000 Wh)

  • Phones, laptops, router
  • LED lighting

Not suitable for fridge long-term.

Apartment Backup (1,500–2,000 Wh)

  • Fridge (intermittent)
  • Lights + router
  • Microwave (short use)

Good balance of portability and runtime.

Extended Backup (3,000+ Wh, expandable)

  • Fridge + freezer rotation
  • Electronics
  • Selective appliance use

Often used in off-grid setups. Still not ideal for electric heating or central AC.

5) Solar Recharge Planning

Battery size is only half the equation. For multi-day outages, solar input matters.

Example:

  • 2,000 Wh battery
  • 400 W solar panels
  • 5 good sun hours

400 × 5 = 2,000 Wh (ideal conditions)

Real-world harvest is typically lower due to angle, clouds, and conversion losses.

If your daily usage exceeds your daily recharge, the battery will slowly drain.

See panel selection and compatibility:

Solar Panels for Power Stations →

Common Solar Generator Mistakes

  • Buying based only on Wh and ignoring inverter watts
  • Ignoring surge requirements for motors
  • Expecting to run electric heaters or central AC
  • Not planning for recharge speed

Solar vs Gas Generator Sizing

Gas generators are sized primarily by watts.

Solar generators must be sized by both watts and battery runtime.

Compare here:

Solar vs Gas Generators →

Next Step

Before choosing a model, calculate your real loads:

Calculate Generator Power Needs →

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