Home backup generator sizing calculator: Watts, LRA, and fuel runtime explained

The starting wattage problem most homeowners miss

When your air conditioner compressor starts, it draws a surge of current called Locked Rotor Amps (LRA) for 0.3–0.5 seconds while the motor accelerates from rest to operating speed. This starting surge can be 4–7× the motor's continuous running current, and it must be supplied by your generator without voltage collapse.

A generator sized to 6,000 watts running capacity might appear adequate for a 2-ton AC unit that draws 1,400 running watts — until the AC compressor starts and demands 8,400 watts for half a second, overloading and stalling the generator.

For complete home resilience planning, see our high-efficiency home infrastructure and resilience guide.

---

Calculating starting wattage for motor loads

The starting wattage for any motor is estimated as:

$$W_{\text{start}} = \frac{\text{LRA} \times V_{\text{nominal}}}{1{,}000}$$

For a single-phase 240V load, running watts and starting watts:

$$W_{\text{running}} = V \times I_{\text{running}} \times \text{PF}$$ $$W_{\text{starting}} = V \times I_{\text{LRA}} = W_{\text{running}} \times \frac{\text{LRA}}{\text{FLA}}$$

LRA/FLA ratios for common residential loads:

Inverter-driven compressors (most modern mini-splits and some whole-house units) start at near-running-speed LRA because the variable frequency drive controls motor ramp-up, dramatically reducing starting surge.

---

Total generator sizing formula

$$W_{\text{generator}} = W_{\text{start (largest motor)}} + W_{\text{running (all other concurrent loads)}}$$

Example sizing calculation — 2,500 sq ft home, summer storm outage:

Peak demand (AC starting, all others running): $W_{\text{peak}} = 12,000 + 180 + 750 + 400 = 13,330W$

Recommended generator size: 14–16 kW (20% headroom above peak demand)

---

Fuel runtime calculations

Propane generator fuel consumption: $$\text{Runtime (hrs)} = \frac{\text{Tank Capacity (gal)} \times 91,600 \text{ BTU/gal}}{\text{Generator Output (kW)} \times 9,580 \text{ BTU/kWh} \times \text{Load Factor}}$$

A 16kW generator at 50% load consumes approximately 1.5 gal/hr of propane. A 500-gallon tank provides ~250 hours (10+ days) of runtime.

Use the Backup Generator Sizing Calculator to input your specific appliance list, motor types, and fuel source for a complete sizing recommendation.