Central AC & Heat Pump Generator Sizing

HVAC Generator Load Calculator

Determine starting surge and running wattage needed for Central Air Conditioners, Heat Pumps, and Electric Furnaces. Size your backup generator accurately with soft start options.

System Specifications

US NEC 240V

System Type

Unit Cooling Size (Tons)

Efficiency (SEER Rating) 13 SEER

10 (Older) 13-14 (Standard) 20+ (High Eff)

Aux / Emergency Heat Strip?

Backup resistive heating element

Soft Start Kit Installed?

Cuts compressor startup surge by ~70%

Result Summary Calculated

Required Starting Surge

11,000 Watts

Peak Inrush Power Required at Startup

Running Load

3,269 W

Amps (@ 240V)

13.6 A

Continuous Running Peak Surge

Running Draw 10% Safety Buffer Added

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HVAC Sizing Formulas & Guide

How We Calculate Loads

Running Watts Formula

(Capacity_BTU / SEER) + Fan_Watts

Example: A 3-Ton (36,000 BTU) 13 SEER unit:
(36,000 / 13) + 500 = ~3,269 Watts

Starting Surge Formula

Running_Watts × Surge_Factor

Standard AC: ~3.5x to 5x running watts
With Soft Start: ~1.5x running watts
Electric Furnace: 1.0x (No Surge) + Fan Surge

Typical AC Wattage Reference (13 SEER)

Size	Running Watts	Standard Start	Soft Start
1.5 Ton	~1,900 W	~5,700 W	~2,800 W
2.0 Ton	~2,400 W	~7,200 W	~3,600 W
3.0 Ton	~3,300 W	~10,000 W	~5,000 W
4.0 Ton	~4,200 W	~14,000 W	~6,300 W
5.0 Ton	~5,100 W	~17,000 W	~7,600 W

*Estimates vary by age, brand, and temperature.