Surge Watt Calculator
Motors draw 3-7x their running watts at startup. Add your loads — find out what surge rating your inverter actually needs.
| Appliance | Running W | Type | Surge W | |
|---|---|---|---|---|
| Refrigerator | 750W | |||
| Sump pump | 2,000W | |||
| Lights | 220W |
How to Use This Calculator
Add your appliances
Click the quick-add presets to add common appliances, or type a custom appliance name, running watts, and type in the add row. Running watts is the power draw during normal operation — not startup. Check your appliance label or look up typical wattage online.
Set the appliance type
The type column is critical because it determines the surge multiplier. Motors and compressors (pumps, saws, AC units, refrigerators) draw 3-7x their running watts at startup. Resistive loads (heaters, toasters, electric kettles) draw only 1-1.1x — virtually no surge. Electronics and LEDs are in between at 1-2x. The default 5x multiplier for motors is conservative and safe for sizing.
Read the result
The calculator shows total surge requirement — this is the peak power draw when the highest-surge appliance starts while all others are running. Your inverter's surge rating (not continuous rating) must exceed this number. The recommended inverter size adds 10-20% headroom above the calculated surge.
The Formula
The key insight: only one appliance starts at a time (the "staggered start" assumption). So total surge equals the worst-case starting appliance's surge watts plus all other appliances' continuous running watts. Inverters are specified with both a continuous rating and a surge rating — the surge rating is typically 2-3x the continuous rating and lasts only a few seconds. Always check both numbers before buying.
Example
Off-grid cabin with well pump and refrigerator
A cabin's electrical system has a 750W well pump, 150W refrigerator compressor, and 200W of LED lights. What inverter do they need?
Worst-case surge
Even though total running watts is only 1,100W, the inverter needs a 5,500W+ surge rating. A 3,000W continuous / 6,000W surge inverter would work well, with 19% headroom. Choosing a 2,000W inverter based only on running watts would result in the inverter tripping every time the well pump starts.