TryBuildCalcGenerator Size Calculator (Running Watts + Starting Watts Surge Method)
Calculate generator size instantly.
🕒 Last updated: August 27, 2026
Inputs
Appliances to Power
Pick a preset for a typical starting point, or choose Custom and enter your appliance's actual nameplate values.
Appliance 1
ℹ️Leave equal to running watts for non-motor loads (heaters, electronics, lighting).
Appliance 2
ℹ️Leave equal to running watts for non-motor loads (heaters, electronics, lighting).
Appliance 3
ℹ️Leave equal to running watts for non-motor loads (heaters, electronics, lighting).
ℹ️Applied on top of your peak load (total running watts + largest single startup surge). 20% is a common default; consider more for continuous heavy-duty use, high altitude, or hot climates.
Cost
Recommended Generator Size: 4,000 W (4 kW)
From 3,300 W peak load, +20% safety margin
Load Breakdown
| Appliance | Running | Starting |
|---|---|---|
| Refrigerator (largest surge) | 700 W | 2,200 W |
| Sump Pump | 800 W | 1,200 W |
| Lighting | 300 W | 300 W |
| Total Running Watts | 1,800 W |
Sizing Calculation
Total running watts: 1,800 W
Largest single starting surge: Refrigerator — 2,200 W (adds 1,500 W above its own running watts)
Peak load: 3,300 W
With 20% safety margin: 3,960 W
Recommended standard generator size: 4,000 W (headroom: 40 W, 1.0%)
Informational Amperage
At 120V: 15.0 A (based on total running watts)
At 240V: 7.5 A (based on total running watts)
Assumptions Used
Only the single largest motor's starting-watts surge is added to total running watts, assuming loads start one at a time (not simultaneously). This is a reference estimate — confirm against actual appliance nameplates and consult a licensed electrician for permanent/whole-house installations.
Sizing your dwelling's full electrical service instead? Panel Size Calculator →
Sizing a breaker for a specific circuit? Circuit Breaker Size Calculator →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Generator Size / Wattage Calculator.
General generator size estimate
This page gives the full combined sizing result — total running watts, peak surge load, and recommended standard generator size — from one set of appliance inputs.
It's the same calculator behind every other generator-sizing variant on this site, just without a specific scenario pre-selected — useful for a general check before finalizing your actual appliance list.
- Applies the standard running-watts-plus-largest-surge sizing method automatically.
- Add up to 20 appliances, each with its own running and starting watts.
- Rounds up to a real standard generator size, from 2,000W portable units to 48,000W large whole-house standby.
Generator Size Formula: How Is Wattage Calculated?
The standard method sums everyone's running watts, then adds only the single largest appliance's incremental startup surge — since equipment doesn't all start at the same instant.
Step 1 — Total Running Watts
Total Running Watts = Sum of every appliance's running watts
This covers the continuous, steady-state power draw of everything you want running at once.
Step 2 — Largest Starting Surge
Incremental Surge = Largest appliance's Starting Watts − its own Running Watts
Only the single largest starting-watts appliance's incremental surge is added — its running watts is already counted in Step 1, so only the extra amount above that counts toward the peak.
Step 3 — Peak Load and Safety Margin
Peak Load = Total Running Watts + Incremental Surge
Target Capacity = Peak Load × (1 + Safety Margin %)
A safety margin (20% by default) covers real-world variance in appliance draw, generator derating, and headroom for anything unaccounted for.
Step 4 — Round to a Standard Generator Size
Recommended Generator Size = smallest standard tier ≥ Target Capacity
Generators are sold in standard wattage tiers — the calculated target is always rounded up, never down.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Appliances | 3 appliances, 1,800 W running | Sets total running watts |
| Largest Surge | Refrigerator — 2,200 W | Sets the incremental surge added to peak load |
| Safety Margin | 20% | Applied on top of peak load |
Steps 1-3 — Running, Surge, and Peak Load
| Calculation | Result |
|---|---|
| Total running watts | 1,800 W |
| Incremental surge (Refrigerator) | +1,500 W |
| Peak load | 3,300 W |
| With 20% safety margin | 3,960 W |
Therefore, for 3,300 W of peak load with a 20% safety margin, you need a 4,000 W (4 kW) generator.
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.