TryBuildCalcGenerator Size / Wattage Calculator (Running + Starting Watts Sizing)
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 →
What Is a Generator Size / Wattage Calculator?
A generator size calculator recommends the correct generator wattage from the appliances you want to power, using the standard industry sizing method: total running watts across everything, plus the extra startup surge from your single largest motor-driven appliance, plus a safety margin — rounded up to a real generator size you can buy.
This is a different, simpler calculation from the Panel Size Calculator (which sizes your home's full electrical service per NEC 220.82) — use this tool when you're choosing a portable or whole-house standby generator based on a specific list of appliances you want to keep running during an outage.
Why getting generator size right matters:
- An undersized generator can stall or trip the moment a motor tries to start, even if it handles the running load fine
- An oversized generator costs more upfront and burns more fuel than necessary for the same job
- Starting watts (motor surge) is the single most commonly overlooked factor — and the biggest driver of generator size
- Whether you include central A/C or electric heat can swing the required size dramatically, from a small portable unit to a much larger whole-house-class generator
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.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Appliance Load Input Accuracy+-
- Actual nameplate running/starting watts used for major appliances, not guessed from a generic chart
- All loads you actually intend to run during an outage are included, including easy-to-forget ones
- Every motor-driven appliance has a starting watts value entered separately from its running watts
✓Starting Watts / Surge Calculation+-
- Confirmed only the single largest motor's incremental surge is added, not every motor's surge summed
✓Safety Margin & Generator Selection+-
- Final recommended generator size confirmed as a genuine standard size, rounded up
✓Installation & Code Compliance+-
- Transfer switch (manual or automatic) confirmed for any generator connected to household wiring
- GFCI protection confirmed for cords and receptacles used with a portable generator, per applicable code
- Generator placement confirmed well away from windows/doors/vents, per carbon monoxide safety guidance
- Whole-house/permanent standby installation's load calculation reviewed by a licensed electrician, not based on this estimate alone
- Permit and inspection requirements confirmed for a permanent generator installation
Full QC Checklist+−
Verification checklist for generator sizing — covering appliance load input accuracy, starting watts/surge calculation, safety margin and generator selection, and installation/code compliance. Use the Essential Checklist for critical checks before purchasing; expand to Full QC Checklist for complete verification.
✓Appliance Load Input Accuracy+-
- Actual nameplate running/starting watts used for major appliances, not guessed from a generic chart
- All loads you actually intend to run during an outage are included, including easy-to-forget ones
- Non-essential loads excluded, rather than adding everything in the house "just in case"
- Every motor-driven appliance has a starting watts value entered separately from its running watts
- Lighting and small electronics load realistically estimated, not omitted entirely
✓Starting Watts / Surge Calculation+-
- Confirmed only the single largest motor's incremental surge is added, not every motor's surge summed
- Risk of two large motors starting at the same moment considered for your actual usage pattern
- Resistive loads (heating elements, incandescent lighting, most electronics) correctly show no significant starting surge
- HVAC compressor (if included) checked as the likely single largest starting-watts driver
- Load-management/sequencing capability of your transfer switch considered — it can reduce the generator size actually needed
✓Safety Margin & Generator Selection+-
- Safety margin percentage confirmed appropriate for the intended duty cycle, not left at a default without thought
- Final recommended generator size confirmed as a genuine standard size, rounded up
- Fuel type and expected runtime/duty cycle considered separately from the wattage sizing itself
- Altitude and ambient temperature derating considered for extreme installation conditions
- Portable vs. whole-house standby generator choice confirmed to match your actual intended use case
✓Installation & Code Compliance+-
- Transfer switch (manual or automatic) confirmed for any generator connected to household wiring
- GFCI protection confirmed for cords and receptacles used with a portable generator, per applicable code
- Generator placement confirmed well away from windows/doors/vents, per carbon monoxide safety guidance
- Whole-house/permanent standby installation's load calculation reviewed by a licensed electrician, not based on this estimate alone
- Permit and inspection requirements confirmed for a permanent generator installation
- Extension cords used with a portable generator are heavy-gauge, outdoor-rated, and adequately sized for the load and distance
Standard Generator Sizes
Common portable and whole-house standby generator wattage ratings actually sold — a recommended size always rounds up to one of these, never to a custom in-between value.
| Standard Sizes (W) | Typical Use |
|---|---|
| 2,000 - 4,000 | Small portable — camping, a few essentials |
| 5,000 - 8,500 | Portable — refrigerator, sump pump, lighting, a window A/C |
| 10,000 - 12,000 | Larger portable/inverter — most essentials, a portable unit's practical ceiling |
| 14,000 - 28,000 | Whole-house standby — most or all of a home's load, including central A/C |
| 30,000 - 45,000 | Large whole-house standby — big homes or a 400A+ service |
| 48,000 | Largest tier — very large homes or light-commercial standby |
Full list of standard sizes: 2,000, 3,000, 3,500, 4,000, 5,000, 6,500, 7,500, 8,500, 10,000, 12,000, 14,000, 16,000, 18,000, 20,000, 22,000, 24,000, 26,000, 28,000, 30,000, 36,000, 38,000, 45,000, 48,000W.
When should you use this generator size calculator?
- Choosing a portable generator for essential circuits during power outages.
- Sizing a whole-house standby generator around your major appliances, including central A/C.
- Checking whether an existing or planned generator purchase actually covers your appliance list.
- Sizing a generator for an RV, campsite, or job site from its specific equipment.
- Comparing how much including or excluding a large appliance (like central A/C) changes the required size.
Quick Generator Sizing Tips
- Use actual nameplate values for major appliances instead of relying on generic presets.
- Don't forget motor-driven loads that are easy to overlook — sump pumps, well pumps, and furnace blowers.
- Leaving central A/C or electric heat off your list can substantially shrink the required generator size and cost.
- A transfer switch is required for any generator connected to your home's wiring — never connect directly to a panel or outlet.
- Never run a generator indoors, in a garage, or near windows/doors/vents — carbon monoxide is a serious hazard.
Common Mistakes
- Sizing only for total running watts and ignoring starting watts entirely — a generator that handles the running load fine can still stall when a motor tries to start.
- Summing every motor's full starting watts instead of just the single largest one's incremental surge — this badly oversizes the generator.
- Connecting a generator directly to a panel or outlet without a transfer switch — a serious safety hazard, not just a code technicality.
- Running a generator indoors or too close to windows/doors — carbon monoxide poisoning is a real and fast-acting risk.
- Forgetting to account for fuel type and runtime separately from wattage — a correctly-sized generator with an inadequate fuel plan still leaves you without power.
Limitations
- Uses the standard appliance-wattage sizing method, not the formal NEC 220.87 historical-demand-data calculation used for some permanent standby installations.
- Assumes only one large motor starts at a time — an installation where two large motors could genuinely start simultaneously may need a larger margin than this calculator's default.
- Does not model fuel type, tank size, or runtime/duty cycle — plan these separately for your actual outage scenario.
- Does not model altitude or ambient temperature derating — check the manufacturer's specific chart for extreme installation conditions.
- This is a planning reference estimate, not a substitute for a licensed electrician's review, especially for a permanent whole-house standby installation.
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.