Backup power · running + surge watts
Generator Size
Calculator
Pick the loads you want to back up and get the right generator size — the one that covers the starting surge, not just the running watts. Portable & whole-house
For homeowners prepping for outages — and pros sizing a standby install.
A fridge, furnace, sump and lights peak near 4,450 W — a 5,500 W portable. Add central A/C and the startup surge alone hits 11,000 W, pushing you to an 18 kW standby. The surge sets the size.
Running + surge
the startup spike, done right
Portable & whole-house
essentials or the full panel
Transfer-switch safe
no dangerous backfeed
Artie has taught physics and electrical theory for over a decade and consulted on real-world electrical design — so every tool here is grounded in both the theory and the field.
University physics lecturer·Consulted on commercial electrical systems·Last reviewed Jul 2026
Generator Size Calculator
Size a backup generator — running load plus the biggest starting surge.
Add loads to size the generator.
Recommended generator
How it's sized
Safety: Never connect a generator to your panel without a transfer switch or interlock — back-feeding the utility line can electrocute lineworkers. Run generators below 80% of rated capacity continuously. Output derates ~3–5% per 1,000 ft of altitude and in high heat.
Compare sizes — tap to check (loaded % at surge)
How to use it
How to size a generator
Pick the loads you want to run — or enter your whole-house calculated load — and read the running watts, the starting surge, and the peak. Add a safety buffer and you have the minimum generator size, portable or standby.
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01
Pick your loads
Choose the appliances to back up — or enter your calculated load for whole-house.
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02
See running + surge
The tool totals the running watts and finds the biggest starting surge.
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03
Get the peak
Running plus the largest surge, with a 20% buffer — the size you actually need.
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04
Pick the unit
Match the size — portable or standby — and wire it safely with a transfer switch.
The core idea
Running watts vs starting watts
Motors need a big spike to start — and that spike sets the size.
Anything with a motor — a fridge, well pump, A/C, furnace fan — draws 2–3× its running watts for a split second at startup. A generator that can't deliver that starting surge stalls or trips, no matter how easily it handles the steady load.
A well pump, starting
So how do you size for surge? You don't add every surge:
- Only one motor starts at a time — they rarely kick on the same instant
- Add the running total, plus the single largest surge
- Summing every surge over-sizes the generator badly
Total running watts + the largest single surge = the peak the generator must cover.
The formula
Size for the surge — the right way
Add the running total plus the single biggest surge. Summing every surge just buys a bigger, costlier generator than you need.
Same loads, 7,500 W vs 12,000 W. The trap costs you a bigger unit, more fuel, and a heavier engine — for surge capacity you'll never use. And run any generator below 80% of its rating continuously to protect the alternator.
Peak = running total + largest single surge · Size = peak × 1.20 → next standard
Worked example
What generator runs the essentials?
A storm-outage kit: refrigerator, furnace fan, sump pump, well pump, and lights. Total the running watts, add the single biggest surge, then check which generator actually covers it.
The calculation
Which generator covers 5,900 W?
7,500 W is the pick — it clears the surge with headroom. To back up the whole house instead (this home's full load ≈ 35 kW), you're into standby territory: ~22–26 kW with a load-management module.
Reference · watts & safety
Appliance watts, and the one safety rule
Typical running and starting watts to size against — plus the wiring rule that isn't optional. Always edit these to your own nameplates.
Typical appliance watts
| Appliance | Running | Surge |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Well pump (½ HP) | 1,000 | 3,000 |
| Sump pump (⅓ HP) | 800 | 1,300 |
| Furnace fan (½ HP) | 800 | 2,350 |
| Window A/C (10k BTU) | 1,200 | 3,600 |
| Central A/C (3 ton) | 3,800 | 11,000 |
| Microwave | 1,000 | 1,000 |
| Electric dryer | 5,400 | 6,750 |
Never backfeed your panel
Connecting a generator to your home's wiring without a transfer switch or interlock can push power back onto the utility line and electrocute lineworkers restoring service. This isn't a preference — it's code, and it's life-safety.
For a standby install: a transfer switch and a licensed electrician are required. Confirm NEC 702 and your local AHJ before any hardwired connection.
Portable vs standby
Portables (~5–10 kW, gasoline) cover essential circuits and cost less. Standby units (~14–26 kW, natural gas or propane) start automatically and can power the whole panel through a transfer switch.
The 80% rule
Never run a generator above 80% of its rating continuously. A 6,500 W unit should carry no more than ~5,200 W steady — it protects the alternator and leaves headroom for surges.
Altitude & heat derate it
Generators lose about 3–5% of output per 1,000 ft of elevation, and more in high heat. Size up if you're at altitude or in a hot climate.
Whole-house sizing
A standby sizes to your calculated load, not the full panel — run the load calculation first. Load management (shedding A/C, dryer, EV) lets a smaller unit cover a big home.
Only one motor starts
Size for the running total plus the single largest surge — not every surge added together. Motors rarely start the same instant, and summing them buys far more generator than you need.
What affects it
What drives the size — and what trips people up
Generator size is mostly about motors and their startup surge. The mistakes are almost always the same: sizing to the steady draw, or over-buying by double-counting surges that never happen at once.
What drives the size up
the big movers
- Central A/CA 3-ton unit surges to ~11,000 W at startup — usually the load that forces a standby.
- Well pumpA ½ HP pump runs at 1,000 W but needs ~3,000 W to start — the biggest surge in most rural homes.
- Whole-house vs essentialsBacking up the whole panel needs a standby sized to the calculated load — many times an essentials kit.
- Altitude & heatOutput derates ~3–5% per 1,000 ft and more in high heat — size up if you're high or hot.
Common mistakes
watch for these
- 01Sizing to running wattsThe most common error. A 5,000 W generator can't start a 5,000 W A/C — it stalls on the surge.
- 02Summing every surgeOnly one motor starts at a time. Adding them all buys a far bigger, thirstier unit than you need.
- 03Ignoring the 80% ruleRunning near full capacity for hours wears the alternator. Keep steady load under 80% of rating.
- 04No transfer switchBack-feeding the panel is illegal and deadly — it can electrocute lineworkers. Never skip it.
The point
Size for the spike, not the steady draw
A generator that comfortably runs your loads can still stall the instant a motor starts. Find the biggest single surge, stack it on the running total, add a buffer — that peak is the number that matters.
Never connect a generator to your panel without a transfer switch or interlock. Backfeed can energize the utility line and electrocute lineworkers. A permanent standby install needs a transfer switch and a licensed electrician.
Quick chart
What size generator?
Enter your running total and the biggest single surge to get the peak and the size — or scan the table for common setups. Essentials plus a well pump peak near 5,900 W → a 7,500 W generator.
Peak = running + the single biggest surge. Size = peak × 1.2, rounded up.
Recommended
7,500 W
peak 5,900 W · covers with headroom
Common setups
| Setup | Running | Peak | Generator |
|---|---|---|---|
| Essentials | 2,900 | 4,450 | 5,500 W |
| + well pump | 3,900 | 5,900 | 7,500 W |
| + central A/C | 7,700 | 14,900 | 18 kW |
| Whole house | — | ~35 kW | 22–26 kW* |
*Whole-house standby sizes to the calculated load; a load-management module lets a smaller unit cover a big home.
| Setup | Running W | Largest surge | Peak W | × 1.2 | Generator |
|---|---|---|---|---|---|
| Fridge + lights only | 1,300 | 1,500 | 2,800 | 3,360 | 3,500 W |
| Essentials (no well) | 2,900 | 1,550 | 4,450 | 5,340 | 5,500 W |
| Essentials + well pump | 3,900 | 2,000 | 5,900 | 7,080 | 7,500 W |
| Essentials + window A/C | 5,100 | 2,400 | 7,500 | 9,000 | 9,000 W |
| Essentials + central A/C | 7,700 | 7,200 | 14,900 | 17,880 | 18 kW |
| Whole house (200 A, managed) | — | — | ~35 kW | — | 22–26 kW |
What size generator do I need for a house?
Add the running watts of everything you'll run at once, then add the single biggest starting surge. Most homes on essentials — fridge, furnace fan, sump, lights, a well pump — peak near 5,000–7,500 W, a common portable range. Whole-house coverage needs a standby sized to your calculated load, often 18–26 kW.
What's the difference between running and starting watts?
Running (rated) watts keep a device going; starting (surge) watts are the brief spike when a motor kicks on — often 2–3× running. A well pump may run at 1,000 W but surge to 3,000 W. Size the generator to cover the running total plus the largest single surge.
What size generator runs a whole house?
A standby unit sized to your calculated load. A typical all-electric 200 A home calculates near 34.7 kW; a 20–26 kW standby with load management covers most homes by shedding big loads (A/C, dryer, EV) so they don't run at once. Get your number from the Electrical Load Calculator.
Will a generator run my air conditioner?
Only if it covers the AC's starting surge. Central AC surges hard — a 3-ton unit can spike near 11,000 W, and that surge usually decides the generator size. Check the nameplate's LRA/MCA and confirm the generator's surge rating clears it.
Portable vs standby — which do I need?
A portable (with a transfer switch or interlock) powers essentials during short outages and costs less. A standby wires into your panel, starts automatically, and can run the whole house on natural gas or propane. Portable = essentials; standby = whole-home, hands-off.
Do I need a transfer switch?
Yes. Never back-feed a generator into your panel through an outlet — it can push voltage onto the utility line and electrocute lineworkers. A transfer switch or interlock kit safely isolates your home from the grid. It's required by code and installed by a licensed electrician.
What size generator for just the essentials?
Fridge, furnace fan, sump pump, and lights total roughly 2,500–3,500 running watts; add a well pump's surge and the peak climbs near 5,000–6,000 W. A 5,500–7,500 W portable covers most essential-only setups with headroom.
What size generator for a 200-amp panel?
A 200 A panel doesn't mean you need 200 A of generator — size to your calculated load, not the panel rating. Most 200 A homes calculate near 34.7 kW; a 22–26 kW standby with load management covers them. Running everything without shedding may need 38 kW+.
Keep going
Related electrical calculators
You’ve got the amps — now put them to work. These pick up from there: convert watts, size the wire for the load, and see the V · I · R · P relationship behind it.
How we keep this accurate
Generator sizing uses typical running and surge wattages — always confirm against equipment nameplates. Large motors and central A/C surge well beyond rated watts. A permanent standby install requires a transfer switch and a licensed electrician; conductor and overcurrent sizing follows the National Electrical Code (NEC 2023). Results are for planning — verify with your AHJ before installation.