Calculator
Generator Sizing Calculator
Add up the starting surge of every motor in the house at once and you will land on a generator far bigger, and far more expensive to buy or rent, than the house actually needs. A refrigerator, a sump pump, a furnace blower and a well pump do not all lurch to life in the same instant during a real outage — they cycle on independently, whenever their own thermostat or float switch calls for them — so the generator only ever has to survive the single largest one starting while everything else is already running.
This calculator adds every appliance's steady running watts together, because those genuinely do run at the same time, and then adds only the largest single surge on top — the extra a motor draws for the fraction of a second it takes to get moving, beyond what it draws once spinning. It also prints what summing every surge would have produced, next to the real number, specifically so the gap between the two is visible rather than something you have to take on faith.
Calculate your quantity
watts
watts — router, TV, phone chargers, microwave
Running load
3,300 watts
Starting load
4,850 watts
- Largest single surge
- 1,550 watts
- If you summed every surge
- 6,850 watts
- Recommended generator
- 5,000 watts
- Fuel burn at this load
- 0.3 gal/hr
- Runtime on 5 gallons
- 16.8 hours
- Summing every surge would have called for 6850 watts against a real requirement of 4850. Motors do not all start at the same instant, and a generator only has to survive the worst one.
Shopping summary
- • A 5000 watt generator (5 kW), sized on the Furnace blower surge
- • A transfer switch or interlock, installed by a licensed electrician
This is an estimate — confirm structural work with a professional.
How this calculation works
Running watts is a straightforward sum: lighting, everything else plugged in and drawing continuous power, plus each selected appliance's own steady run wattage. That part of the math is genuinely additive because a generator has to carry all of it simultaneously for as long as the outage lasts, not just for a moment.
Starting watts is different by design. Each motor's surge is its run wattage plus a short-lived extra needed to overcome the load of getting a compressor or impeller moving from a standstill — call that extra amount the appliance's surge premium. Only one appliance is realistically starting at the exact instant every other motor is already running, so the generator's true worst case is the running total plus the single largest surge premium among the appliances present, not the sum of all of them. A furnace blower's premium and a well pump's premium never actually stack, because they do not start together.
- Never backfeed a generator into a household outlet with a double-ended cord — it energizes the utility line outside the house and can kill a lineworker who believes the circuit is dead, which is why it is treated as a serious safety violation, not just a code technicality.
- A transfer switch or interlock, sized for the generator's actual output and installed by a licensed electrician, is what makes a generator-to-house connection safe and legal. That is wiring work, not a DIY afternoon project.
- Fuel burn scales with the load actually running, not the generator's rated size — an oversized unit run near idle burns fuel inefficiently and can foul a small engine over time.
- The run and surge figures here are typical values, not a substitute for the actual data plate on your specific appliance — a close sizing call should be checked against real nameplate or locked-rotor amperage, not a general estimate.
The formula
running = lightingWatts + otherWatts + sum(count_i x run_i); startingWatts = running + max(surge_i - run_i); sumOfAllSurges = running + sum(count_i x (surge_i - run_i))
- lightingWatts + otherWatts
- The continuous baseline load with no motors in it: lighting circuits plus everything else drawing steady power — routers, a TV, phone chargers, a microwave while it runs.
- sum(count_i x run_i)
- Each selected appliance type's steady running wattage, multiplied by how many you have, added to the baseline. This is the part of the total that is genuinely additive.
- max(surge_i - run_i)
- The single largest surge premium among the appliance types present — not summed across types, because only one appliance is realistically starting at the moment the generator is under its worst load.
- sumOfAllSurges
- What the total would be if every appliance's surge premium were added together instead of taking only the largest — printed alongside the real figure specifically to show how much smaller a generator the correct method actually calls for.
- recommended generator
- The smallest standard portable or standby generator wattage class at or above the starting-watts figure, from the sizes those units are actually sold in.
Where these numbers come from
- 700/2,200 W fridge, 800/1,300 W sump pump, 800/2,350 W furnace blower, 1,000/3,000 W well pump (run/surge)
- Manufacturer-published: typical run and surge figures drawn from portable generator makers' own appliance wattage guides. A specific appliance's data plate or locked-rotor amperage is the more precise source for a sizing decision that is close to a boundary between two generator sizes.
- 3,500 through 26,000 W standard generator sizes
- Manufacturer-published: the wattage classes portable and standby generators are actually sold in. A calculated starting-watts figure between two of these steps should round to the size above it, never below.
- Largest-single-surge sizing method
- Trade convention among electricians and generator installers, based on how household motors actually cycle — independently, not simultaneously — rather than a citation from a specific code section.
- 0.09 gal/hr per kW of running load (gasoline)
- Trade convention approximating typical gasoline generator fuel consumption at roughly half load. Actual burn rate varies by engine and load fraction, and a specific model's fuel consumption chart is more precise for planning a fuel budget.
Worked examples
A standard household with one of each essential appliance
| Refrigerators / freezers | 1 |
|---|---|
| Sump pumps | 1 |
| Furnace blowers | 1 |
| Well pumps | 0 |
| Lighting | 400 |
| Other running load | 600 |
| Running load | 3300 watts |
|---|---|
| Starting load | 4850 watts |
| Largest single surge | 1550 watts |
| If you summed every surge | 6850 watts |
| Recommended generator | 5000 watts |
| Fuel burn at this load | 0.3 gal/hr |
| Runtime on 5 gallons | 16.8 hours |
The furnace blower has the largest surge premium here — 1,550 watts above its own run wattage, beating the fridge's 1,500 and the sump pump's 500 — so starting load is 3,300 plus 1,550, not the 6,850 you would get by adding every appliance's premium together. That difference is the gap between a 5,000 watt generator and one nearly 40% larger.
5,000 watts is the smallest stock size that clears the real 4,850 watt requirement, with a little headroom left over. Renting or buying to the summed-surge figure instead would mean paying for capacity this household will never actually draw, on any outage, ever.
The same household plus a well pump and a second fridge
| Refrigerators / freezers | 2 |
|---|---|
| Sump pumps | 1 |
| Furnace blowers | 1 |
| Well pumps | 1 |
| Lighting | 500 |
| Other running load | 800 |
| Running load | 5300 watts |
|---|---|
| Starting load | 7300 watts |
| Largest single surge | 2000 watts |
| If you summed every surge | 12350 watts |
| Recommended generator | 7500 watts |
| Fuel burn at this load | 0.48 gal/hr |
| Runtime on 5 gallons | 10.5 hours |
Adding a well pump changes which appliance sets the size, not just the total. Its 2,000 watt surge premium overtakes the furnace blower's 1,550, so the largest-single-surge rule now points at the well pump — even though the second fridge also raised the running load. Two things moved at once, and only one of them actually drove the surge figure.
Summed-surge sizing here would call for 12,350 watts against a real requirement of 7,300 — the gap between renting a 16,000 watt unit that may not even be readily available and the 7,500 watt size that actually covers this house, which is a meaningfully different rental cost for a multi-day outage.
Common mistakes
- Adding up every appliance's starting surge instead of taking the largest single one, which routinely calls for a generator 30 to 50% bigger — and more expensive — than the house needs.
- Backfeeding a generator into a dryer or range outlet with a double-ended cord. It energizes the utility line outside the house, can injure or kill a lineworker, and is treated as a serious safety violation rather than a shortcut.
- Forgetting continuous loads that are not the headline appliances — a sump pump running nearly nonstop during the same storm that caused the outage, or a well pump's steady draw once it is already started, both belong in the running total.
- Assuming the generator's rated wattage is available continuously in any condition. Real output derates with heat and altitude, and most portable units are not built to run at their full rated load for hours on end without a shortened service life.
- Installing a transfer switch or interlock that is not rated for the generator actually purchased, or skipping the permit and electrician entirely — both are code and safety issues on a system that connects directly to household wiring, not paperwork to skip.
- Flipping every big breaker back on at the same moment once the generator is running. Even a correctly sized unit can stumble if several loads start in the same instant — bring them on one at a time regardless of total capacity.
Shopping summary
Buy or rent to the recommended wattage this page produces, and budget separately for a transfer switch or interlock kit sized to that same generator and installed by a licensed electrician — that connection is what makes backfeeding unnecessary and the whole setup legal to use.
If an EV charger is one of the loads you are planning to run during an outage, size its circuit on the EV charger circuit calculator first — a Level 2 charger's continuous draw is large enough to change which appliance sets your generator's running load, not just its surge.
A gas or electric tankless water heater is a similarly large, easy-to-miss load; check its input requirement on the water heater size calculator before assuming your generator has room for it alongside everything else. If backup power is a recurring need rather than an occasional storm, the solar array calculator is worth comparing against a generator's fuel and runtime economics for the same load.
FAQ
Why not just add up every appliance's starting watts to be completely safe?
Because household motors do not start at the same instant — a fridge's compressor and a furnace blower are controlled by separate thermostats and cycle independently. Summing every surge assumes a coincidence that essentially never happens in practice, and it routinely inflates the required generator size by a third or more, which costs real money whether you are buying or renting.
Can I ever safely backfeed through a dryer outlet if I'm careful about which breakers are off?
No. A double-ended backfeed cord energizes wiring outside the house on the utility side, where a lineworker restoring power has every reason to believe the line is dead. This is not a matter of doing it carefully — it is a genuinely dangerous practice and the reason transfer switches and interlocks exist as the correct alternative.
Do I need a full transfer switch, or is a cheaper interlock kit enough?
Both prevent the generator and utility power from ever being connected at the same time, which is the safety requirement either one satisfies. A transfer switch lets you choose specific circuits to power and switch between sources without going near the panel; an interlock is simpler and cheaper but requires you to manually flip the main breaker. Either one needs to be sized to the generator and installed by a licensed electrician.
Why did adding a well pump change which appliance sets the generator size, even though a second fridge also raised the total?
The running load and the surge premium are answering different questions. The second fridge adds to running load because both fridges genuinely run continuously together. The well pump's 2,000 watt surge premium is larger than the furnace blower's 1,550, so it becomes the one that governs starting load — the two changes happened together but only one of them decided which appliance the sizing depends on.
How long will a generator actually run on a tank of gas at this load?
Roughly the tank size divided by the fuel burn figure this page estimates for your running load — five gallons at a typical half-load burn rate is commonly in the 10 to 17 hour range depending on how big the running load actually is, which is why the runtime figure here drops noticeably between a light load and a heavier one.
Should I size for the whole house or just a handful of essential circuits?
Most portable generators are sized for essential circuits — refrigeration, a sump or well pump, furnace blower, some lighting and outlets — rather than an entire home's full electrical service, which would need a much larger standby unit. This calculator is built around that essentials list; add anything else you specifically want to run, like an EV charger, as its own line item rather than assuming it is already covered.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.