Generator Sizing Calculator
Work out the minimum backup generator for a list of appliances — everything running at once, plus the extra kick of starting the largest motor, which is the part that catches people out.
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Generator Sizing Calculator
Minimum generator
How this was derived
- Running loadeverything on at once
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- Largest starting surgeone motor at a time
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- Required output
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- Next available sizerunning watts, not starting
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How this is calculated
A generator has to do two things at once, and only one of them is obvious.
It has to carry everything that is running. Add up the running watts of every load you want on during an outage. That is the easy half, and on its own it produces a generator that fails the first time something with a motor tries to start.
It has to absorb one motor's starting kick on top of that. An induction motor at rest draws two to three times its running current until it comes up to speed. The utility absorbs that without noticing; a small engine turning an alternator does not. So the requirement is the running total plus the largest single surge — the difference between one motor's starting and running figures, not the whole starting figure, because that motor's running draw is already counted.
One surge, not all of them. This is where the model earns its keep. Motors do not start in unison: a compressor cycles on while the pump is already running steadily, half a second apart at worst. Adding every motor's surge together is arithmetically simple and produces a set one or two rungs larger than anything the house will ever ask for. The calculator shows that stacked figure alongside the real one, because seeing the gap is more convincing than being told about it.
Then round up to something you can buy, and compare against the right column of the spec sheet. Generators advertise a starting rating and a running rating, the larger one more prominently. The answer here is a continuous requirement and it belongs against the running figure.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
A modest outage list: a refrigerator at 800 W running and 1,600 W starting, a well pump at 1,000 W running and 3,000 W starting, and600 W of lights and odds and ends with no motor in them.
Running total. 800 + 1,000 + 600 = 2,400 W. Everything on at once, which is the state the generator has to hold indefinitely.
The surges. The fridge's is 1,600 − 800 = 800 W. The pump's is 3,000 − 1,000 = 2,000 W. The lights have none. The largest is the pump's, and that is the only one that counts.
Required output. 2,400 + 2,000 = 4,400 W, which rounds up to a 5,000 W set — 88% loaded, with 600 W to spare.
What the two wrong models would have given. Sizing on running watts alone gives 2,400 W and buys a 3,000 W set, which carries the house perfectly well right up until the pump tries to start and stalls it. Stacking both surges instead gives 2,400 + 800 + 2,000 = 5,200 W and buys a 7,500 W set — a whole size larger, for a scenario where the fridge and the pump happen to start in the same half-second.
Note which appliance drove the answer. The fridge and the pump draw almost the same running watts, but the pump has twice the surge, and swapping it for a smaller one would drop the requirement to 3,200 W and a 5,000 W set to a comfortable 64%. On a list this short, one motor decides the machine.
Visual comparison
Load total: 4400 W
| Load | WattsW |
|---|---|
| Refrigerator | 800 |
| Well pump | 1000 |
| Lights and misc | 600 |
| Starting surge — Well pump | 2000 |
Source: Running watts summed; surge is the largest single starting delta
Reference tables
The first table is the ladder of commonly available sizes with the step to the next rung spelled out, because the gaps are wide and uneven — 3,000 to 5,000 is a 67% jump. Use it to see what a marginal result actually costs before deciding whether to trim the load list or pay for the next size.
The second table is typical running and starting figures for the loads that turn up on nearly every backup list. They are a starting point and nothing more: a showroom of refrigerators varies by a factor of three, and the nameplate on yours beats every figure here. Use them to build a first draft, then go and read the labels.
| Running output | Step to nextW | Typically |
|---|---|---|
| 3,000 W | +2,000 | Small portable or inverter set. A fridge, some lights and a few outlets — not a well pump. |
| 5,000 W | +2,500 | The common mid-size portable. Enough for a fridge, lights and one motor load starting at a time. |
| 7,500 W | +2,500 | Large portable, usually on wheels. Handles a well pump plus general household circuits. |
| 10,000 W | +2,000 | Top of the portable range or the smallest standby sets. A small central air conditioner becomes possible. |
| 12,000 W | +3,000 | Small permanently installed standby set, typically on a transfer switch. |
| 15,000 W | +5,000 | Mid-size standby. Most of a modest all-electric house except the range and the dryer together. |
| 20,000 W | +2,000 | Large residential standby. Whole-house coverage for a typical single-family dwelling. |
| 22,000 W | — | The top of the common residential range. Above this, sets are commercial and sized by a different process. |
| Load | RunningW | StartingW | Surge deltaW |
|---|---|---|---|
| Refrigerator | 800 | 1600 | +800 |
| Chest freezer | 500 | 1000 | +500 |
| Well pump (1/2 HP) | 1000 | 3000 | +2000 |
| Sump pump (1/3 HP) | 800 | 2000 | +1200 |
| Furnace blower | 800 | 2000 | +1200 |
| Window air conditioner | 1200 | 3000 | +1800 |
| Lights and general outlets | 600 | 600 | none |
| Microwave | 1000 | 1000 | none |
| Electric water heater | 4500 | 4500 | none |
| Space heater | 1500 | 1500 | none |
Notes and exceptions
Two motors that really could start together. The one-at-a-time model is right for the ordinary case and it is an assumption, not a law. A well pump and an air conditioner on the same circuit after a power cut may genuinely energise within the same second, and a house with two large compressors on one thermostat cycle is the case to think about. Where that is a real risk, size for the two largest surges deliberately — the calculator shows you that stacked figure, so you can choose it rather than arrive at it by accident.
Soft starters change the arithmetic. A soft start module on an air conditioner or a variable-speed pump can cut inrush dramatically, sometimes enough to drop a generator a whole size. If the surge column is what is driving your answer, pricing a soft starter against the next generator up is often the cheaper move.
None of this covers the installation. Article 702 governs optional standby systems: the transfer equipment, the interlock that prevents back-feeding the utility, and the requirement that the system be installed so it cannot be energised in parallel with the normal source. A correctly sized generator connected through a suicide cord is a lethal installation, and no wattage calculation says anything about that.
Watts, not watt-hours. This sizes the machine, not the fuel. A 5,000 W set running a 3,000 W load burns fuel at a rate the tank capacity has to sustain for as long as the outage lasts, and run time is a separate question with a separate answer. It is also where generator sizing and battery-bank sizing genuinely part company: a tank can be refilled at 2 a.m., a battery cannot.
Common mistakes
Sizing on running watts alone
The classic, and it fails on the first cold start. A well pump drawing 1,000 W running needs 3,000 W for the half-second it starts. A set sized to the running total bogs down, the voltage sags, and the pump either fails to start or cooks its windings trying.
Stacking every motor’s surge on top of each other
The over-correction, and it costs real money. Motors start one at a time — a compressor cycles on while the pump is already running, not in the same instant. Adding every surge together typically buys a set one or two sizes larger than anything the house needs.
Comparing the answer against a generator’s starting rating
Sets are advertised with two numbers and the bigger one is on the front of the box. "5,500 starting / 4,500 running" is a 4,500 W generator for sizing purposes. Matching a 5,000 W requirement to that set leaves it 500 W short of the continuous load.
Ignoring the fuel derate
The nameplate figure is usually gasoline. The same set makes roughly 10% less on propane and less again on natural gas, and altitude takes more. A set that just fits on paper can be genuinely undersized once it is plumbed to the fuel it will actually run on.
Backing up the whole house by reflex
Every load added pushes the set up the ladder and the ladder is not cheap. An electric range and a water heater together can double the requirement. Decide what genuinely has to run during an outage — heat, refrigeration, water, some lights — and leave the rest off.
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
What size generator do I need for a refrigerator and a well pump?
Around 5,000 W for a typical pairing with some lighting. An 800 W fridge, a 1,000 W well pump and 600 W of lights come to 2,400 W running; the pump’s starting surge adds another 2,000 W on top of that, giving 4,400 W. The next commonly available size is 5,000 W. The pump drives the answer even though the fridge draws almost as much running.
Why do starting watts matter so much?
An induction motor at rest looks almost like a short circuit for the first fraction of a second, drawing two to three times its running current until it comes up to speed. A generator is a small engine turning an alternator, and unlike the utility it has no reserve to draw on — the engine simply bogs down, voltage sags, and the motor stalls. That transient sets the size of the machine even though it lasts half a second.
Do I add up the starting watts of every motor?
No, and doing so is the most common way to oversize a generator. Only the single largest surge counts, added to the running total of everything else. Motors do not start in unison — a compressor cycles on while the pump is already running at its lower running draw. Size for two motors starting together only if you have a specific reason to think they will, such as two units on one thermostat.
Is this the same as sizing a battery bank?
No, and the difference is not a detail. A generator is sized in watts — instantaneous power, dominated by the worst transient. A battery bank is sized in watt-hours, for energy and autonomy: how much you draw multiplied by how long you need it, adjusted for depth of discharge. A bank that comfortably delivers your peak power can still be flat by 3 a.m., which is a failure mode generator sizing simply does not have.
Does the generator size tell me what transfer switch to use?
Not directly. The transfer switch is rated in amperes and has to suit the service it is switching, not just the generator feeding it — and whether you need a full-service switch or one serving a small critical-loads panel is a separate decision about which circuits get backed up. Article 702 covers optional standby systems and the interlock requirements that stop a generator back-feeding the utility.
Should I run a generator at 100% of its rating?
Not continuously. Most sets quote a running rating for sustained output and a higher starting figure for a few seconds, and sitting at the running rating all night is loud, thirsty and leaves nothing for the load you forgot. Landing under about 80–90% of the running rating is a comfortable place to be, which is why this calculator flags a result that arrives right at the top of a size.