A generator is sized by two different numbers at once, and confusing them is the most common mistake in the whole exercise. The first is running watts — the steady draw of everything switched on together. The second is starting watts, the brief spike an electric motor pulls in the fraction of a second it takes to get moving. A refrigerator that hums along at 700 watts can demand two or three times that for a moment when the compressor kicks in, and a generator that cannot supply the spike will stall or trip even though the average load sat comfortably within its rating.
This generator size calculator handles both. Arb Digital built it as part of a free tools library for people doing practical arithmetic without specialist software. You enter each load once, tell the tool whether it contains a motor, and it returns a running requirement and a peak requirement separately, along with the spare capacity left on a generator you already own.
What This Generator Size Calculator Does
You enter up to four loads — or four groups of loads, since each row has a quantity field — as running watts plus a start factor. The start factor is a multiplier: a resistive load such as a lamp, a kettle or a heater draws the same current at switch-on as it does a minute later, so it sits at 1x. Anything with an induction motor behind it draws far more, and the multiplier expresses how much more. The calculator sums all the running watts, works out the extra watts each motor would add at the instant it starts, takes the single largest of those extras, and adds it to the running total to produce the peak figure.
It then applies two adjustments. Headroom is spare running capacity you deliberately leave unused so the engine is not pinned at its full rating for hours on end; the default of 20% is a planning allowance rather than a rule. Derate is the reduction in output engine manufacturers publish for altitude and high ambient temperature — specific to the engine and the fuel, so this calculator takes it as an input from your manual rather than inventing a figure.
Two tools on this site sit next to this one and the boundary is worth stating plainly. The appliance wattage calculator is the lookup step: it works out what a single appliance actually draws from its amperage, voltage or nameplate rating. This page is the sizing step: it takes wattage figures you already have and turns a whole list of them into a generator specification with surge included. The electricity bill calculator answers a third question entirely — what the same loads cost over a month on mains power — and the electrical power calculator handles the underlying volts, amps and watts relationship.
How to Use It
- List the loads you actually need during an outage. Not everything in the house. A backup generator is normally sized for a refrigerator, some lighting, a furnace fan or well pump, and a couple of outlets — not for an unchanged lifestyle.
- Enter the running watts for each. Take the number from the appliance nameplate, or calculate it from amps multiplied by volts. Where the nameplate gives amps only, remember that most household circuits in the United States run at 120 volts and large appliances at 240.
- Set the start factor honestly. Resistive loads are 1x. Small fans and hand tools sit near 2x. Compressors, pumps and air conditioners are the reason surge ratings exist at all.
- Set headroom and derate. Headroom is your choice. Derate comes from the generator's manual and depends on your elevation and summer temperatures.
- Read both numbers. The generator must meet the running requirement continuously and the peak requirement momentarily. A unit that meets one but not the other is the wrong unit.
The Formula / How It's Calculated
The arithmetic is deliberately simple, because the difficulty in generator sizing is getting the inputs right rather than doing the sum.
Total running watts is the sum of each row's running watts multiplied by its quantity. Each row's surge extra is the running watts multiplied by the quantity, multiplied by the start factor minus one — the "minus one" matters, because the running watts are already counted and the surge is the additional demand on top of them. Peak starting watts is the total running watts plus the single largest surge extra in the list. The recommended running figure is the total running watts increased by the headroom percentage, and both figures are then divided by one minus the derate fraction.
Worked example, which is what the page loads with. A refrigerator at 700 running watts with a 3x start factor, a sump pump at 800 watts also at 3x, 400 watts of lighting at 1x, and a 1,000-watt microwave at 1x. Total running watts is 700 + 800 + 400 + 1,000 = 2,900 W. The refrigerator's surge extra is 700 x (3 − 1) = 1,400 W; the pump's is 800 x 2 = 1,600 W, which is the larger of the two. Peak starting demand is therefore 2,900 + 1,600 = 4,500 W. With 20% headroom and no derate, the recommended continuous rating is 2,900 x 1.20 = 3,480 W. A generator rated 3,600 running and 4,500 peak covers this list; one rated 3,600 running but only 4,000 peak does not.
Why Only One Surge Is Counted
Stacking every appliance's starting watts together produces an enormous number and sends people out to buy a generator two or three times the size they need. It is also physically wrong in almost every case. Motor starting takes somewhere between a fraction of a second and a couple of seconds, and the chance that two independent thermostats or float switches close in exactly that window is small. Standard practice for standby power is to add the largest single starting load to the total connected running load, which is what this calculator does.
There is a real exception. If two appliances are switched by the same control — a well pump and a pressure booster on a common relay, or two compressors on one thermostat — they start together by design. Combine them into a single row: add their running watts together and give the combined row the higher of the two start factors.
Running Watts, Peak Watts and What the Box Says
Generator marketing leads with the bigger number. A unit sold as a "4,500 watt generator" is usually quoting the peak or surge rating, and its continuous running rating might be 3,600 or 3,700. That gap is normal and not dishonest — the engine genuinely can deliver the higher figure briefly — but it means the headline number is not the number you compare against your running total. Look for the wording "rated", "continuous" or "running" on the specification plate, and use that instead.
Inverter generators complicate the comparison slightly. Their surge capability comes from the inverter electronics rather than from engine inertia and is usually available for a shorter time than a conventional generator's, so where your load list is dominated by motor starts, the duration of the surge matters as much as its magnitude. That figure lives in the manual rather than on the box.
Power factor is the third complication. A motor with a power factor of 0.8 that delivers 800 watts of real power draws 1,000 volt-amperes from the alternator. Where a specification sheet gives kVA, multiply by the stated power factor before comparing it here; the power converter handles other unit mismatches.
Altitude, Temperature and Fuel Type
An internal combustion engine makes less power in thin air, and generator manufacturers publish derate curves for exactly this reason. The derate field on this page exists so you can apply the figure from your own manual instead of a generic assumption, because the correction differs between naturally aspirated and turbocharged engines and between fuel types. High ambient temperature causes a further reduction, and the two effects combine.
Fuel choice changes the available output too. The same engine typically produces less power on propane than on gasoline, and dual-fuel units publish separate ratings for each fuel. If you are planning to run on propane because it stores indefinitely, size against the propane rating. The natural gas converter helps when a specification is given in energy units rather than watts, and the fuel cost calculator covers running costs.
None of this touches the far more serious point about generators, which is exhaust. Engine-driven generators emit carbon monoxide, and every safety authority is unambiguous that they must never be run indoors, in a garage, in a basement or in any partly enclosed space, regardless of ventilation or open doors. The Ready.gov power outage guidance and OSHA's carbon monoxide hazard page both cover it. Sizing arithmetic is no substitute for reading them.
Sizing a Circuit List Versus Sizing a Whole House
There are two philosophies here and they produce very different answers. Circuit-list sizing is what this calculator supports: you decide what must stay running, add it up, and buy a generator that covers it. Whole-house sizing works from the electrical service instead — a 200-amp, 240-volt service could in principle draw 48,000 watts — and then applies demand factors that reduce the figure substantially, because no household ever runs everything at once.
For a portable generator, circuit-list sizing is almost always the right approach, and it is worth noticing which loads dominate the list. Anything that makes heat is expensive in watts: an electric water heater, an oven or a clothes dryer will each swamp everything else combined, which is why the electricity bill calculator shows them at the top of a monthly bill as well. Anything that makes cold is expensive in surge. Lighting is almost free by comparison, especially on LED, which the LED savings calculator quantifies. Heating capacity itself belongs in the furnace size calculator.
National consumption data shows where the watts go in aggregate: the US Energy Information Administration's breakdown of electricity use in homes puts air conditioning, space heating and water heating at the top of the residential list. Those are precisely the loads most outage plans leave off the generator, which is why a unit in the 3,000 to 5,000 watt range covers so many households.
Connecting It Matters More Than Sizing It
A correctly sized generator connected incorrectly is more dangerous than an undersized one. Feeding power into a house through a dryer outlet or any other double-ended cord energises the utility side of the meter and bypasses every overcurrent protection in the panel. Cord sizing is its own calculation. Long runs at high current lose voltage, and a motor fed at low voltage draws more current and runs hot, which is the failure mode that kills refrigerator compressors during outages. The voltage drop calculator and the wire size calculator handle that arithmetic. If your generator is charging a battery bank rather than feeding appliances directly, the battery life calculator is the relevant tool for runtime.
Arb Digital maintains a large free tools library across energy, construction, finance and measurement, and our team is happy to point you at the right one.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Comparing your load total against the peak rating on the box — the number that must cover continuous load is the rated or running watts, which is always the smaller figure.
- Adding every appliance's starting watts together — only one motor normally starts at a time, and stacking surges inflates the requirement enormously.
- Using the nameplate maximum as the running draw — a nameplate states the most an appliance can pull, not what it typically pulls.
- Ignoring the altitude and temperature derate — a generator rated at sea level in mild conditions will not deliver its plate rating on a hot day at elevation.
- Sizing with no headroom at all — running an engine continuously at its full rating leaves nothing for the load you forgot and shortens the service life of the unit.
Related Free Tools From Arb Digital
Start with the appliance wattage calculator to establish what each item draws, then come back here to size the generator. Use the electricity bill calculator for mains running costs, the solar panel calculator if you are comparing a generator against a solar and battery setup, the battery life calculator for runtime from stored energy, and the watts to horsepower converter when a specification is quoted in horsepower instead. Everything else is in the tools hub.
Frequently Asked Questions
Running watts is the steady draw of an appliance once it is operating. Starting watts is the much larger, very brief demand an electric motor makes as it begins to turn. A generator must supply the running total continuously and the peak figure momentarily, and the two requirements are checked separately.
Because motor starts last a fraction of a second and rarely coincide. Standard practice is to add the single largest starting load to the total running load. If two motors are switched by the same control and genuinely start together, combine them into one row so their surge is counted as one event.
The tool defaults to 20% spare running capacity, which is a planning allowance rather than a standard. More headroom means the engine spends less time at full load and there is room for a load you forgot; less headroom means a smaller, cheaper and quieter unit. It is a judgement call, not a rule.
The figure from your generator's own manual for your elevation and expected ambient temperature. Derate curves differ between engines and fuels, so this calculator does not assume one. Leave it at zero if you do not have the manufacturer's number and treat the result as an uncorrected sea-level figure.
It changes the size the generator can deliver rather than the size you need. The same engine usually produces less power on propane than on gasoline, and dual-fuel units publish separate ratings for each. Compare your requirement against the rating for the fuel you will actually use.
That tool works out what one appliance draws, from its amperage, voltage or nameplate. This one takes several wattage figures you already have and turns them into a generator specification with surge and headroom included. They are the two halves of the same job.
It will give you a defensible figure for a list of circuits you choose, which is how portable generators are normally sized. Whole-house standby systems are sized from the electrical service with demand factors applied and are wired through a transfer switch, which is a different calculation and a job for a licensed electrician.
This tool performs load arithmetic only. It is not an electrical design, does not replace the National Electrical Code or your local wiring rules, and does not address generator installation, transfer switching or exhaust safety. Engine-driven generators produce carbon monoxide and must never be operated indoors or in any enclosed or partly enclosed space. Have any permanent connection to building wiring installed by a licensed electrician.
