Generator Sizing Calculator

A generator has two capacity numbers, and confusing them is the most common sizing mistake. The running (continuous) watt rating is what the engine and alternator can deliver hour after hour. The starting (surge) watt rating is a brief overload the unit can absorb for a second or two while a motor spins up.

Correct sizing means both of these are satisfied at once: enough continuous capacity for everything you intend to run simultaneously, and enough surge headroom that your worst-starting motor can get going without collapsing voltage. A generator that is comfortable on continuous load can still stall or brown out every time the well pump kicks on.

This calculator builds that picture from your loads. It is a planning tool — the authoritative numbers are the nameplates on your own equipment and the specification sheet for the generator you are considering.

Shop with both, not one. A candidate generator must meet or exceed recommended running capacity on its continuous rating and meet or exceed recommended starting capacity on its surge rating. Whichever of the two is harder to satisfy is the one that determines the size you buy.

• Running-limited — lots of steady load, no big motors (resistive heat, lighting, electronics). The continuous rating is your binding constraint and the surge rating comes along for free.
• Surge-limited — modest steady load but one stubborn motor, typically a well pump or a compressor. This is the common rural case, and it is why people end up buying a generator that looks oversized on paper.
• Approximate current is a rough check that your cordset, inlet, and breaker are in the right class. It divides recommended running watts by the voltage you entered and ignores power factor, so treat it as an order-of-magnitude figure only.

Note also that a generator's published surge rating is a short-duration figure. If your calculated starting requirement is only barely under a unit's surge rating, that unit will be working at its limit on every pump start for the life of the machine.

Total running watts — the sum of continuous watts for everything you actually intend to have on at the same time. This is where load planning matters more than arithmetic; see the load table below.

Largest motor running watts — the continuous draw of your single hardest-starting motor, not its surge. The calculator derives the surge from it.

Starting surge multiplier — how many times its running draw that motor pulls at startup. Motor type drives this: a capacitor-start induction motor behaves very differently from a modern variable-speed unit. Get the value from the nameplate, the locked-rotor amps, or the equipment manual rather than assuming.

Headroom % — deliberate margin above your computed need. It covers loads you forgot, altitude and high-temperature derating, fuel and engine wear over time, and the reality that continuous operation at 100% of rating shortens service life. 20–25% is a defensible planning margin.

Circuit voltage — the nominal voltage of the branch you are checking: 120 V for a single general-purpose circuit, 240 V for a whole-panel figure or a 240 V appliance.

Only one motor's surge is added, because starting two large motors in the same instant is something you avoid by sequencing rather than by buying a bigger machine. If two large motors genuinely can start together and you cannot prevent it — for example two independent thermostat-driven loads — sum both surge adders manually and size to that.

Illustrative figures only. Every wattage below must be replaced with the nameplate value from your own equipment before you buy anything.

Adding only the simultaneous loads gives roughly 3,550 W — call it 3,500 W. The well pump at 1,200 W with a 3× multiplier adds 2,400 W of surge, so peak starting demand is about 5,900 W. With 20% headroom that becomes roughly 4,200 W continuous and 7,100 W starting. You are shopping for a unit that clears both — and this is exactly the scenario where a nominally "7,000 W" generator is the honest answer to a 3,500 W house.

Two decisions did more work here than any arithmetic: excluding the range and dryer entirely, and agreeing to run the water heater only when the pump is idle. Load discipline is usually cheaper than engine displacement.

Generator capacity is not a fixed budget you must buy up front — it is a schedule you can manage. On a transfer-switch install with individually switchable circuits, or simply with a habit, you can run a much bigger house off a much smaller machine.

• Start big motors first, on a light generator. Bring the pump or compressor up before you load the machine with everything else.
• Never let two large motors start in the same window. If the well and the sump can both call at once during a storm, either size for both surges or actively manage one.
• Batch the resistive hogs. Water heating, well pumping to a pressure tank, laundry, and any electric cooking are all deferrable. Do them one at a time, deliberately.
• Shed before you strain. If the engine bogs, lights dim, or the unit hunts, drop load immediately rather than waiting for the overload to trip.
• Beware "always on" loads you forgot. Well pressure switches, freezers, HVAC controls, septic pumps, and livestock waterers all cycle without you touching anything.

Most portable generators produce 240 V split into two 120 V legs. Total wattage alone does not tell you whether a given load mix will work.

• 240 V loads need genuine 240 V output. A well pump, a deep-well control box, or a 240 V compressor cannot be run from a 120 V-only generator regardless of its wattage rating.
• Per-leg capacity is roughly half the total. Piling all your 120 V load onto one leg can overload that leg while the machine's total output looks only half used. Spread 120 V circuits across both legs.
• Check the outlet, not just the number. A generator's usable output is limited by the receptacles and breakers it actually has — a large-capacity unit with only 120 V 20 A receptacles cannot deliver its rating to one appliance.
• Neutral bonding and grounding rules differ between standalone use and transfer-switch use. This is one of several reasons the connection side belongs to a licensed electrician.
• Frequency and voltage quality matter for electronics and motors. Inverter generators hold cleaner output; some sensitive equipment and variable-speed motors care about this.

The calculator is only as good as the numbers you feed it. Before ordering, physically confirm:

• Well pump: nameplate voltage, running amps or watts, service factor, and locked-rotor amps or the manufacturer's stated starting requirement. Deep-well pumps are the single most common reason a generator turns out undersized.
• Furnace or boiler: blower motor rating plus control and ignition loads, from the unit's data plate — not the fuel input rating.
• Refrigeration: nameplate running watts on each fridge and freezer, remembering that they cycle independently of your wishes.
• Anything with a compressor: starting requirement stated by the manufacturer, plus whether it uses a hard-start kit.
• Generator side: continuous rating (not just the headline surge number), fuel type and consumption at your expected load, receptacle configuration, altitude and temperature derating, and rated duty cycle.
• Any equipment with soft-start or variable-speed drive: its surge behavior may be far lower than a classic induction motor's, which can save you a generator size.

• It uses a single surge multiplier on one motor. Real motors vary widely, and hard-start kits, soft starters, and VFDs all change the picture.
• It ignores power factor, so the current figure is approximate and reactive loads may stress the alternator more than the watt number implies.
• It does not derate for altitude or high ambient temperature — both reduce real output, sometimes substantially.
• It does not size fuel supply, run time, propane vaporization rate, wire, breakers, transfer equipment, or grounding.
• It is not a load calculation for permitting purposes, and it is not a substitute for an electrician's assessment of your service and panel.

• Sizing to the headline surge number on the box and discovering the continuous rating is far lower.
• Forgetting the well pump entirely, then losing water on day one of an outage.
• Summing every appliance in the house instead of only what runs simultaneously — buying twice the machine you need.
• Loading one 120 V leg while the other sits idle.
• Running the unit at or near 100% of rating for days and shortening its life.
• Planning a whole-house backup with no transfer switch and improvising a connection during the outage.
• Running a generator in a garage, shed, breezeway, or under an open window.

If your goal is keeping a small number of critical 12 V loads alive rather than backing up a house, a battery and solar setup is often the better fit — start with the 12V wire voltage drop calculator and the batteries and 12V hub.