Generator Size Calculator
Generator sizing goes wrong in one specific way: adding up the starting watts of every appliance. Motors do not all start in the same instant. The realistic worst case is everything already running while your single largest motor kicks in, and that number is far smaller. This calculator uses that method, and shows what the naive sum would have told you so you can see the gap.
- Generator size needed
- 8600 W Limited by motor starting surge. About 8.6 kW.
- Running requirement
- 6,000 W 5,000 W plus 20% headroom
- Starting surge requirement
- 8,600 W Everything else running (3,800 W) plus the largest motor starting (4,800 W)
- Largest motor
- 1,200 W → 4,800 W Capacitor start — well pump, compressor — starts at about 4× its running draw
- If you summed every surge
- 20,000 W The common mistake. Motors do not all start in the same instant, so this oversizes by about 133%.
Why motors surge and resistive loads do not
An induction motor at rest has no back-EMF opposing the supply, so the winding looks close to a short circuit at the moment power is applied. Current rushes in at six to eight times the running value until the rotor accelerates and the motor begins generating opposition to its own supply.
How long that lasts depends on what the motor is turning. An unloaded fan reaches speed in a fraction of a second. A compressor starting against head pressure, or a well pump lifting a column of water, can take two or three seconds — long enough for a marginal generator to sag, stall or trip.
Resistive loads have no surge at all. A heating element, an incandescent lamp or a kettle draws its rated current the instant it is switched on and never more. This is why a 5,000 watt heater is easier for a generator than a 1,200 watt well pump.
The starting multiplier varies by motor design. Permanent split capacitor motors, common in fridges and small fans, start around three times running. Capacitor-start motors used in pumps and compressors reach four. Older split-phase motors can hit five and are the hardest thing most generators will be asked to start.
Why summing every surge is wrong
The naive method — multiply the whole running load by the starting factor — assumes every motor in the house starts at the same instant. For that to happen, the fridge compressor, the well pump and the furnace blower would all have to be triggered within the same fraction of a second by independent thermostats and pressure switches.
It is not impossible, but it is rare enough that no sizing standard accounts for it. The accepted worst case is everything already running plus the single largest motor starting, and that is what this calculator uses.
The difference is not marginal. A 5,000 watt load with a 1,200 watt capacitor-start motor needs about 8,600 watts by the correct method and 20,000 by the naive one. That is the difference between a portable generator and a standby unit, and between a few hundred dollars of fuel a year and considerably more.
Oversizing also carries a specific penalty on diesel units. An engine running lightly loaded does not reach operating temperature, unburned fuel accumulates in the exhaust and the unit wet stacks — a fouling problem that shortens engine life and is expensive to reverse.
Soft starters change the arithmetic
A soft starter or variable frequency drive ramps voltage or frequency up rather than applying full supply instantly, which cuts starting current dramatically — often to twice running current rather than four times.
On a system where a single large motor is driving the generator size, fitting a soft start device to that one motor is frequently cheaper than the next generator size up. Air conditioning compressors are the usual candidate, and aftermarket soft start kits for residential units are common in the RV and off-grid markets for exactly this reason.
If you fit one, enter the reduced starting figure rather than the standard multiplier. The calculator has no way to know, and the running watts do not change.
What this is based on
- Accepted practice: surge = total running load − largest motor running + largest motor starting
- Typical locked-rotor multipliers by motor type (PSC, capacitor start, split phase)
An estimate for planning. Actual starting draw varies by appliance and can exceed these multipliers, particularly on hard-start compressors. Confirm against nameplate data, and have a transfer switch installed by a licensed electrician.
Frequently asked questions
Why does only the largest motor count for surge?
Because starting surge lasts a fraction of a second. For two motors to surge together they would have to start within the same instant, which is possible but rare and not worth sizing for. The accepted method is everything running plus the single largest motor starting.
What is the difference between running and starting watts?
Running watts is the steady draw once a motor is up to speed. Starting watts is the brief spike as it overcomes inertia, typically three to five times higher depending on motor type. Resistive loads such as heaters and incandescent bulbs have no surge at all.
Why leave headroom if the generator is rated for it?
Continuous operation at full nameplate output runs hot, shortens engine life and leaves nothing for an unexpected load. Most manufacturers rate a lower continuous output than peak, and 20 to 25% headroom is the usual planning figure.
Is undersizing or oversizing worse?
Both cost you. Undersized, the generator stalls or trips when a motor tries to start. Oversized, you pay more, burn more fuel, and a diesel running lightly loaded suffers wet stacking — unburned fuel fouling the exhaust.
Do I need to size for my whole panel?
Only if you intend to run everything. Most backup installations use a transfer switch covering selected circuits, so you size for that subset rather than the service rating.