MundoKit

Generator Size Calculator

Choose editable appliance estimates or custom manufacturer ratings. Select an operating mode to compare running and surge targets with a candidate generator.

Inputs
Initial-only sizing requires turning other loads off as needed before every restart. Automatic cycling cannot be assumed to follow the initial sequence.
Honda estimates are approximate. The sump-pump preset is flagged as requiring 240 V; the refrigerator preset is not. Verify nameplate voltage, phase and circuits for your actual appliances.
Use manufacturer total starting watts, not additional surge watts. Up to 50 rows. Default motor values are hypothetical.
Blank starts each row in sequence. Otherwise enter one group per row, separated by commas, e.g. 1,2,3. Equal groups start together; groups start in ascending order. Preset order: refrigerator, then sump pump.
%
W
W

Results

Selected operating-mode surge target with margin
2,280 W
Continuous running target with margin
1,080 W
Simultaneous startup target with margin
3,000 W
Single unit restarting with all others running
1,900 W (before margin)
Selected operating-mode apparent-power estimate
2,280 VA
Candidate watt-rating comparison
Enter both candidate ratings
Startup sequence before margin (earlier groups keep running)
GroupPeak watts at startupRunning watts after startup
11,500500
2600600
31,500900
  • Controlled mode uses the larger of initial staged startup and a single-unit restart with all other loads running. Restarts must not overlap.
  • VA here assumes the same minimum power factor for all loads and startup states. Use actual manufacturer VA and surge-duration specifications when available. Check voltage, phase, circuit limits and environmental derating separately. A surge rating must sustain actual startup duration.
  • The margin is a user assumption, not a manufacturer recommendation. Portable generators must be used outdoors away from openings; use a qualified electrician for a home connection and never backfeed a wall outlet.

Results update as you type. The share link stores your inputs after the # in the address, which is not sent to our server, but anyone you send it to can read them.

How it works

Honda distinguishes starting from running requirements and recommends managing loads. Enter manufacturer values for your equipment rather than multiplying all motor watts by an assumed factor. Total starting watts already include running watts.

Running load = sum(quantity × running watts) Group startup peak = earlier groups’ running load + current group’s total starting load Simultaneous peak = sum(quantity × total starting watts) Target = modeled load × (1 + chosen margin ÷ 100) Estimated VA = watts ÷ assumed power factor

Groups start in ascending order. Earlier loads keep running, later loads remain off. The displayed single-unit restart assumes all other units are already running, including other units on the same row. Controlled nonoverlapping restarts use max(initial staged peak, single-unit restart peak) before margin. Uncontrolled overlap uses simultaneous startup. Initial-startup-only mode requires explicit load shedding before later restarts, as Honda’s power-management guidance describes. Startup-group editing is available in the two staged modes. Margin is your assumption. Neither watt capacity nor VA capacity alone establishes compatibility; duration, voltage and circuit ratings also matter.

Custom default motor loads are hypothetical. The editable household presets use Honda’s approximate table: refrigerator/freezer 1,200 starting W and 132–192 running W (192 prefilled); ⅓-hp sump pump 1,300 starting W and 800 running W. Honda flags the pump as requiring 240 V, but does not flag the refrigerator. Verify actual nameplate voltage and manufacturer ratings. Calculations retain full precision; displayed W and VA use up to two decimals.

Worked examples

Three staged loads

Motor A runs at 500 W and starts at 1,500 W in group 1. Lights use 100 W in group 2. Motor B runs at 300 W and starts at 900 W in group 3. Startup peaks are 1,500, 600 and 1,500 W. Total running load is 900 W; simultaneous startup is 2,500 W. A 20% margin gives an initial-only staged target of 1,800 W and continuous target of 1,080 W. Motor A restarting with the other loads running needs 1,900 W before margin, so controlled-restart mode requires 2,280 W surge. Uncontrolled simultaneous startup requires 3,000 W with margin.

Quantity and apparent power

Two identical devices each running at 100 W and starting at 300 W in the same group require 200 running W and 600 startup W. With zero margin and assumed power factor 0.8, startup apparent power is 750 VA.

Frequently asked questions

Do I add running watts to starting watts?
Only if the manufacturer explicitly provides additional surge watts. This worksheet takes total starting watts, so it does not add running watts again for a starting device.
Does staggering reduce generator size?
It can reduce the initial peak. Connected loads still need capacity for single-unit restarts; controlled mode includes this. Use simultaneous mode when starts can overlap. Initial-only sizing requires shedding loads before restarts.
Is there one motor starting multiplier?
No. Use equipment-specific manufacturer data and the required surge duration. Different motors and starting systems vary.
What does the candidate result mean?
It compares continuous and surge watt ratings with the selected operating-mode targets. It does not check VA rating, surge duration, wiring, voltage or environmental derating.
Can this size a home connection?
It is a load worksheet. A qualified electrician must assess the connection, transfer equipment and circuit requirements. Never connect a generator by backfeeding a wall outlet.

Sources

Last reviewed 2026-11-14. The method is written out above so you can check our math.