Generator Size Calculator
Build a load list with running and total starting watts. The calculator automatically finds the single largest incremental startup demand.
Use the nameplate or manufacturer running wattage when available.
Enter total watts during startup, not only the extra surge. Leave blank for a non-motor load.
Use simultaneous only when the listed loads can realistically energize or restart together.
How to Use
- Enter loads that can run together - add running watts for the equipment you expect the generator to carry at the same time.
- Enter total starting watts per motor load - for a refrigerator, pump, compressor, or other motor-driven load, enter its total startup watts next to its running watts. The calculator subtracts running watts automatically to find the incremental surge.
- Leave starting watts blank when there is no surge - resistive and electronic loads that do not have a meaningful startup surge can use running watts only.
- Use reserve only if you want it - reserve changes the continuous planning target. It does not replace altitude, temperature, fuel, voltage, phase, or manufacturer derating.
- Choose the startup scenario - use the staggered model when one motor starts at a time. Use simultaneous only when multiple listed surge loads can realistically start or restart together.
- Match two generator ratings - the actual generator should satisfy both its continuous/running rating and its short-duration starting/surge rating.
Generator sizing has two separate limits. Total every load that may run at the same time. Then find the one appliance or tool with the largest difference between starting watts and running watts. In the normal staggered-start mode, the peak-starting target is total running watts plus the single largest incremental surge. If several surge loads can start together, switch to the simultaneous scenario and the calculator adds their incremental starting demands. CalcShed calculates both from the per-load values automatically.
Why Starting Watts Are Not Simply Added Together
Running Watts vs Total Starting Watts
Running watts are the steady load after equipment is operating. Starting watts are the temporary total draw during startup. The incremental surge is starting watts minus running watts for that same load.
Example: a pump rated 1,000 W running and 3,000 W starting adds 2,000 W of incremental startup demand. If the other simultaneous loads total 2,500 W, the peak-starting target is 4,500 W, not 5,500 W. The pump's 1,000 running watts were already included in the 2,500 W total.
The calculator checks each entered load and automatically uses the largest incremental surge. That avoids the common double-counting mistake and makes it obvious which item sets the starting requirement.
When One-Surge-at-a-Time Is Not Conservative Enough
The calculation assumes the selected loads are already running and one motor starts at a time. That is a common practical planning model, and load management can intentionally stagger motor starts.
If a transfer sequence, automatic control, or process can start more than one large motor at the same time, do not use this one-surge assumption blindly. Model the actual simultaneous-start sequence or use manufacturer/engineering data for the equipment package.
Likewise, altitude, high ambient temperature, fuel type, harmonic/nonlinear loads, motor starting method, voltage dip limits, and generator alternator/engine characteristics can change the required equipment rating.
Worked Load-List Example
| Load | Running watts | Total starting watts | Incremental surge |
|---|---|---|---|
| Refrigerator example | 200 W | 1,200 W | 1,000 W |
| Sump-pump example | 800 W | 2,000 W | 1,200 W |
| Lighting / electronics | 300 W | 300 W | 0 W |
Illustrative numbers only. Replace every value with the actual nameplate, manual, or manufacturer data for the equipment you plan to run. In staggered-start mode, the example totals 1,300 W running and 2,500 W peak starting because the 1,200 W sump-pump increment is the largest single surge.
Generator Sizing Formulas
For the entered load list:
Incremental surge per load = max(starting watts − running watts, 0)
Largest startup increment = Maximum incremental surge among entered loads
Staggered-start target = Total running load + largest single startup increment
Simultaneous-start target = Total running load + sum of all entered startup increments
Continuous target = Total running load × (1 + optional reserve %)
Governing planning target = Larger of continuous target and starting target
Continuous target apparent power = Continuous target ÷ (1000 × PF)
Match the actual generator's continuous/running output to the continuous target and its temporary starting/surge capability to the starting target.
Authoritative References
- Honda publishes running and starting wattage guidance and recommends load management or staggered starts when practical rather than assuming all starting loads hit at the same time.Honda Power Equipment - Generator Wattage Estimation Guide
- Generac's official wattage worksheet totals simultaneous running watts and then adds the single highest additional starting-watt requirement, which matches the default staggered-start method used here.Generac - Home Standby Generator Buyer's Guide Wattage Worksheet
- Portable generators produce carbon monoxide. CDC guidance says to operate a generator outside and at least 20 feet from windows, doors, and vents.CDC - Carbon Monoxide Poisoning Prevention
Next Steps
After estimating wattage, check the electrical side of the installation:
Related Calculators
Generator wattage is only the source-capacity question. Browse the electrical calculator collection for current, conductor, protection, and voltage-drop checks.
FAQ
Should I add every appliance's starting watts together?
Not in the normal staggered-start planning model. Add all simultaneous running watts, then add the largest single incremental startup demand. If your system can start multiple large motors at once, model that actual sequence separately.
What if starting watts are not listed?
Check the equipment manual, nameplate, or manufacturer support data. Do not invent a surge multiplier when the generator purchase or installed design depends on the result.
Why is reserve optional?
There is no universal rule that every generator calculation must add the same 20% or 25%. Reserve can be useful for future load or operating preference, but site derating and manufacturer limits should be handled from actual product data.
Does this choose a specific generator model?
No. It gives continuous and starting wattage targets. Verify the chosen model's running output, surge capability, voltage, phase, receptacles, transfer equipment, fuel/derating data, grounding/bonding instructions, and installation requirements.
Can a portable generator run inside a garage with the door open?
No. Carbon monoxide can build up quickly. CDC recommends operating portable generators outside and at least 20 feet from windows, doors, and vents.