Sizing and cost calculators
Generator size calculator
No result yet. Enter your values, then calculate.
Method and printable worksheet
Inputs are your assumptions unless a document is supplied. Check source numbers and compatibility in the matching manufacturer manual.
For group G, R = sum(quantity × running_W). One-at-a-time S = R + max(max(starting_total_W - running_W,0)) for one starting unit; simultaneous-start S = sum of total starting watts for starting units plus running watts for all others. Quantity and selected starting-unit counts must be explicit. Apply user headroom h as R×(1+h/100) and S×(1+h/100). Never add total starting watts to the same motor running watts twice. Compare continuous and surge requirements separately; transient duration compatibility is a separate manual check.
Synthetic arithmetic check, not appliance data: rows A run 600 W/start 1800 W and B run 400 W/start 1200 W, each quantity 1. R=1000 W, sequential S=2200 W, simultaneous S=3000 W, with h=0. Use your own appliance ratings for a capacity estimate.
| Item / exact model | Quantity and units | Low / running | High / starting | Source and date |
|---|---|---|---|---|
| Record 1 | ________ | ________ | ________ | ________ |
| Record 2 | ________ | ________ | ________ | ________ |
| Record 3 | ________ | ________ | ________ | ________ |
Worksheet: https://bestgeneratorforhome.com/what-size-generator-do-i-need/generator-size-calculator/
Review the planning guide. Check the exact manual before using any result. Report a method correction.
The short answer
Use the generator size calculator to plan simultaneous loads. It reports running watts and startup watts separately. Enter nameplate or manufacturer data for each row, then choose a startup scenario. Missing running data blocks a result. Missing surge data allows only a labeled subtotal. Check surge duration, voltage, receptacles and fuel derating separately for any generator.
Last updated October 2026
What's covered8
This worksheet adds up the loads you plan to run at the same time. It gives two numbers: running watts and startup watts. It starts with a sourced, editable example. If a key number is missing, it says so.
A generator size calculator adds the running watts of every load in one group. It then adds startup demand for the scenario you choose. That can be one motor starting at a time, or several starting together. If a running value is unknown, there is no result. If a surge value is unknown, you get only a labeled running subtotal.
Generator size calculator inputs and scenarios
| Item | One motor starts at a time | Motors start together |
|---|---|---|
| Running total (R) | Sum of quantity × running watts | Same R |
| Startup total (S) | R plus the largest single start gap | Full starting watts of starting units plus running watts of all others |
| Unknown surge | Running subtotal only, labeled incomplete | Running subtotal only, labeled incomplete |
| Headroom h | R × (1 + h/100) and S × (1 + h/100) | Same rule |
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Each row holds a label, quantity, running watts, total starting watts and voltage. It also holds a group, a source or model, and a known or unknown status. The initial rows use Honda's generic fridge/freezer estimate and a 60 W bulb example. The fridge uses the upper 192 W running estimate and 1,200 W total start.
These are estimates, not your actual loads. Replace them with exact model data. Voltage stays unknown.
Headroom is optional, and the default is 0 percent. It is a planning choice, not a code rule. When you add it, it applies to R and S on their own. The example source is the undated Honda US estimation guide.
Interactive results need JavaScript. Without it, the formulas and a blank printable worksheet still appear.
How the math avoids double counting
A start gap is a unit's total starting watts minus its running watts. A negative gap counts as zero. Its running watts are already in R, so only the gap is added for startup. Adding the full starting value on top of R would count that motor twice.
The one-at-a-time scenario assumes you can stagger starts. Choose the together scenario if motors may start at once. Do not assume they never will. You can read more in our guide to running wattage vs starting wattage.
Some rows come from volts times amps. That gives volt-amps (VA), not watts, unless the power factor is known. The worksheet asks for an explicit power factor before it converts VA to watts.
Worked example (illustrative only)
These numbers are made up to show the arithmetic. They are not appliance data. Row A runs at 600 W and starts at 1,800 W. Row B runs at 400 W and starts at 1,200 W.
Each has a quantity of one.
R is 1,000 W. Row A's gap is 1,200 W, and Row B's gap is 800 W. One at a time, S is 1,000 W plus 1,200 W, or 2,200 W. Starting together, S is 1,800 W plus 1,200 W, or 3,000 W.
Headroom is 0 percent here.
When data is missing
If any running value is unknown, the tool gives no complete capacity result. If a motor's surge is unknown, it shows a running subtotal marked incomplete. A size built on a guessed surge is not a confirmed size.
Some loads cannot be inferred from general facts. Heat pump auxiliary heat cannot be derived from tonnage. Pump startup cannot be derived from well depth. Use the exact nameplate and manufacturer documents for these rows.
Each row should record whether its value is observed, derived or assumed.
Checking a generator against your result
You can enter a generator's continuous and surge ratings. The tool then marks continuous, surge and voltage as supported, insufficient or UNKNOWN. Each field is checked on its own, and a unit can pass one and fail another. This is not a full compatibility result or a product pick.
Voltage differs between listed models. The Honda EU2200i is a 120 V unit rated 1,800 W and 2,200 W maximum. The Champion 201176 lists 120/240 V output. A basic adapter cannot create a voltage the generator does not supply.
Fuel changes output. The Champion 201176 lists 7,250 W running on gasoline. It lists 6,525 W on propane and 5,875 W on natural gas. The Westinghouse WGen11500TFc lists 11,500 W running on gasoline and 9,500 W on natural gas.
Enter the rating for the fuel you will use. Surge duration and receptacle fit come from the exact manual.
Battery stations add an energy limit. The BLUETTI AC200L lists 2,400 W rated output and 2,048 Wh capacity. Watts limit what runs at once. Watt-hours describe stored energy.
Home connections are a separate job. Generac states that the generator, switch, inlet, panel and circuits must match. Licensed electrical trades install and service that equipment. A connection part has its own limits.
For example, the Reliance TF152W is a one-circuit, 15 A switch rated for 1,875 generator running watts.
Sources, method and limits
Unit rules come from the U.S. Department of Energy style guide. Placement rules come from CPSC.
Model figures come from each manufacturer's US page.
The CPSC Carbon Monoxide Information Center says to run engine generators outdoors. Keep them at least 20 feet from homes, with exhaust pointed away. Never run one indoors or in a garage, even with the door open. The CPSC safety alert adds rules on dry surfaces and refueling.
It also warns never to backfeed through a wall outlet. Standby clearances come from the unit's manual and local rules.
This worksheet does not cover whole-house installation, fuel lines or permits. For broader planning, start at what size generator do I need.
Generator size calculator FAQs
What numbers does the worksheet need?
Each row needs quantity, running watts and total starting watts for motor loads. It also needs voltage and a source. Use the nameplate or exact manufacturer documents. If running watts are missing, there is no result.
If surge is missing, you get only a labeled running subtotal.
Why do the two scenarios give different totals?
One at a time adds only the largest single start gap to the running total. Starting together adds every starting unit's full starting watts, plus running watts for everything else. Choose the together scenario if motors may start at once. Never assume they cannot.
Can I enter volts times amps?
Yes, but that product is volt-amps, not watts. The worksheet converts it to watts only when you enter a documented power factor. Without one, the row stays marked as VA. Treating VA as watts can misstate demand in either direction.
Does a passing result mean a generator fits?
No. The tool checks continuous, surge and voltage fields separately. You still need to confirm surge duration, receptacles and the rating for your chosen fuel. A home connection also needs matched equipment installed by licensed trades.
Watts alone do not prove a fit.
Primary documents
- CPSC portable engine generator placement
- Honda EU2200i exact US specifications
- Champion 201176 operator's manual specifications
- BLUETTI AC200L exact US specifications
- Westinghouse WGen11500TFc US product specification
- Reliance Controls TF152W single-circuit transfer
- CPSC generator dry-surface and refueling safety
- Power and energy units
- Generac home connection safety
- Honda US approximate appliance demand
- BLUETTI AC200L cycle/storage record
Generator size calculator Verdict
Generator size calculator: Use this worksheet to plan one group of loads you will run at the same time. Treat its output as confirmed only when every row has documented running and starting data. If a generator passes on watts, check voltage, receptacles, fuel derating and surge duration in its manual.
Continue your backup plan
Choose your next step
Once you have a size, read the generator sizing guide and follow its link to the home generator picks. Match the exact fuel, voltage and starting limits to your result before you choose.