Skip to content
Calcapedia
Menu
Home & Garden

Generator Size Calculator

Find the backup or portable generator size you need from the running and starting watts of the appliances you want on at once.

Level: Grade 4

  • Common Core standard 4.OA.A.3Solve multistep word problems posed with whole numbers and having whole-number answers using the four operations, including problems in which remainders must be interpreted. Represent these problems using equations with a letter standing for the unknown quantity. Assess the reasonableness of answers using mental computation and estimation strategies including rounding.
Printable worksheet

Generator Size Calculator

Inputs

Spare capacity so the generator isn't run at its limit. 10–25% is a common allowance.

Result

Starting (peak) watts needed

4,550 W

Look for a generator rated at least 3,660 W running and 4,550 W starting (peak) watts.

Total running watts
3,050 W
Suggested running rating (with 20% headroom)
3,660 W
Largest starting surge
1,500 W
Suggested running rating in kilowatts
3.66 kW
Share of the running load
  • Refrigerator or freezer70023%
  • Sump pump (1/2 hp)1,05034.4%
  • Furnace fan (1/2 hp)80026.2%
  • Lights (LED, several rooms)2006.6%
  • TV, internet router and phone chargers3009.8%

See the step-by-step solution ↓

Appliances included
Appliances included
ApplianceRunning wattsAdditional starting watts
Refrigerator or freezer7001,500
Sump pump (1/2 hp)1,0501,150
Furnace fan (1/2 hp)8001,350
Lights (LED, several rooms)2000
TV, internet router and phone chargers3000

Step-by-step solution

  1. Add the running watts of everything that may be on at once, since they all draw power together

    700+1,050+800+200+300=3,050 W

  2. Find the biggest start-up burst: the refrigerator or freezer needs 1,500 W extra for a moment as it starts

    1,500 W

  3. Peak load: add the one biggest start-up burst, since motors rarely start at the same time

    3,050+1,500=4,550 W

  4. Add 20% headroom (spare room) so the generator is not run at its limit. This is its running (rated) watts

    3,050×1.2=3,660 W

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how big a generator you need to run the things that matter most during an outage. Every generator has two ratings worth knowing. Its running (rated) watts are what it can supply hour after hour. Its starting (peak or surge) watts are a short burst of extra power it can give for a few seconds. Appliances with motors need that burst to get going. Think refrigerators, sump pumps, furnace fans, well pumps and air conditioners. The burst can be well above what they use once they're running.

This calculator follows the usual sizing worksheet. First it adds up the running watts of everything you want on at once. Then it adds the one biggest starting burst, because motors rarely all start in the same instant. Typical figures for common household items are filled in to get you started. You can change every one, so swap in the numbers from your appliances' labels or manuals.

Where it's used

Why this math matters outside the classroom:

  • Storm preparation. A homeowner adds 700 + 1,050 + 800 + 200 + 300 = 3,050 running watts and a 1,500 W surge to choose a 4,550 W peak generator.
  • Camping and RVs. Campers check that a 1,200 W microwave and a 300 W fan fit a 2,000 W portable generator with 500 W to spare.
  • Job sites. A crew adds the running watts of saws and lights, then the largest motor surge, before renting a generator.
  • Multistep word problems. Students add several loads and then the largest of a second list, a two-step problem with whole numbers.

Grade level and standards: Grade 4; Common Core State Standards for Mathematics 4.OA.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Tick each appliance you want to keep running during an outage.
  2. Check the running watts and additional starting watts for each one against its label or manual. Change them if needed. Use Other appliance for anything not listed.
  3. Set a headroom percentage (spare capacity) so the generator isn't run at its limit.
  4. Compare the running and peak figures with the two ratings on any generator you're considering.

What the inputs mean

Refrigerator or freezer
Starting watts here are the extra watts needed for a moment at start-up, on top of the running watts. That's how many generator charts list them.
Other appliance: running watts
If a label lists amps instead of watts, multiply amps by volts to get running watts. A US home outlet is usually 120 V.
Headroom on the running load
Running a generator near its maximum shortens its life and leaves no room for one more load. 10–25% spare capacity is a common allowance.

Formula and method

total running watts = sum of running watts of the chosen appliances
peak watts = total running watts + largest additional starting watts
suggested running rating = total running watts × (1 + headroom ÷ 100)
running watts from a label = volts × amps

Running watts are the steady power each appliance uses, and additional starting watts are the extra it needs for a moment to start. Headroom is the spare capacity you chose, as a percent. Volts and amps come from an appliance's label, and multiplying them gives watts.

Everything you run at once draws its running watts the whole time, so those add up. A motor's starting burst lasts only a second or two. Two motors rarely start at exactly the same moment. So only the largest burst is added to find the peak the generator must handle.

Choose a generator whose running rating is at least the suggested running rating. Its starting rating should be at least the peak watts. Sometimes two large motors could start together, for example a well pump and an air conditioner on the same thermostat cycle. In that case, add both bursts to be safe.

Worked example

During an outage you want to run a refrigerator, a 1/2 hp sump pump, a 1/2 hp furnace fan, LED lights, and a TV with the internet router. You use the typical figures and 20% headroom.

  1. Running watts: 700 + 1,050 + 800 + 200 + 300 = 3,050 W.
  2. Largest additional starting watts: the refrigerator's 1,500 W.
  3. Peak watts: 3,050 + 1,500 = 4,550 W.
  4. Suggested running rating: 3,050 × 1.2 = 3,660 W.

Result: Look for a generator rated at least 3,660 running watts and 4,550 starting watts.

Understanding the result

The peak figure is the one people most often guess too low. If the generator's starting rating is too small, a motor may stall as it starts. Or it may trip the generator's breaker, even though the running load is fine.

If the numbers are higher than you want to pay for, take turns instead. Run the well pump or air conditioner only when other large motors are off. Untick them here to see the smaller generator you'd need.

Assumptions and limitations

  • Only one motor starts at a time.
  • All ticked appliances may run at the same time.
  • Additional starting watts are entered on top of running watts, not as a total.
  • The preset figures are typical starting points, not measurements of your appliances. Labels and manuals vary widely.
  • The calculator doesn't cover transfer switches, wiring, fuel use, power loss at high altitude (derating) or voltage needs such as 240 V well pumps. Connecting a generator to home wiring should follow local code. It's a job for a licensed electrician.
  • Sensitive electronics may need an inverter generator with clean, steady output. Wattage alone doesn't address that.

Frequently asked questions

What size generator do I need to run a house?

It depends entirely on what you run. Essentials such as a refrigerator, sump pump, furnace fan, lights and electronics often fit within a few thousand running watts. Central air conditioning, electric water heaters and ranges add far more. Tick your essentials and read the totals.

What is the difference between running watts and starting watts?

Running watts are the steady power an appliance uses once it's going. Starting watts are the brief extra power a motor needs to get moving, which lasts a few seconds. Generators list both: a running rating and a higher starting or peak rating.

How do I find an appliance's wattage?

Look for a label or plate on the appliance, or check its manual. If it gives amps, multiply by the voltage: a 120 V appliance drawing 5 A uses about 600 W. Starting watts are listed less often. For motors, a manufacturer's chart or the generator maker's guide can help.

Why add headroom?

A generator running near its maximum all the time works hard. It uses more fuel for each useful watt and leaves no room for an extra light or a phone charger. Sizing the running rating 10–25% above the load gives you a margin.

How this calculator is checked

Method: Standard generator sizing worksheet: sum of running watts plus the largest single additional starting load; headroom as a percentage of the running load. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

Embed this calculator

Put this Generator Size Calculator on your own website or blog for free. Paste this code where you want it to appear: