Generator Size Calculator
Find the generator size your home needs — running watts plus motor starting surge, sized to a real standard generator — with U.S. pricing, fuel cost for an outage, a size comparison, and a downloadable PDF load list.
Wattages are typical U.S. averages — check the nameplate on your own equipment. Motors (AC, well pumps, compressors) draw a large starting surge. Estimates use 2024-2025 U.S. prices. For planning purposes only — a licensed electrician must size and connect a standby unit.
| Size | Continuous Watts | Surge Watts | Headroom | Est. Unit Cost |
|---|
For your 0 W running load and 0 W peak surge, at $400/kW. Surge column assumes a 1.25× starting rating.
How to use the generator size calculator
Four quick steps turn a list of the appliances you want to power into a recommended generator size — running watts, starting surge, and headroom all worked out — with a scenario comparison and a downloadable PDF.
List your appliances
Tick the items you want to run at once, or add custom ones with their watts.
Check the watts
Each item’s running and starting watts are filled in, or enter your own from the label.
Set the headroom
Keep the standard ~20% safety margin or adjust it to how hard you’ll run the unit.
Read the size & save a PDF
Get the recommended watts and kVA, a load breakdown, and a downloadable summary.
How the generator size is calculated
Sizing rests on two numbers per appliance — running watts and starting watts. You add all the running loads, add the single biggest starting surge, then add headroom. Here is the exact math.
Worked example — essential home backup
Running: fridge 700 + furnace 800 + sump 1050 + lights 400 + misc 200 = 3,150 W
Biggest surge: furnace fan starts at 2,350 − 800 = +1,550 W
Peak demand: 3,150 + 1,550 = 4,700 W
+20% headroom: 4,700 × 1.20 ≈ 5,640 W
≈ 5,640 W peak with headroom → a 6,000 W (6 kW) generator is the safe fit.
Generator sizing charts
Handy lookups for the questions people ask most — appliance watts, what each generator size runs, and how to convert watts to kVA and amps. Wattages are typical; always check your own appliance labels.
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Microwave | 1,000 | 1,000 |
| Lights (10 × LED/incand.) | 100–600 | same |
Motors (fridges, pumps, AC) surge on start; resistive loads (lights, microwave, heaters) don’t, so their starting and running watts match. Always confirm with the nameplate.
| Generator size | Roughly powers | Best for |
|---|---|---|
| 2,000 W (2 kW) | Lights, laptop, phone, small fridge | Camping, tailgating |
| 3,500 W | Fridge + lights + small items | Bare essentials |
| 5,000–7,500 W | Fridge, furnace, sump, well pump | Essential home backup |
| 10,000–12,000 W | Adds window AC and more circuits | Larger backup |
| 18,000–26,000 W | Most of the house, central AC, range | Whole-house standby |
Guidance only — add up your specific loads, since a single big item like central AC or a well pump can push you to the next size up.
| Watts | kW | kVA | Amps @240V |
|---|---|---|---|
| 2,000 | 2.0 | 2.5 | 8.3 |
| 3,500 | 3.5 | 4.4 | 14.6 |
| 5,000 | 5.0 | 6.2 | 20.8 |
| 7,500 | 7.5 | 9.4 | 31.2 |
| 10,000 | 10.0 | 12.5 | 41.7 |
| 22,000 | 22.0 | 27.5 | 91.7 |
kVA = kW ÷ power factor (~0.8 for generators). Amps = watts ÷ volts — halve the amps by using 240V instead of 120V for big loads.
| Type | Typical size | Best for |
|---|---|---|
| Inverter | 1–4 kW | Quiet, clean power for electronics |
| Portable | 3–10 kW | Home backup on a budget, manual start |
| Standby | 10–26+ kW | Automatic whole-house, wired in |
Standby units start automatically through a transfer switch and must be installed by an electrician. Portables need a proper interlock or transfer switch to power house circuits safely.
starting spike
running watts
~20% spare
Sizing a generator is three moves: add the steady loads, cover the biggest surge, and leave a little headroom.
- Running watts — add up everything that will be on at the same time.
- One surge — add only the largest starting spike, since motors rarely all start at once.
- Headroom — add ~20% so the generator isn’t run flat out, which extends its life.
Everything the calculator works out
One appliance checklist gives you a complete sizing answer — the loads, the surge, the recommended size, and the conversions.
Key figures behind generator sizing
Built for anyone sizing a generator
From a camping inverter to a whole-house standby, the same calculator scales to the loads you need to keep running.
Wants to keep the fridge, heat, and a few lights on during outages without buying more generator than needed.
- Tick your must-run appliances
- Note the biggest motor surge
- Round up to the next size
Sizing a portable or inverter unit for a camper, tailgate, or job site and watching weight and noise.
- List only what runs at once
- Mind AC and microwave surges
- Pick an inverter for electronics
Specifying a standby generator and transfer switch and needs the load and kVA figured before quoting.
- Total the circuits to cover
- Convert watts to kVA & amps
- Have an electrician wire it in
7 tips for sizing a generator right
Small habits keep your sizing accurate and your generator safe, whether it’s a portable or a standby.
Generator size calculator FAQ
The sizing, running-vs-starting, conversion, and whole-house questions people ask most.
Add up the running watts of everything you want to power at once, add the single largest starting surge among them, then add about 20% headroom. That total in watts is the minimum generator size you need.
A typical essentials backup of a fridge, furnace fan, sump pump, and lights comes to roughly 4,700 watts of peak demand, so a 6,000-watt generator is a safe fit. Bigger loads like central AC or a well pump push you higher.
Running watts are the continuous power an appliance draws while operating. Starting watts, also called surge watts, are the brief spike needed to start a motor or compressor, often two to three times the running watts.
A fridge might run at 700 watts but need 2,200 to start. A generator has to cover both the total running load and the biggest single surge, which is why sizing uses both numbers.
List every appliance you want to run at the same time with its running and starting watts. Add all the running watts together, then add only the largest starting surge, since motors rarely all start at once.
That gives your peak demand. Add roughly 20% headroom so the generator isn’t run flat out, and round up to the next available size. The calculator does this from a simple appliance checklist.
A 5,000-watt generator can typically run a refrigerator, several lights, a furnace fan, a sump pump, and small electronics at the same time, which covers most essential home backup needs.
It usually can’t also run central air conditioning or an electric range, which are large loads on their own. Add up your specific appliances rather than relying on a rule of thumb, since starting surges can eat into that 5,000 watts quickly.
Motors and compressors draw a large surge of current at the instant they start, because they have to overcome inertia and magnetic resistance to get moving. Once up to speed, they settle to their lower running draw.
That’s why fridges, pumps, air conditioners, and power tools list a starting watt figure well above their running watts. Purely resistive loads like lights and heaters have no surge, so their starting and running watts are the same.
Divide watts by 1,000 to get kilowatts (kW). To get kVA, divide the kW by the power factor, which is about 0.8 for a typical generator, so kW divided by 0.8 gives kVA. For amps, divide watts by the voltage.
For example, 5,000 watts is 5 kW, about 6.25 kVA, and roughly 21 amps at 240 volts. Generators are often rated in both watts and kVA, so it helps to convert between them.
Powering a whole house, including central air, electric range, water heater, and all circuits, usually needs a standby generator in the range of about 18 to 26 kW, wired through a transfer switch by an electrician.
Many people instead choose a smaller generator that runs only essential circuits, which is far cheaper. Add up the specific loads you must keep running, since not every home needs a full whole-house unit.
A modest amount of extra capacity is good, and adding about 20% headroom over your peak demand is standard, so the generator isn’t constantly run at full load. That improves efficiency and lifespan and leaves room to add a device.
But a hugely oversized generator wastes fuel and money and can run inefficiently under light load. Size it to your real peak demand plus a sensible margin rather than buying the biggest unit available.
General Estimating Notice: This calculator estimates a recommended generator size from the appliances you select, their running and starting watts, and a headroom margin, using typical wattages and standard conversions. Actual appliance draw varies — always check the nameplate ratings — and correct sizing depends on your specific loads and how they run together. Generator installation, transfer switches, and any connection to house wiring must be done by a qualified electrician to local code, and generators must be run outdoors, away from living spaces, because of carbon monoxide. For planning purposes only.