Generator Size Calculator
Find the right generator size in watts and kilowatts — based on what you need to power during an outage.
Your Generator Estimate
- List the appliances you need — use the wattage table below to find running watts for each item. Add only what you'd actually run at the same time.
- Enter total running watts — this is the sum of all simultaneously-running appliances — not everything in your house, just what you'd need in an outage.
- Add surge separately — motors (AC, fridge, well pump) start at 2-3x their running watts. Your generator's surge capacity must cover the largest motor start.
- Keep headroom on — running a generator continuously at 100% shortens its life significantly. The 25% buffer is a real operating guideline.
- Match to standard sizes — generators come in fixed wattage steps — 3,500W, 5,500W, 7,500W, 10,000W. Always round up to the next standard size.
Running watts vs starting watts — the number most people miss
Every generator has two ratings: running (continuous) watts and starting (surge) watts. Running watts are what the generator delivers continuously. Starting watts are the brief spike it can handle when a motor first kicks on — typically for 1 to 2 seconds. A refrigerator compressor that runs at 200 watts can demand 600 to 800 watts at startup. An AC unit that runs at 2,400 watts might need 5,500 at startup.
Your generator must meet both requirements: its running capacity must cover all your appliances running simultaneously, and its surge capacity must cover the largest single motor starting up. If the math is close, size up to the next generator tier — a generator that stalls on motor startup is not actually doing its job during an outage.
Common appliance wattage reference
Use these as starting points. Always check the nameplate label on your actual appliances — watts vary significantly by age, brand, and efficiency rating:
| Appliance | Running watts | Starting watts | Priority during outage |
|---|---|---|---|
| Refrigerator / freezer | 150-400 W | 800-1,200 W | Essential |
| Central AC (2.5 ton) | 2,400 W | 5,500-7,200 W | Optional |
| Window AC (10,000 BTU) | 900-1,200 W | 2,200-3,000 W | Optional |
| Well pump (1/2 HP) | 750-1,000 W | 1,500-2,000 W | Essential (if on well) |
| Sump pump (1/3 HP) | 400-800 W | 1,000-1,400 W | Essential if flood risk |
| Gas furnace (blower + ignition) | 400-600 W | 1,200 W | Essential in cold weather |
| Microwave | 800-1,200 W | 800-1,200 W | Convenience |
| LED lighting (10 bulbs) | 60-100 W | 60-100 W | Essential |
| Phone / laptop chargers | 25-100 W | 25-100 W | Essential |
| Electric water heater | 3,500-5,500 W | 3,500-5,500 W | Skip — use on-demand instead |
| CPAP machine | 30-60 W | 30-60 W | Essential if medically needed |
| Electric dryer | 5,000-6,000 W | 6,500-8,000 W | Skip during outages |
Generator size guide by use case
| Generator size | Best for | What it powers | Fuel type |
|---|---|---|---|
| 2,000-3,500 W | Camping / minimal backup | Lights, phone charging, small appliances, TV | Gas (portable) |
| 3,500-5,500 W | Essential home circuits | Fridge, furnace blower, lights, chargers | Gas (portable) |
| 5,500-7,500 W | Most of the house | Above + window AC, well pump, sump pump | Gas (portable) |
| 7,500-10,000 W | Whole-home portable | Above + small central AC or electric water heater | Gas (portable) |
| 10-14 kW | Whole-home standby (small) | Entire home except large appliances | Propane / NG |
| 16-22 kW | Whole-home standby (mid) | Full home including central AC and well | Propane / NG |
| 22-30 kW | Large home / backup heat | Full home with electric heat or large AC | Propane / NG |
Standby generators start automatically when power is lost and run on natural gas or propane — no manual setup. Portable generators must be run outdoors, at least 20 feet from any door, window, or vent.
Generator sizing formulas
The core calculations behind generator sizing:
Required kW = Required watts / 1,000
Amps at 120V = Watts / 120
Amps at 240V = Watts / 240
Surge check = Generator surge capacity must exceed largest single motor starting watts
Always round up to the next standard generator size — running a generator above 80% rated capacity for extended periods reduces engine life significantly. For whole-home standby, a licensed electrician must install the transfer switch. Improper wiring can backfeed the utility line and endanger line workers.
Generator purchase cost by type and size (2025 US)
| Type | Size range | Price range | Key advantage |
|---|---|---|---|
| Portable inverter | 1,000-3,500W | $300-$1,500 | Quiet, fuel-efficient, safe for electronics |
| Portable conventional | 3,500-7,500W | $400-$1,800 | Higher output, lower cost per watt |
| Portable dual-fuel | 4,000-10,000W | $500-$2,500 | Runs on gas or propane |
| Home standby (small) | 7-14 kW | $3,500-$8,000 + install | Auto-start, natural gas or propane |
| Home standby (whole-home) | 16-22 kW | $6,000-$12,000 + install | Full home including AC and well |
| Home standby (large home) | 22-30 kW | $9,000-$18,000 + install | Electric heat, large AC loads |
Standby generator installation adds $2,000-$5,000 for transfer switch, electrical work, gas line connection, and concrete pad. Annual maintenance runs $150-$300. Total installed standby cost typically ranges $5,000-$20,000. Generac, Kohler, and Briggs and Stratton are the dominant residential standby brands.
Related Calculators
Generator sizing connects to load calculations and wire sizing. Browse the full electrical calculator collection.
Frequently Asked Questions
What size generator do I need to run a house?
For essential circuits only (refrigerator, furnace, lights, chargers), a 5,500 to 7,500W portable generator covers most average homes. For a whole-home standby with central AC and a well pump, plan for 16 to 22 kW. Add up your appliances using the wattage table above and add 25% headroom to get your minimum required capacity.
Can a 7,500 watt generator run central air conditioning?
It depends on the AC unit. A typical 2.5-ton central AC needs 2,400 running watts plus 5,500 to 7,200 starting watts. A 7,500W generator has roughly 9,375W surge capacity — enough to start that AC unit, but it leaves little room for anything else to be running simultaneously. In practice, you'd need to manage loads carefully, turning off other significant draws before the AC compressor starts.
How far from the house does a generator need to be?
Portable generators must be at least 20 feet from doors, windows, vents, and attached garages. Carbon monoxide from generator exhaust is odorless and lethal — it can accumulate in a house within minutes if a generator is run too close. Standby generators installed by electricians are permanently mounted and positioned per manufacturer specifications, usually 18 to 60 inches from the home depending on fuel type and local code.
How long can I run a generator continuously?
Most portable generators can run 8 to 12 hours on a tank of gas. Standby generators on natural gas or a large propane tank can run for days. Change the oil every 50 to 100 hours of operation. Never run a portable generator until it runs out of fuel with connected appliances — fuel starvation can damage the carburetor and magnetize the alternator.
Do I need a transfer switch for a generator?
Yes, if you want to power circuits in your house. A transfer switch or interlock kit safely disconnects your home from the utility before connecting the generator — this prevents backfeed that can injure utility workers. Never plug a generator into an outlet (the "suicide cord" method) — it is illegal in most jurisdictions and extremely dangerous. A licensed electrician can install a transfer switch in a few hours.