Exploring
Add up the running watts of everything you want on at once, then add the single largest starting surge on top. Essential circuits for a typical home — fridge, freezer, furnace fan, well pump, lights and electronics — land at about 5,000–7,500 watts. Whole-house backup with central air needs 12,000–22,000. A 30-amp RV is fully served by 3,000–3,600 watts, and a modest off-grid cabin by 2,000–3,500.
Running Watts vs Starting Watts
Every generator carries two ratings. Running (rated) watts is what it can supply continuously. Starting (peak or surge) watts is what it can supply for a few seconds while a motor spins up. Anything with a compressor or a motor — a refrigerator, a well pump, an air conditioner, a sump pump — draws two to five times its running wattage at start.
The sizing rule is simple: total running watts of everything on at once, plus the largest single starting surge. You do not add every appliance's surge, because they will not all start in the same second — but you do have to cover the biggest one, and the well pump or the air conditioner is usually it.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator / freezer | 150–400 | 1,000–1,200 |
| Chest freezer | 100–300 | 800–1,000 |
| Well pump, 1/2 HP | 800–1,000 | 2,000–3,000 |
| Well pump, 1 HP | 1,500–2,000 | 4,000–6,000 |
| Sump pump, 1/3 HP | 800 | 1,300–2,000 |
| Furnace fan (gas furnace) | 600–900 | 1,500–2,500 |
| Window AC, 10,000 BTU | 1,200 | 1,800–3,600 |
| Central AC, 3 ton | 3,500–5,000 | 8,000–12,000 |
| Electric water heater | 4,500 | 4,500 |
| Electric range (one burner) | 1,500–2,500 | — |
| Microwave | 1,000–1,500 | — |
| Space heater | 1,500 | — |
| LED lights (whole house) | 100–200 | — |
| TV, router, laptop, phones | 150–300 | — |
| RV rooftop AC, 13,500 BTU | 1,300–1,500 | 2,800–3,500 |
The loads that catch people out: Resistive loads — heaters, kettles, electric water heaters, ranges — have no surge but are enormous. An electric water heater alone eats 4,500 watts. In an outage, heat water with propane and cook on gas; it halves the generator you need.
Worked Example 1: Essential Circuits for a Home
Most households do not need to run everything — they need the fridge, the freezer, heat, water, lights and a way to charge phones. Take a gas-furnace home on a well:
| Load | Running W |
|---|---|
| Refrigerator | 300 |
| Chest freezer | 200 |
| Furnace fan | 800 |
| Well pump, 1/2 HP | 1,000 |
| Lights, TV, router, chargers | 400 |
| Microwave (occasional) | 1,200 |
| Running total | 3,900 |
| + largest surge (well pump) | +2,500 |
| Peak needed | 6,400 |
A generator rated around 5,500 running / 7,000 starting watts covers this comfortably with the microwave off while the pump starts. That is the sweet spot for most portable backup, and it is why 6,500–9,500-watt dual-fuel portables are the best-selling home-backup class. Connect it through a transfer switch or an interlock — see our guide on connecting a generator to your house — rather than extension cords.
Worked Example 2: Whole-House Backup With Central Air
Add a 3-ton central air conditioner (4,000 running, 10,000 starting), an electric water heater (4,500) and an electric range to the essentials above, and the running total passes 12,000 watts with a 10,000-watt surge on top. That is standby generator territory — 18,000 to 22,000 watts — or a large portable with a soft-start kit on the AC compressor, which cuts its surge by 60–70% and can bring the requirement down to a 12,000–15,000-watt unit.
Before sizing for whole-house, decide honestly whether you need central air during an outage. Dropping the AC and the electric water heater turns a 20 kW standby into a 7 kW portable, and the fuel consumption difference over a three-day outage is dramatic. Our whole-home battery backup guide covers the alternative that runs silently overnight and lets the generator only run to recharge.
Worked Example 3: An RV or Camper
A 30-amp RV can draw at most 3,600 watts from its shore cord, so a generator larger than that is wasted on it. The load that decides everything is the rooftop air conditioner: a 13,500 BTU unit runs at about 1,400 W and starts at up to 3,500. With the AC running, the microwave or a hair dryer is the only other big load you can add.
So: 3,000–3,600 watts covers a 30-amp RV with one air conditioner. Two inverter generators paralleled, or a single 4,500-watt class inverter unit, covers a 50-amp RV with two ACs. A soft-start kit on the AC lets a 2,200-watt inverter generator start a 13,500 BTU unit, which is the trick that makes the small, quiet generators viable for camping. See our inverter generator picks and the dual-fuel comparison.
Worked Example 4: An Off-Grid Cabin With Solar
On a solar-and-battery cabin the generator has two jobs: run the big occasional loads the inverter cannot, and recharge the battery bank in a cloudy week. Its size is set by the charger. A 48V inverter-charger with a 60-amp charge rate pulls about 3,000 W from the generator while charging, plus whatever the house is using — so a 3,500–4,500-watt inverter generator is the usual match, and a 2,000-watt unit will only charge at a reduced rate.
Bigger is not better here. A generator runs most efficiently at 50–75% load; a 9,000-watt unit charging a battery at 3,000 W burns far more fuel per kWh than a 4,000-watt unit doing the same job. Match the generator to the charger, and run it hard for a few hours rather than lightly all day. Our inverter charger guide covers the charge rates.
Generator Sizing Chart
| Situation | Generator size | Type |
|---|---|---|
| Phone charging, lights, a fan (camping) | 1,000–2,000 W | Small inverter |
| 30-amp RV with one AC | 3,000–3,600 W | Inverter (or two paralleled) |
| 50-amp RV with two ACs | 4,500–7,000 W | Large inverter or open-frame |
| Off-grid cabin, battery charging + loads | 3,500–4,500 W | Inverter |
| Home essentials, gas furnace, well pump | 5,500–7,500 W | Dual-fuel portable |
| Home essentials + window AC or heat pump | 7,500–10,000 W | Dual-fuel portable |
| Whole house with central AC | 12,000–22,000 W | Large portable + soft start, or standby |
Sizing Tips Most Guides Skip
Derate for altitude and fuel. Engines lose about 3% of output per 1,000 feet above sea level, and propane delivers roughly 10% less power than gasoline in a dual-fuel unit. A 7,500-watt gasoline rating becomes about 6,000 on propane at 5,000 feet.
Load it to 50–75%. Generators are least efficient and dirtiest at light load, and a portable idling all night to run a fridge is the classic waste of fuel. If your overnight load is small, a battery power station charged from the generator by day is a far better fit — see our portable power station guide.
Split-phase 240V. A well pump, a central AC or a dryer needs 240V, which means a generator with a 240V outlet (L14-30 or 14-50). Most inverter generators under 4,000 W are 120V only, which is fine for an RV and a problem for a house.
Soft-start kits change the math. A soft starter on an air conditioner compressor cuts its starting surge by more than half. It is the cheapest way to make a smaller, quieter generator start a big AC, and it also makes the AC easier on an inverter.
Two smaller generators can beat one big one. Paralleled inverter generators give you a quiet 2,000-watt unit for everyday and 4,000+ watts when you need it, with redundancy if one fails.
Gear We Recommend
With the wattage settled, these guides cover the generators and the hardware to connect them.
- Best Portable Generators for Off-Grid & Home Backup (2026) — dual-fuel open-frame generators for home essentials and whole-house
- Best Inverter Generators: Quiet Power for Camping & Backup (2026) — quiet inverter generators for RVs, cabins and paralleling
- Best Generator Transfer Switches: Manual & Automatic (2026) — manual and automatic transfer switches
- How to Connect a Generator to Your House Safely (2026) — interlocks, inlet boxes and the safe methods
- Best Generator Covers for 2026 (While Running & Storage) — running a generator safely in rain and snow
- Best Whole Home Battery Backup Systems (2026) — the battery alternative for silent overnight backup
Frequently Asked Questions
What size generator do I need to run a house?
For essential circuits — refrigerator, freezer, furnace fan, well pump, lights and electronics — a 5,500–7,500-watt generator covers most homes. Whole-house backup that includes central air conditioning and an electric water heater needs 12,000–22,000 watts, which is standby generator territory unless you add a soft-start kit to the AC.
What size generator do I need to run a refrigerator?
A refrigerator runs at 150–400 watts but needs 1,000–1,200 watts to start the compressor, so any generator rated 2,000 watts or more will run it with room for lights and phone charging. A 1,000-watt unit may struggle to start it.
Will a 7,500-watt generator run a house?
It will run the essentials in almost any house — fridge, freezer, furnace fan, a well pump, lights, electronics and a microwave — and a window air conditioner on top. It will not run central air, an electric water heater and an electric range at the same time; those need 12,000 watts or more.
What size generator do I need for a 30-amp RV?
A 30-amp RV can draw at most 3,600 watts, so a 3,000–3,600-watt generator is the right size and anything larger is wasted. The rooftop air conditioner is the load that matters — it starts at up to 3,500 watts — and a soft-start kit lets a 2,200-watt inverter generator handle it.
What size generator do I need for a well pump?
A 1/2 HP well pump runs at about 1,000 watts but surges to 2,000–3,000 at start, and it needs 240V. A generator with at least 3,500 starting watts and a 240V outlet will run it alone; to run it alongside a fridge, freezer and furnace fan, size for 5,500 running watts or more. A 1 HP pump roughly doubles the requirement.
Is it better to oversize a generator?
Only slightly. A generator runs cleanest and most efficiently at 50–75% load; one that is far too big burns much more fuel, wet-stacks on diesel, and costs more to buy and maintain. Size for your realistic peak plus 20% margin, not for the largest unit you can afford.
Related Guides
How We Research
Picks are based on manufacturer specifications, current Amazon availability, and patterns across verified owner reviews. Specs, capacities, and included accessories vary by listing — confirm the current details at the retailer before buying.
Note: Appliance wattages are typical ranges. Check the data plate on your own well pump, air conditioner and furnace — locked-rotor amps times voltage gives the real starting watts — before buying a generator.