What Size Generator to Run a House?
A typical essentials list comes to about 2,100 running watts with a 3,700 watt starting spike, which a 5,000 watt generator covers comfortably. Add a well pump or central air conditioning and the answer doubles or worse.
Size a generator for your house
Generator
5,000 W generator
Needs at least 2,650 W continuous and 3,700 W to start.
Battery power station
21,700 Wh
5 × 5,000 Wh and up (expandable) — needs 2,100 W of inverter and 3,700 W of surge.
A home backup running for 24 hours draws 2,100 W with everything on, peaks at 3,700 W when the sump pump (1/3 hp) starts, and uses 16.1 kWh over the session.
What this assumes
- Running watts assume everything can be switched on at the same time. That is the safe way to size a generator, and it is why the number looks high.
- Starting watts are the running total plus the single largest start-up surge — not every surge added together, because two motors rarely start in the same instant.
- Energy uses each item's duty cycle: a fridge compressor runs about a third of the time, an espresso boiler idles at a fraction of its element rating between shots.
- Battery sizing allows 15% for inverter losses and keeps 15% in reserve so you do not finish the day at empty. In the cooler bands it also allows for the capacity lithium gives up, which is about 85% of rated around freezing and roughly 60% in severe cold.
- Generator sizing never plans to sit above 80% of the continuous rating, which is the same thing as adding 25% to your running total. In the warmer bands it adds roughly 2% per 10 °F, or per 5.5 °C, above 77 °F (25 °C) for the output an engine loses in hot air.
- Fuel burn is modelled from published inverter-generator run times and is accurate to roughly ±20% in the real world.
Start with the circuits you actually need
The mistake that doubles a generator budget is sizing for the house rather than for the outage. Your home's electrical service is rated for everything running at once on the hottest day of the year. An outage is not that. What you genuinely need is refrigeration, heat, water, a few lights, the internet and your phones, and on that list the largest single motor decides your answer.
For the load in the calculator above, the essentials come to 2,100 W running. The starting figure is 3,700 W, and the gap between the two is one appliance: the sump pump, which draws 800 W while pumping but demands 2,400 W for the fraction of a second its motor spins up. Take the sump pump off the list and the generator you need gets smaller, even though the running total barely moves.
The household loads that decide your answer
These are the items that most often set the size. Everything else on a typical essentials list is small by comparison.
| Appliance | Running | Starting | Average | Energy per hour |
|---|---|---|---|---|
| Window air conditioner (5,000 BTU) | 500 W | 1,500 W | 350 W | 350 Wh |
| Window air conditioner (8,000 BTU) | 720 W | 2,150 W | 500 W | 500 Wh |
| Window air conditioner (12,000 BTU) | 1,100 W | 3,300 W | 770 W | 770 Wh |
| Portable air conditioner (10,000 BTU) | 1,000 W | 2,800 W | 750 W | 750 Wh |
| Central A/C (2 ton) | 2,500 W | 7,500 W | 1,500 W | 1.50 kWh |
| Central A/C (3 ton) | 3,500 W | 10,500 W | 2,100 W | 2.10 kWh |
| Gas furnace blower (1/2 hp) | 800 W | 2,350 W | 290 W | 290 Wh |
| Electric space heater | 1,500 W | 1,500 W | 900 W | 900 Wh |
| Dehumidifier | 480 W | 1,450 W | 290 W | 290 Wh |
| Sump pump (1/3 hp) | 800 W | 2,400 W | 65 W | 60 Wh |
| Sump pump (1/2 hp) | 1,050 W | 3,150 W | 85 W | 80 Wh |
| Well pump (1/2 hp) | 1,000 W | 3,000 W | 40 W | 40 Wh |
| Well pump (1 hp) | 2,000 W | 6,000 W | 80 W | 80 Wh |
| Electric water heater (tank) | 4,500 W | 4,500 W | 900 W | 900 Wh |
| Clothes washer | 500 W | 1,400 W | 200 W | 200 Wh |
| Clothes dryer (electric) | 5,000 W | 6,750 W | 4,000 W | 4.00 kWh |
| Hand-wash station water heater | 1,400 W | 1,400 W | 140 W | 140 Wh |
Two patterns are worth noticing. First, the electric items with no starting column at all, such as the water heater and the dryer, are purely resistive: they never surge, but they are enormous and steady, which is why they are the first things to leave off a generator plan. Second, the pumps and compressors have modest running figures and brutal starting figures. Those are the ones that make a generator stall even though the label says it has plenty of capacity.
Sizing rules that actually apply
Three rules produce the number, and the tool applies all three:
- Add every running watt. Assume the fridge, the freezer, the furnace and the sump pump can all be on together, because at 3 a.m. in a storm they will be.
- Add one surge, not all of them. Take your running total and add only the largest single start-up gap. Adding every surge together is the most common sizing error, and it routinely doubles the generator someone thinks they need.
- Leave the top 20% alone. A generator's continuous rating is not a target. Planning to sit at 80% of it keeps the engine out of trouble and leaves room for the load you forgot.
Read starting watts versus running watts for the full explanation of why the second rule works, and how long a generator runs on a tank for the fuel side of the decision.
Portable, inverter or standby
A conventional portable in the 5,000 to 9,500 watt range is the cheapest way to cover the essentials. It is loud, it runs at fixed engine speed whatever the load, and you refuel it yourself.
An inverter generator throttles its engine down when the load is light, which makes it dramatically quieter and much more efficient at the light loads a house actually presents most of the night. It costs more per watt and tops out lower, but for a fridge, a freezer and some lights it is the better machine.
A standby unit is installed permanently, fed from a gas line, and starts itself when the power drops. It is the right answer for medical dependence, frequent outages or a house you are not there to look after, and the wrong answer for one storm a year.
What to do before the next outage
Put your real list into the calculator and note two numbers: the running total and the starting total. Then check the label on the generator you own or are about to buy, and compare against both, because a unit that comfortably carries your load can still refuse to start it. If the starting figure is the problem, the cheapest fix is almost never a bigger generator: it is a soft starter on the air conditioner, or simply not running the well pump and the furnace in the same minute.
If you are weighing an engine against a battery, the generator versus power station comparison works through the cost per watt and the cost per stored kilowatt-hour with real figures.
Frequently asked questions
What size generator do I need to run my whole house?
A true whole-house generator that carries central air, an electric water heater and a well pump usually starts around 14,000 running watts and is normally a permanently installed standby unit. Most people do not need that. The essentials, meaning refrigeration, heat, a sump pump, lights and internet, come to roughly 2,100 running watts and fit a 5,000 watt portable generator that costs a fraction as much.
Will a 5,000 watt generator run my house?
It will run the essentials in most homes: a refrigerator, a chest freezer, a gas furnace blower, a sump pump, lights and internet come to roughly 2,100 running watts with a starting spike near 3,700 watts. It will not run central air conditioning at the same time, and it will not run an electric range, an electric water heater or an electric dryer, each of which alone can exceed its capacity.
Do I need a transfer switch?
If you want to power circuits through your existing wiring rather than running extension cords, yes, and in most places it is required by code. Back-feeding a generator into a dryer outlet is dangerous and illegal: it can energize the utility line outside and kill a lineworker. A transfer switch or an interlock kit is an electrician job, and it is the one part of this you should not improvise.
How much fuel will it use per day?
Switch to the "How long will it run?" tab with your own list. As a guide, a 5,000 watt generator carrying a typical essentials load burns roughly half a gallon an hour, which is about 12 gallons over a full day of continuous running. Most people cut that substantially by running the generator in blocks, since a full refrigerator and freezer hold their temperature for several hours with the power off.
Can a battery power station back up my house instead?
For a few hours, comfortably. For a multi-day outage, the arithmetic gets expensive fast: a typical essentials load uses around 16 kilowatt-hours a day, which is four or five of the largest portable batteries sold. Batteries make the most sense for the quiet overnight stretch, for medical equipment, or paired with solar and a generator that tops them up during the day.
Does central air change the answer that much?
Yes, more than anything else in the house. A 3 ton central air conditioner runs at about 3,500 watts and surges to around 10,500 watts starting. Adding it to an essentials list can push a 5,000 watt requirement past 12,000. A soft starter cuts that surge by roughly two thirds and is often far cheaper than the larger generator you would otherwise have to buy.