What Size Generator Do I Need?

Guide · Updated August 5, 2026

Buy too small a generator and it stalls the moment your furnace kicks on; buy too big and you overpay and waste fuel. Sizing it right comes down to one idea: add up the watts you need to run at once, then leave room for the biggest motor's start-up surge. Here's how to do it properly.

Running watts vs. starting watts

Every appliance has two numbers that matter:

Running (rated) watts — the steady power it draws once it's going.
Starting (surge) watts — the brief spike when a motor starts, often 2–3× the running figure.

Resistive loads — lights, heaters, chargers — have no surge. Anything with a motor or compressor (fridge, furnace blower, well pump, AC) does, but only for a second or two. You size the generator to cover all running watts plus the single largest surge.

Step 1 — List what must stay on

Appliance	Running watts	Starting watts
Refrigerator / freezer	150–800	1,200–2,200
Furnace blower (gas heat)	600–1,000	1,500–2,350
Sump pump (1/3 hp)	800	1,300–2,900
Well pump (1/2 hp)	1,000	2,000–4,000
Window AC (10k BTU)	1,200	1,800–2,600
Lights + phone / laptop	200–400	no surge
Microwave	1,000–1,500	no surge

Step 2 — Do the math

Add the running watts of everything you'll power at the same time. Then add the highest single starting surge from your list — in practice only one large motor starts at a time. That total is your minimum generator size.

Generator size ≥ (sum of running watts) + (largest single starting surge)

Worked example — riding out an outage

Say you want to keep the fridge, furnace blower, sump pump, and some lights running:

Fridge 600 W + furnace 800 W + sump pump 800 W + lights 300 W = 2,500 running watts.
The sump pump adds the biggest surge — about 2,100 W extra at start — so peak demand is roughly 4,600 watts.

A 5,000-running-watt generator (with ~6,000+ starting watts) covers this with headroom; a 3,500 W unit would trip the moment the pump kicks on.

Common generator sizes and what they run

Generator (running W)	Typically powers
2,000–2,400 W	Fridge OR a few lights + electronics; RV / camping essentials
3,500–4,000 W	Fridge + lights + furnace fan (not all with big surges at once)
5,000–6,500 W	Fridge, furnace, sump / well pump, lights — a comfortable "essentials" backup
7,500–9,000 W	Adds a window AC or well pump plus most kitchen loads
10,000 W+	Whole-home essentials incl. central AC or electric range (with transfer switch)

Check it against your circuits

Generators are rated in watts, but breakers and outlets are rated in amps. To confirm a load won't overload a generator's outlet, convert watts to amps with our Watts to Amps calculator (Amps = Watts ÷ Volts). On a 120 V outlet, an 1,800 W load draws 15 A — right at a standard outlet's limit.

Safety — read this before you plug in

Never "backfeed" by plugging a generator into a wall outlet — it can electrocute utility workers. Use a transfer switch or interlock installed by an electrician.
Run generators outdoors only, well away from windows and doors. Carbon monoxide is odorless and deadly.
Don't exceed 80% of rated output continuously; like breakers, generators need headroom.

Common Questions

Will a 5,000 watt generator run a house?

It runs the essentials of most homes — refrigerator, furnace blower, sump or well pump, and lighting — but not central air conditioning or an electric range at the same time. For those, look at 7,500 W and up with a transfer switch.

How many watts does a refrigerator use?

A modern fridge runs on roughly 150–800 running watts but spikes to 1,200–2,200 watts for a second or two when the compressor starts. Always size for that starting surge, not the running figure.

What size generator do I need for a 200-amp panel?

Panel size is the maximum, not the load. Size the generator to the watts you actually intend to run during an outage, not to the panel rating. Most homes back up their essentials with a 5,000–10,000 W unit.

More Guides

Or head back to the Ohm Calculator home to run the numbers.