Generator
Tick what has to stay on. It adds the running watts, finds the biggest motor start, and names the generator that covers both.
Buy this generator
0
watts · 0 W running- Running load
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- Largest surge
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- Peak draw
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- At 240 V
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Sized above the peak, not the average.
Two numbers decide a generator
The first is easy: add the running watts of everything you want on at once. The second is the one that catches people out. Anything with a motor — a fridge compressor, a well pump, a furnace blower, an air conditioner — draws two to three times its running load for the moment it starts turning. Only one of them will start at a time, so the peak your generator must survive is the running total plus the single largest surge.
That is why a 3,500 watt list often needs a 5,500 watt machine, and why generators are advertised with two numbers on the box. Buy on the running figure alone and the unit will stall the first time the well pump kicks in behind the fridge.
What people usually back up
An outage list is shorter than a wish list. Refrigeration, the heating system's blower, a sump pump if the basement needs it, lights, and enough outlets for phones and a laptop — that is a comfortable few thousand watts. Heat by resistance (water heater, space heaters, an electric range) is where wattage explodes; those are the loads worth doing without for a day, or the reason to buy a standby unit instead of a portable.
Fuel and runtime
A portable generator burns roughly half a gallon of gasoline per hour at half load, and more than that flat out. Sizing with headroom does not just protect the machine — a generator loafing at 60 % of its rating uses noticeably less fuel per hour than one straining at 95 %, and stored fuel is the real limit in a long outage.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Appliance watts
- Typical nameplate figures — check your own labels, they vary widely
- Starting surge
- The single largest motor start added on top of the running total
- Headroom
- 20 % above peak by default, adjustable in the form
- Sizing
- Rounded up to a wattage generators are actually sold in
Questions from the aisle
What size generator runs a house?
A fridge, furnace blower, sump pump, lights and a few outlets come to about 2,650 running watts, and 4,200 with the furnace blower starting behind them — a 5,500 watt portable with headroom to spare. Add an electric water heater or a range element and it doubles; add central air and electric heat and you are shopping standby units at 14,000 watts and up.
Running watts or starting watts?
Both, and they are different numbers. Running watts is what everything draws steadily. Starting watts is that total plus the surge of the single biggest motor coming on — anything with a compressor or a pump pulls two to three times its running load for a second or two.
Why leave 20% headroom?
A generator held at its rating all night runs hot, burns fuel hard and has nothing left for the surge you forgot. Sizing 20% above your peak is the difference between a machine that idles through an outage and one that trips its overload at 3am.
Do I need an interlock or transfer switch?
Yes, for anything wired into the panel. Backfeeding through a dryer outlet energises the utility line outside and kills linemen. A mechanical interlock kit or a transfer switch is the legal and safe way, and it is inspected work.
The reasoning behind this one
Does my panel have room for it
Where the assumptions above come from, what changes them, and the mistakes that cost a second trip. Free to read, no account needed.