Generator and Transfer Switch Sizing
What size generator your house actually needs — sized on starting watts, derated for altitude and heat, with the backfeeding warning it deserves.
Never backfeed through a dryer outlet
A male-to-male cord from a generator into a dryer or range outlet energises the utility drop through your service transformer, which steps it back up to line voltage on the wires outside — where a lineman who has grounded the circuit and believes it dead is working on it. It also leaves live pins exposed on whichever end is unplugged. These cords are sold; they are called suicide cords; they have killed people. A breaker interlock kit costs a few hundred and makes it physically impossible.
What do you need to run?
Essential
Heating & cooling
Water
Kitchen
Comfort
Medical
Starting watts, not running watts
Generators carry two ratings and people read the bigger one off the box as though it were the continuous figure. It is the other way round: the smaller number is what it can sustain, and the larger is a brief surge allowance. A motor draws three to eight times its running current for the first second, so a well pump that runs at 750 W needs 3,000 W to start. A generator sized on running watts alone stalls the first time it is asked to do the job it was bought for.
- Altitude and heat derate a generator, and nobody reads that page. Roughly 3.5% per thousand feet and 1% per ten degrees above 77 °F. A 7,500 W unit at 5,000 feet is a 6,200 W unit — enough to turn a comfortable margin into a stall.
- Neutral bonding catches people out. Most portable generators bond neutral to ground internally, and running one through a transfer switch that does not switch the neutral gives two bonds and current on the grounding conductor. Match the generator to the switch — it is a real fault, not a formality.
- Carbon monoxide kills more people every storm than the electrical hazards do. Outdoors, well away from windows and doors, never in a garage even with the door open, and fit a CO alarm indoors.
- A lightly loaded generator is not a happy one. Below about a quarter load a diesel glazes its cylinder bores over time and a petrol unit simply wastes fuel. If the real load is small, a smaller generator is cheaper to buy and to run.
- Work out the fuel before the storm. Three days of continuous running is a lot of petrol, and the filling station will have no power either.
How to use
- Tick what you need to keep running during an outage.
- Pick a generator and set your altitude and expected temperature.
- Check BOTH the running and the starting figures — starting is what fails.
- Choose a connection method: an interlock kit is the cheap compliant one.
Frequently asked questions
Can I just plug the generator into a dryer outlet?
No, and this is the one thing on this page that kills people. A male-to-male cord backfeeds your panel, which energises the utility drop through your service transformer, which steps it back up to line voltage on the wires outside — where a lineman who has grounded the circuit and believes it dead is working on it. It also leaves live pins exposed on whichever end is unplugged. These cords are sold, they are illegal everywhere, and a breaker interlock kit costs a few hundred and makes the whole thing physically impossible.
What is the difference between running and starting watts?
Running watts is what a generator can sustain; starting watts is a brief surge allowance, usually 20 to 25 percent higher. It matters because motors draw three to eight times their running current for the first second or so. A well pump running at 750 watts needs about 3,000 to start, so a generator sized on the running total alone will carry everything happily and then bog down the first time the pump kicks in. Size on the starting figure.
Do I need a transfer switch or is an interlock kit enough?
An interlock kit is enough and is usually the better buy for a portable generator. It is a listed metal plate that makes it mechanically impossible to close the generator breaker and the main breaker at the same time, it costs a few hundred installed, and it gives you access to every circuit in the panel rather than the six or ten a manual transfer switch pre-selects. It must be a kit listed for your specific panel — a homemade plate will not pass inspection and should not.
How much does altitude affect a generator?
More than most people expect, and it is on the page of the manual nobody reads. Roughly 3.5 percent of rated output per thousand feet of elevation, plus about 1 percent per ten degrees Fahrenheit above 77. A 7,500 watt unit at 5,000 feet on a hot day is delivering closer to 6,100 watts, which is easily enough to turn a comfortable margin into a stall on the one day the thing actually matters.
What is neutral bonding and why does it matter?
Most portable generators bond the neutral to the frame internally, which is correct when you plug appliances straight into them. Connect one through a transfer switch that does not switch the neutral and you now have two neutral-to-ground bonds in the system, which puts normal load current onto the grounding conductor. Some generators are sold with a floating neutral for exactly this reason. Check which yours is and match it to the switch — this is a genuine fault rather than a paperwork detail.
How much fuel will I get through?
A 7,500 watt portable at moderate load burns roughly three quarters of a gallon an hour, so about 18 gallons a day of continuous running and 50 or more across a three-day outage. That is worth working out before the storm rather than during it, because the filling station will have no power either. A permanently piped natural gas standby unit sidesteps the problem entirely, at the cost of depending on the gas network staying up.
Is it bad to run a generator at low load?
For a diesel, yes — sustained light loading glazes the cylinder bores and causes wet stacking, and manufacturers recommend loading them to at least 30 to 40 percent. A petrol unit will not damage itself but simply wastes fuel, since it turns at 3,600 rpm regardless of what you ask of it. If your real load is a fifth of the generator, a smaller one would be cheaper to buy, cheaper to run and quieter.
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