Generator Sizing Calculator

There is a well-known trick involving a cable with a male plug on both ends, run from a generator to a dryer outlet or any convenient socket. It works. It is also one of the most dangerous things a homeowner can do to a house.

Power pushed into a socket flows back through your panel, out through your meter, and into the utility transformer, which steps it back up to several thousand volts on the line outside. Utility crews working to restore your power expect that line to be dead. People have been killed this way. It also bypasses every overcurrent protection in the house, which means the wiring in your walls is being fed by something with no idea what it is connected to.

The correct answer is a transfer switch or an approved panel interlock, fitted by a licensed electrician. It physically prevents the generator and the utility supply from being connected at the same time. A manual transfer switch for eight to twelve circuits is typically eight hundred to twelve hundred dollars installed, which is a real cost and is included in the comparison on this page rather than left out to make the numbers look better.

The alternative, which is entirely legitimate, is extension cords rated for the load running to individual appliances, with nothing connected to the house wiring at all. That works for a fridge, a few lamps and phone charging. It does not power a furnace, a sump pump or anything hard-wired.

Anything with a motor — a compressor, a pump, a fan — draws several times its running current for a fraction of a second as it starts. That is the locked-rotor surge, and a generator must survive it or the breaker trips and everything goes dark.

The naive approach adds every surge to every running watt. On a modest essentials list that turns a genuine 4,000-watt requirement into 9,000 watts, which is the difference between a mid-sized portable and a large one, or between a large portable and a standby unit. It is an expensive error and it is extremely common.

The realistic approach recognises that surges are momentary and effectively never coincide. Your fridge compressor and your sump pump both starting inside the same half-second is possible but rare, and designing around it means paying for capacity that sits idle for the machine's entire life. So: everything's running watts, plus the largest single surge, plus headroom.

Headroom matters separately. Running a generator continuously at its maximum rating shortens its life, leaves nothing for the load you forgot, and means every start-up is marginal. Twenty per cent is the usual allowance and is what this tool applies by default.

A portable generator costs a fraction of a standby unit and does most of the job. It also requires somebody to be at home, to wheel it out, to start it, and to refuel it every eight to ten hours including through the night. In a three-day outage that is a genuine imposition, and in a week-long one it becomes the dominant problem — particularly since petrol stations need power to pump.

An automatic standby unit starts itself within seconds of the supply failing, runs on the natural gas main indefinitely, and works whether or not anyone is home. It also costs five thousand dollars or more for the unit, a similar figure for installation, and needs a gas connection sized for it.

The decision is less about watts than about circumstances. If outages where you live are rare and brief, a mid-sized portable and a manual transfer switch is a sound, proportionate answer. If they are frequent, long, or if anyone in the house depends on powered medical equipment, the case for standby is much stronger — and in the medical case a portable that needs manual starting is not really a plan at all.

One thing that changes the arithmetic more than people expect: a soft starter fitted to a central air compressor can cut its inrush substantially, sometimes bringing whole-home cooling within reach of a large portable. If air conditioning is what pushes you toward standby, ask an HVAC contractor whether your unit will take one before spending five figures.

Naturally aspirated engines lose roughly three per cent of their rated output per thousand feet of elevation, because there is less oxygen to burn. At five thousand feet — Denver, Albuquerque, much of the Mountain West — a generator rated 7,500 watts at sea level delivers closer to 6,400.

That is not a rounding error, and manufacturers rate their machines at sea level. The tool applies the derate to your requirement so the recommendation reflects what the machine will actually do where you live. If you are near sea level it changes nothing.

A portable generator at half load typically runs eight to twelve hours on a tank. That sounds fine until it means getting up at three in the morning in a storm to refuel it, or discovering that the local petrol station cannot pump fuel because it has no power either. Storing enough petrol for a multi-day outage means stabilised fuel, correct containers and somewhere safe to keep it.

Dual-fuel models that run on propane solve some of this. Propane stores indefinitely without degrading, which petrol does not, and a couple of twenty-pound cylinders is a far more practical reserve than jerry cans.

Whatever you buy, it needs to be run periodically. A generator that has sat untouched for two years will very often not start on the night you need it, usually because the carburettor has gummed up. Monthly is ideal; twice a year is the minimum that makes it dependable — and that is a maintenance-calendar item, not something to leave to memory.