Sizing a standby generator: what actually has to stay on

Most conversations about standby power start with a model number, and that is the wrong end of the problem. The generator is the last decision, not the first. What determines whether an installation is sensible or wildly over specified is a question nobody enjoys answering: when the street goes out, what genuinely has to keep running?

The two honest answers for a house

The first is everything. Whole house standby means you barely notice the outage. It is the most expensive option, it needs the largest unit, and for some households, particularly ones with medical equipment, a well, or somebody who works from home without any alternative, it is straightforwardly the right call.

The second is the essentials. Heating, the refrigerator and freezer, a well pump or sump pump, some lighting and a few outlets. In a Michigan winter that list is mostly about not losing the house: keeping the heating on and keeping water out of the basement. It is a much smaller generator, and for a lot of homes it covers the outcomes people actually worry about.

Picking between them is a budget and tolerance question rather than a technical one, and it is worth deciding before anyone quotes.

For a business, ask what makes restarting cheap

Commercial and industrial sites have a more useful version of the same question. It is rarely worth running the whole site on standby power. What is worth protecting is whatever makes coming back from an outage fast and cheap.

Sometimes that is refrigeration or a process that spoils. Sometimes it is the control systems, so that a line can be restarted cleanly instead of being recovered. Sometimes it is security lighting and access control, so the site is not sitting dark and open. Working out which of those applies to you usually shrinks the generator considerably.

Starting loads matter more than running loads

A common surprise is that the total of everything's running consumption is not what sizes the machine. Motors draw substantially more at the moment they start than they do once turning, and a generator has to absorb that surge without the voltage dropping far enough to upset everything else connected to it.

That is why a well pump, a furnace blower, an air conditioner or a compressor can have more influence on the size of the unit than a whole floor of lighting. It is also why staggering what starts, rather than letting everything come back at once, is sometimes the cheapest way to get the size down.

The parts of the job that are not the generator

• A pad or base for it to sit on, level and clear of where water collects.
• A transfer arrangement so that the generator and the incoming supply can never feed the property at the same time.
• A conduit run back to the service, sized for the distance involved.
• Fuel. Natural gas, if the property has it, removes the refuelling problem entirely, and that often decides the whole approach.
• Somewhere to put it where the noise and the exhaust are not aimed at a bedroom window or a neighbor.

What a standby generator will not do

It is worth being clear about the gap, because it is where disappointment usually lives. A standby unit does not keep the power on continuously. There is a pause while the outage is detected and the generator starts and takes over, typically a matter of seconds, and anything that cannot tolerate a brief interruption still needs its own battery backup behind it.

It also does not make the property independent of its fuel supply. A unit running on natural gas depends on that supply staying up, and one running on propane depends on how much is in the tank and how long the outage lasts. Neither of those is a reason not to fit one. They are just the questions worth asking before you decide the outage is solved.

Exercise it, or it will disappoint you

A standby unit that has not run in a year is not standby power, it is a large object in the yard. Most run themselves briefly on a schedule for exactly this reason. Letting that happen, and noticing if it stops happening, is most of the ownership requirement.

Working it out with somebody

Whether the property is a house, a commercial premises or an industrial site, the sizing conversation is worth having before the shopping. Brandon Electric installs generators across Oakland and Macomb County and estimates are free, so working out what has to stay on costs nothing but the phone call.