Standby Generator Sizing: What Actually Needs to Run

The most common sizing error in this trade is reading the main breaker and multiplying. A 200 amp service at 240 volts implies 48 kilowatts of theoretical capacity, but no house draws its full service rating. The panel is sized for the worst possible simultaneous case plus code margin, not for what the house does on a hot Tuesday evening.

The correct method is a load calculation following the National Electrical Code, which accounts for the actual connected loads and applies demand factors reflecting how appliances get used in practice. A house is not running the range, dryer, water heater, air conditioner, and well pump simultaneously, and the calculation accounts for that.

Two categories drive the answer. The first is continuous draw, the steady load while everything runs. The second, and the one that catches people out, is startup surge. Motors pull far more current in the first fraction of a second than they do while running, and the generator has to absorb that without the voltage sagging enough to stall other equipment.

Residential standby systems are typically designed to tolerate a voltage dip of up to about thirty percent when a motor starts. Homes with sensitive electronics sometimes need that held tighter, which means a larger and more expensive machine. That is a genuine tradeoff worth discussing rather than discovering after the equipment is set.

Once calculated demand is established, a reserve margin is added so the generator is not running at its ceiling every time the air conditioner starts. For a house with stable loads and few motors, five to fifteen percent is reasonable. For a house with several large motors, a well pump, a pool, or a planned vehicle charger, fifteen to twenty-five percent is the better choice.

Margin is also where honest future planning belongs. If you intend to add a heat pump water heater, a pool, or vehicle charging within a few years, saying so during sizing costs nothing. Discovering it later means either load management or replacing the generator, and the second option is expensive.

Whole home backup means the generator carries the entire panel and nothing has to be prioritized. It is the simplest experience and the most expensive equipment. Critical circuit backup puts the loads that matter on a subpanel, which lets a smaller generator deliver most of the practical benefit at a lower price.

A middle path uses load management, where the system sheds or staggers large loads so a smaller generator can serve a whole house without tripping. The air conditioner and the electric water heater, for instance, are prevented from running at the same time. Done well the homeowner never notices. Done carelessly it produces confusing behavior during outages, so it belongs in writing before installation.

Natural gas is convenient where a service line exists, because the fuel supply is effectively unlimited and there is no tank to manage. Much of the rural Upstate has no gas main, which makes propane the practical option and introduces a tank sizing decision. A generator that runs for days consumes propane at a rate worth calculating before the outage rather than during it.

As a rough planning figure, a mid sized air cooled standby unit under moderate load burns propane fast enough that a 250 gallon tank is a sensible minimum for multi-day resilience, and 500 gallons is common where the homeowner wants to ride out a week without a refill. We size the tank alongside the generator rather than treating it as an afterthought.