How to Size a Home Generator (Without Over- or Under-Buying)

Before any math, decide the goal. Essential-circuit backup keeps the things you truly need through an outage — usually the furnace, refrigerator, well pump or sump pump, some lighting, and outlets for phones, internet, and medical devices. It is far cheaper, uses a smaller and quieter generator, and covers the scenarios most people care about. Whole-home backup runs everything, including air conditioning, electric ranges, and every outlet, as if nothing had happened — but it demands a much larger generator, a bigger fuel supply, and a heavier install.

Most homeowners are best served by honestly ranking what matters. In a Colorado winter outage, heat and a working sump or well pump usually top the list, while central air can wait. Choosing essential backup where it genuinely meets your needs is the single biggest lever on both the size and the cost of the system. Whole-home backup is the right call when you have loads you cannot do without — a home medical setup, a business run from the house, or simply a strong preference for total normalcy.

Every electrical device has a running wattage — the steady power it draws while operating — and the generator has to supply the sum of everything running at once. But motor-driven equipment also has a start-up surge: for a brief instant when it kicks on, a well pump, furnace blower, sump pump, or air conditioner can pull several times its running wattage. The generator has to cover the largest of those surges landing on top of everything already running, or it will bog down and stall.

In practice an electrician lists each load you want backed up, notes its running watts, identifies the motor loads and their surge demand, and then sizes the generator so that the worst-case moment — say, the well pump and furnace blower both kicking on while the fridge is already running — still leaves headroom. This is why two homes of the same size can need very different generators: a home on a deep well with a big pump has a much larger surge to cover than a home on city water. It is also why 'just buy the biggest one' is not actually the safe default — see below.

A few loads deserve individual attention because they drive the sizing. Well pumps are often the single largest surge in a rural or semi-rural home, and they are non-negotiable — no pump means no water. Furnace blowers run for long stretches in a Colorado winter and carry their own start-up surge. Sump pumps matter most exactly when the power is likely to fail, during heavy snowmelt or a storm, so backing them up prevents a flooded basement at the worst possible time.

Medical equipment changes the calculation entirely. Oxygen concentrators, CPAP machines, powered wheelchairs, or any life-supporting device should be treated as must-run loads and sized with generous margin and reliability in mind, not trimmed to save on capacity. If someone in the home depends on powered medical equipment, tell your installer up front — it can shift the recommendation toward a larger unit, a more robust transfer setup, or a battery for the instant, gap-free switchover that even a fast-starting generator cannot quite match.

The transfer switch is what safely disconnects your home from the grid and connects it to the generator, and its type is bound up with sizing. An essential-circuit (or 'managed') transfer switch backs up a chosen set of circuits and can even shed or rotate loads so a smaller generator covers more — a cost-effective match for essential backup. A whole-home (service-rated) transfer switch backs up the entire panel and pairs with the larger generators needed for whole-home coverage. Choosing the switch and choosing the generator size are really one decision.

Then there is altitude, which is easy to overlook and genuinely matters here. A combustion engine makes less power as elevation rises because the air is thinner, so a generator carries a lower real-world output at Centennial-area elevation than its sea-level nameplate suggests. A properly sized system accounts for this derate up front — otherwise a unit that looked adequate on paper turns out to be undersized in practice, running hot and struggling under exactly the winter loads you bought it for. A local installer who sizes for altitude routinely will build this in without you having to ask.