Sizing Comes Before Shopping
Most generator regret is sizing regret, and it cuts both ways. Buy too small and the machine tripping off line the moment the air conditioner compressor kicks in turns every outage into an argument with a breaker. Buy too large and you pay for capacity you never use, burn more fuel in every outage, and often get a louder unit sitting outside your bedroom window. The way out of both traps is the same: walk your own house with a notepad before you look at a single model. A properly sized standby generator is not a specification you accept from a brochure — it is a number your own home hands you, room by room, and the walk-through below takes about an hour with a calculator and a phone flashlight.
Start With the Question: What Must Run?
Before adding anything, decide what an outage actually requires in your household. There are three honest tiers. Essential mode keeps refrigeration, a few lights, the internet, phone chargers, and either the furnace fan or a couple of window units going — most households are genuinely comfortable here, and the number usually lands between 5,000 and 8,000 running watts. Comfortable mode adds the kitchen (microwave, coffee maker, a television) and one more climate zone, landing in the 9,000 to 12,000 range. Whole-home mode runs everything including central air, electric water heating, and the dryer, which is where 16,000 to 22,000 watts and larger units enter the picture. Write your tier down first, because the room walk is about filling in that tier's details, not cataloging everything you own. Households that try to size for “everything, just in case” almost always oversize; households that guess at “the basics” often undersize by forgetting exactly two things, which the walk will catch.
Walk the House: The Room-by-Room Sheet
Now the notepad earns its keep, room by room. Kitchen: refrigerator (roughly 150–400 running watts, but see the starting-watt warning below), freezer (same range), microwave (800–1,500), coffee maker (900–1,200), dishwasher if it is on your essential list (1,200–1,500 for heating). Living areas: internet router and modem (under 50 combined), television (100–400), a few LED lights (10 each, count the fixtures you would actually switch on). Laundry if included: washer (500–800 running) and electric dryer (3,000–5,000 — the single biggest optional load in most homes, and the first thing to strike from the list if your tier is essential). Office or work-from-home corner: laptop (60), monitor (30–50), desktop tower (200–500 under load). Bathroom and utility: sump pump (see below), well pump (see below), a handful more lights. Heating and cooling: this is the tier-divider. A gas furnace only needs its fan and controls (400–800 running watts), which is why gas-heat homes can run essentials cheaply in winter. Air conditioning is the expensive line: a central AC unit commonly needs 2,000–4,000 running watts and a hard starting surge on top, while window units run 500–1,200 each and can be added one at a time. Medical equipment goes on the sheet first, in red pen, before any of this arithmetic — CPAP machines, oxygen concentrators, refrigerated medication — with a healthy margin added that no optimization is allowed to touch.
Running Watts Versus Starting Watts
Every line on your sheet has two numbers, and the second one is where sizing mistakes live. Running watts is what an appliance draws steadily. Starting watts is the brief surge motors and compressors draw for the first second or two as they spin up — commonly two to three times the running figure, and worse for hard-starting compressors. A refrigerator that runs at 300 watts may ask for 900 at startup. A central AC that runs at 3,000 may demand 8,000 or more for a moment. Your generator has a running rating and a higher surge rating, and the sizing rule is simple: total running watts across everything you will run simultaneously must fit under the running rating, and the single largest starting surge, added on top of that total, must fit under the surge rating. You do not add every appliance's starting watts together, because motors do not all start at the same moment — but you must assume the worst realistic coincidence: the AC compressor and the refrigerator compressor kicking over within the same few seconds happens more often than you would guess, and staged starts (planning which big motor starts first) are a legitimate sizing tool rather than a hack.
Do Not Forget the Two Pumps
The two loads most often missing from first-time sizing sheets are the well pump and the sump pump, and both are exactly the loads that matter most when the power is out. A well pump commonly draws 1,000–2,000 running watts with starting surges well beyond, and without it a rural household has no water at all — no toilet refill, no drinking water, nothing. A sump pump draws little (300–800 running) but its starting surge matters, and its job is precisely the one storms interrupt: keeping the basement dry during the worst weather of the year. If either pump is on your sheet, add its surge to your worst-case arithmetic deliberately, and consider whether both pumps plus the furnace fan ever coincide — in a storm, they will.
From Load Sheet to Shortlist
Add up your tier's running watts, add the largest realistic starting surge, then add a working margin of about twenty percent. That margin is not padding for its own sake: generators lose capacity as they age, earn derating in real heat, and should not spend their whole life pinned at ninety percent load. The final number is your shortlist filter, and it is worth cross-checking against a dedicated sizing guide such as the home standby generator resource before you let a salesperson's square-footage rule of thumb talk you into a size your own sheet does not support. Whole-home coverage for a typical gas-appliance house often lands near a 14–18 kW unit; all-electric homes with central air routinely need 20 kW and up. Your sheet, not the average house, decides.
The Mistakes That Cost Real Money
Four errors account for most sizing regrets. Sizing from square footage instead of loads, which treats your house as the unit instead of your habits. Forgetting the pumps, which turns the outage you bought the machine for into the outage it fails. Ignoring starting surges entirely and then watching a perfectly adequate unit stumble on compressor kick-in. And sizing for a fantasy tier — everything running at once — that no real evening of your life has ever contained. Do the one-hour walk, write honest numbers, apply the surge rule, add the margin, and the machine you finally choose will feel almost boringly correct: quiet in its work, unstressed in a storm, and exactly as big as your house actually needed.