I've been handling generator install and service orders for an electrical contractor for nine years. In that time I've personally made six mistakes big enough to write down — roughly $14,000 in rework, gas line re-dos, and one very uncomfortable phone call. Now I keep the pre-survey checklist our team runs before anyone signs a work order.
Someone asks me "what size generator do I need?" probably ten times a month. And the honest answer is: there isn't one. It depends on what you want running when the power's out, for how long, and what your house can actually support. But out of the roughly 400 installs I've helped move through, almost every one falls into three buckets. And I can give you a fairly specific answer for each.
Here's how I split them:
- Scenario A: You want to survive a few hours to a day or two — fridge, a few lights, the router, your phone
- Scenario B: You want the house to keep functioning without anyone babysitting it, but you can live without the dryer
- Scenario C: Big house, big loads, or a business running out of the garage
Scenario A: You just want the lights on and the fridge cold
This is the most common one, and it's also the one people overthink the most. If your actual need is the fridge, some lights, the router, maybe a sump pump, you're looking at two words: inverter generator.
Inverter units — basically a small generator plus an electronic module that converts AC to DC and back — give you cleaner power and variable engine speed, which means quieter and more fuel-efficient. Most sit around 50 to 65 dB and fit in a trunk. Compare that to a conventional portable generator, which is louder, thirstier, and usually cheaper, but rougher on sensitive electronics.
The generator vs inverter thing isn't just spec-sheet trivia, by the way. If you're powering a laptop, a furnace with an electronic ignition board, or a modern fridge, my experience is that the cleaner output genuinely matters. You pay for it, but you pay for it once.
Speaking of which — if you end up with a gas portable, pay attention to the plug. A lot of small units ship with an NGK BR9ES heat range, and people swap in whatever's on the shelf. The engine starts hard, misfires under load at normal running speed, and everyone blames the carburetor. It isn't the carburetor. Use the plug on the label or in the manual; if you can't find it, cross-reference it properly, don't just grab the closest number.
And please don't run a portable in the garage or on a covered porch. Carbon monoxide. I've had service calls that started because someone smelled gas near a bedroom window and the machine was three feet away on the other side of it.
Scenario B: You want the house to run itself while you sleep
This is the home standby band — roughly 7.5 to 26 kW, an automatic transfer switch, natural gas or propane. Based on what I've seen, most single-family homes land between 14 and 24 kW once you account for AC, the furnace, and a well pump. Above that, you're in a different conversation.
The hard part in this scenario isn't picking the unit. It's the transfer switch and the main breaker panel. Not every 200-amp panel has room for a whole-house transfer. Some can only take a managed or selective setup — meaning you pick which circuits get backup and which go dark.
And per NFPA 70 (NEC) Article 702, an optional standby system either has to carry the entire load, or it needs load management that's been calculated and documented. Translation: you can't just jam a 22 kW unit onto a panel and hope.
I've made that exact mistake. In 2018 I signed off on a 22 kW unit for an older four-bedroom without confirming gas meter capacity or doing a real load calc — because "the neighbor has a 22 kW too." The utility said no. Re-running the gas line plus the re-inspection came to $1,180 and a one-week delay before the house had power.
Now the prevention piece. Fifteen minutes on the survey to measure the service entrance, read the gas meter tag, and count breakers on the panel — that's what actually saves the money later. If your load calculation lands right on the edge, drop a size down and accept that you'll shed a couple of appliances at runtime. That's cheaper than a unit the gas line can't feed and the panel can't accept.
Scenario C: Large home, heavy loads, or a shop in the garage
Once you're past 30 kW, a lot changes. A 30kW Generac generator is typically a liquid-cooled unit rather than air-cooled. That means a different noise profile, a different maintenance schedule — oil, coolant, belts — and a much larger footprint. Generally, the 30 to 38 kW range exists for homes over 4,000 square feet with multiple HVAC systems, well pumps, EV chargers, or shop equipment.
Circuit breaker panels are where the real money goes in this scenario. It's common to be looking at a 400-amp service upgrade, or a load management center that can handle whole-house transfer. If your panel already has tandem breakers or a meter-main, expect the electrical scope to run higher than you guessed.
The most frustrating part of this whole bracket: customers fixate on the unit price, but the electrical scope is what drives the quote. The unit is right, the site is wrong, and the answer is still wrong.
One thing that genuinely helps — pull twelve months of gas and electric bills and look at your actual peak demand. Don't estimate off square footage. Use a real number.
How to tell which scenario you're in
Honestly? This is simpler than the checklist I just wrote. Ask yourself three questions in order:
- What's your single largest load? Furnace, central AC, well pump, EV charger, tools. Look at the nameplate for starting wattage, not running wattage.
- What are you willing to shed? If you're fine not running the dryer, the oven, or the EV charger during an outage, a Scenario B unit will carry a surprisingly large house.
- How long are you staying? Five-plus years, and overbuying capacity usually pays off. Renting or selling soon, and a portable inverter is probably enough.
If all three still leave you guessing, size for one large load plus your baseline, then check the fuel math twice. That's the part that actually bites people.
I don't have hard data on exactly how often mis-sizing causes rework across the industry — we didn't track it from the start, and I wish we had. What I can say anecdotally is that in the last 18 months, running this check before every order has caught four mismatches that would have become site-day problems.
The checklist before anything gets installed
Whatever scenario you land in, run this:
- Service entrance capacity — main breaker amps and available panel spaces
- Fuel type and actual fuel availability — gas meter capacity or propane tank size
- Starting watts vs running watts for every major load
- Placement clearances — from combustibles, windows, and air intakes
- Transfer switch type — whole-house or managed load panel
- Local permit requirements — in most jurisdictions this needs a licensed electrician and a separate permit
Each of those items is either a few hundred dollars or a week of waiting. There's no free lunch on any of them.
If you're still stuck between scenarios, measure first, calculate second, buy third. Doing it in the other order is the most expensive way to do it.
