If you're shopping generator quotes right now, you've probably noticed the numbers all over the place and the explanations pretty thin. Here's what actually goes into sizing and installing one correctly, so you know what to ask before you sign anything.
How big of a generator do you actually need?
There are really two options, and any contractor should ask which one you want before quoting a size.
Essential circuits only — refrigerator, sump pump, some lights and outlets, maybe your furnace blower — usually runs 5,000 to 10,000 watts. This is the cheaper option and it's enough for most people who just want the house livable during an outage.
Whole-house — everything, including central air — runs 14,000 to 26,000 watts depending on the size of your home and your AC unit. This costs more but you never have to think about what's on and what's off during an outage.
Here's the part most homeowners don't know, and it's the single biggest reason DIY sizing charts get it wrong: appliances with a motor — your AC, your fridge, your sump pump, your well pump if you have one — pull a lot more power for a second or two when they *start* than they do while running. A generator sized only for "running watts" can power your AC just fine once it's on, and still fail every time it tries to start it. If a quote doesn't account for starting watts, the size on that quote is a guess.
The right way to size a generator isn't a rule of thumb off a chart — it's a real load calculation based on your actual panel and what's on it. That's a few extra minutes for whoever's bidding the job. If they're not doing it, they're guessing, and you're the one who finds out it was wrong the first time the power goes out.
Natural gas or propane?
This usually isn't a choice — it's whatever's already available at your property. If you have natural gas service, that's the simple answer: connected to the line, no tank, never runs out. If you don't, you're on propane, and that means a tank sized to both the generator and how many days of runtime you want without a refill. A smaller 250-gallon tank gets you a few days at typical load; most homeowners doing a whole-house unit end up on a 500-gallon tank for a week or more of cushion.
One thing to plan for: the propane tank and gas line are usually a separate company from your electrician. That's not a delay anyone's hiding from you, it's just how the trades split — worth asking about up front so you're not surprised.
Why you can't just plug it in
This is the one non-negotiable part of the job, and it's worth understanding why. A generator has to connect to your house through a transfer switch — a piece of equipment that makes sure your house is either running off the utility or off the generator, never both at the same time. Without it, power from your generator can travel backward out onto the utility lines during an outage. That's dangerous for the linemen working to restore power, who have every reason to believe those lines are dead. It's also a code violation, full stop. Any contractor telling you that you can skip the transfer switch, or that you can just plug the generator into an outlet, is telling you something that isn't legal to install.
Where the generator actually goes matters more than people think
A generator has to sit far enough from windows, doors, and anything that pulls air into your house — vents, dryer exhaust, HVAC intakes. The standard clearance is 5 feet, and the reason isn't arbitrary: generator exhaust is carbon monoxide, and carbon monoxide is odorless. That clearance is the thing standing between normal operation and CO getting pulled into your home. National safety data consistently shows portable generators — not properly installed standby units — as the leading cause of generator-related carbon monoxide deaths, almost always from units run too close to the house or indoors during an outage. A correctly sited, professionally installed unit with the right clearance is specifically what avoids that.
You'll sometimes see contractors or manufacturers advertise an 18-inch clearance instead of 5 feet. That's real, but it only applies to specific fire-tested units with a listing that allows it — not a default you get to assume. If a bid quotes 18 inches, ask them to show you why that specific unit qualifies for it.
Beyond the safety clearance, the unit also needs room for a technician to actually service it later, clearance overhead, and it needs to respect your property line setback, which varies by local rules. All of this needs to be worked out before the pad gets poured, not after.
What a permit is buying you
Generator installations require a permit and an inspection — this is normal, not a red flag, and any contractor skipping it is cutting a corner that becomes your problem, not theirs, if something goes wrong later. The inspection checks the sizing, the transfer switch, the clearances, and the grounding. It's the second set of eyes confirming the job was actually done right.
Questions worth asking whoever's bidding your job
- Did you run a load calculation, or are you sizing off a chart?
- What size transfer switch, and is it automatic or manual?
- Where exactly will the unit sit, and does that meet the 5-foot clearance from openings?
- Is a permit included in the price, or extra?
- Natural gas or propane, and if propane, what tank size and who's installing it?
If a contractor has clear answers to all five without hesitating, that's a good sign you're dealing with someone who actually does this work correctly, not just someone selling a generator.
If you'd rather just have it done right
Red Cedar Electric installs standby generators across La Vergne, Smyrna, Murfreesboro and the surrounding Middle Tennessee communities. Every install starts with a real load calculation off your actual panel — not a chart — and the permit and inspection are part of the job, not an add-on you find out about later.
If you already have a bid in hand and want a second opinion on whether the sizing holds up, that's a conversation worth having before you sign.
Request service or call 615-625-2163.
