How to Size a Home Standby Generator

Picking the right size is the single most important decision in a standby generator project. Too small and the unit struggles — or refuses — to carry your home when you need it during a hurricane-season outage. Too big and you’ve paid for capacity and fuel you’ll never touch. The sweet spot for most Charleston-area homes is narrower than people expect, and getting there comes down to understanding a few basics and then letting a pro do the math.

Quick reminder of who we are. Lowcountry Generator Pros connects you with a single vetted local installer who performs the actual load calculation on-site. We’re a homeowner resource, not a contractor, and we don’t post star ratings or invented reviews. This guide is here so you walk into that conversation already knowing the language.

kW Basics: What You’re Actually Buying

Standby generators are rated in kilowatts (kW) — a measure of how much electrical power the unit can supply at once. Your house doesn’t draw a fixed amount; it draws whatever happens to be running at a given moment. The lights, the refrigerator, the well pump kicking on, the AC compressor starting up — those loads stack on top of each other.

Two numbers matter:

• Running watts — the steady power something uses while it’s on.
• Starting (surge) watts — the brief spike when a motor first starts. Air conditioners, well pumps, and refrigerator compressors can pull two to three times their running wattage for a split second at startup.

Sizing is about making sure the generator can handle your realistic peak — the worst-case moment when several things run and a big motor starts — not just the gentle average. This is exactly where do-it-yourself estimates go wrong.

Whole-Home vs. Managed (Load-Shedding) Coverage

There are two philosophies for sizing, and the right one shapes everything.

Whole-home coverage

The generator is large enough to run essentially everything in the house at once, just like the grid does. It’s the simplest experience — you never think about what’s on — but it usually means a bigger, more expensive unit.

Managed / load-shedding coverage

Instead of sizing for “everything at once,” the installer adds smart management — load-shedding modules or a smart transfer switch that prioritizes circuits. If demand spikes (say two AC units want to start together), the system briefly sheds a lower-priority load so the generator never gets overwhelmed. This lets a smaller, more affordable generator comfortably protect a large home, because it intelligently manages the peaks instead of brute-forcing them.

For most Lowcountry homes, a smart managed setup hits the best balance of cost, fuel use, and coverage — especially in larger Mount Pleasant and Daniel Island homes with multiple HVAC systems.

The Charleston AC + Humidity Surge Factor

This is the part generic sizing advice misses, and it’s the most important factor here.

Air conditioning is almost always the single biggest load in a Lowcountry home, and it runs hard for a huge chunk of the year. Our summers aren’t just hot — they’re saturated with humidity, which means your AC works overtime to pull moisture out of the air, not just to cool it. The compressors cycle frequently and stay loaded.

Two things follow from that:

1. The AC startup surge dominates the sizing math. Central AC compressors draw a heavy spike when they kick on. The generator has to absorb that surge while still powering everything else that’s already running.
2. You’ll actually be using the AC during outages. In much of the country, a backup generator might never run the air conditioning. Here, an August outage with no AC is a safety problem, so the AC must be part of the plan — which pushes the required size up compared to a cooler climate.

A good installer accounts for the surge of your specific compressor(s) and the reality that, in Charleston, the AC isn’t optional backup load — it’s a primary one.

Typical Size Tiers

Every home is different, but standby units tend to cluster into a few ranges. Use these as a rough orientation, not a quote.

~14–18 kW — smaller homes and essential coverage

A good fit for compact homes, condos with the right setup, or homeowners who want to reliably cover the essentials — refrigerator, well/septic pump, lights, internet, and a single AC system, often with load management smoothing the peaks.

~22–26 kW — the workhorse range for most Lowcountry homes

This is where a lot of Charleston-area single-family homes land. It comfortably handles a full set of essentials plus central air, typically with smart load management, and suits the majority of Mount Pleasant, West Ashley, and Summerville households.

27+ kW — large homes and whole-home, no-compromise coverage

Larger or older homes with multiple HVAC zones, pools, electric ranges, and big appliance loads — or anyone who simply wants true whole-home power with everything running at once — move into this tier.

How a Pro Does the Load Calculation

A real sizing visit goes well beyond a square-footage rule of thumb. Here’s what the installer is doing:

1. Inventories your major loads — every AC compressor and air handler, well and septic pumps, water heater, range/oven, dryer, and any standout appliances.
2. Captures starting surges — especially for AC compressors and pumps, since those spikes drive the peak.
3. Reviews your electrical panel — service size, breaker layout, and what’s realistically going to run together.
4. Decides the coverage strategy — whole-home vs. managed load-shedding, and which circuits get priority.
5. Factors in the local climate — Charleston’s heat and humidity mean the AC is treated as primary, sustained load, not an afterthought.
6. Sizes for your real peak with headroom — enough to handle the worst realistic moment, without padding the unit far beyond it.

The result is a recommendation built on your actual home, not a guess.

Don’t Oversize

It’s tempting to think “bigger is safer.” It usually isn’t — it’s just more expensive and, ironically, can run worse.

• You pay more upfront for capacity you’ll never use.
• You burn more fuel. During a multi-day Dominion outage, an oversized unit running well under its efficient load wastes natural gas or propane you’d rather stretch.
• Lightly loaded generators can run inefficiently and don’t reach their ideal operating point, which isn’t great for long-term engine health.

The goal isn’t the biggest generator on the block. It’s the right-sized unit — usually a properly sized model paired with smart load management — that carries your home through the outages this region actually delivers.

Next Steps

Once you know roughly what size you’re looking at, the next decisions are fuel and connection:

• Natural Gas vs. Propane — which fuel suits your property, and how it affects runtime and installation.
• Generator Transfer Switch Guide — how your generator safely takes over from the grid, and where load management lives.

Ready for a real load calculation on your home? Start on our homepage and we’ll connect you with our vetted local installer, or see what we cover in Mount Pleasant and across the Charleston Lowcountry.