“What size generator do I need?” is the first real question of any standby project, and it is the one people get wrong most often. Undersize the unit and it strains, trips, or refuses to carry the house during a hurricane-season outage. Oversize it and you have paid for capacity and fuel that never get used. For most Charleston-area homes the right answer sits in a narrower band than shoppers expect, and reaching it comes down to a little math and an honest look at what actually runs during a Lowcountry outage.
Quick note on who we are. Lowcountry Generator Pros connects you with a single vetted, licensed local installer who performs the real load calculation on site. This is a homeowner resource, not a contractor, and it carries no star ratings or invented reviews. The point of this guide is to get you fluent before that conversation. Want a fast ballpark first? Try the generator size calculator.
What Size Generator to Run a Whole House
The honest answer to “what size generator do I need to run a house” is that it depends on your loads, not your address. That said, a few reference points help orient the decision before an installer measures anything.
Standby generators are rated in kilowatts (kW), a measure of how much power the unit can deliver at once. A house never draws a fixed amount; it draws whatever is running in that moment, and the peaks are what sizing has to survive.
Two numbers drive everything:
- Running watts, the steady power a device uses while it is on.
- Starting (surge) watts, the brief spike when a motor starts. Air conditioners, well pumps, and refrigerator compressors can pull two to three times their running wattage for a split second at startup.
Whole-house standby generator sizing is about covering your realistic peak, the worst moment when several things run and a large motor kicks on, not the gentle average. That gap between average and peak is exactly where do-it-yourself estimates fall apart.
Sizing by Square Footage (a Rough Starting Point)
Square footage is a blunt tool, but it gives a first guess when you have nothing else. As a very loose orientation for Lowcountry homes with central air:
- Under ~1,500 sq ft: often lands in the 14 to 18 kW range for essentials plus one AC system.
- ~1,500 to 3,000 sq ft: frequently the 20 to 26 kW workhorse range.
- Over ~3,000 sq ft or multiple HVAC zones: commonly 27 kW and up, sometimes liquid-cooled.
Treat these as conversation starters, not quotes. Two homes of identical size can need very different generators depending on AC tonnage, electric versus gas appliances, well and septic pumps, and whether load management is used. Square footage ignores all of that, which is why the appliance-level math below matters more.
Sizing by Appliance: Adding Up the kW
A more reliable method is to tally the major loads that will run during an outage, then add the single largest startup surge on top. Rough running figures for common Lowcountry loads:
- Refrigerator or freezer: ~0.7 kW running, ~1.4 kW to start
- Central AC (3 ton, ~1,500 sq ft): ~4.5 kW running, up to ~7.5 kW to start
- Well pump (1 HP): ~2 kW running, ~4 kW to start
- Electric water heater: ~4.5 kW
- Electric range or oven: ~3 kW
- Electric dryer: ~5.4 kW
- Lights, outlets, Wi-Fi, TV: ~2 kW combined
Add the running watts of everything you want on at once, then add only the biggest single surge (motors rarely all start at the same instant). That total, in watts divided by 1,000, is roughly the kW you need. The generator size calculator does this arithmetic for you and applies the AC startup surge automatically.
The Charleston AC and Humidity Surge Factor
This is the part generic sizing advice skips, and here it is the single most important variable.
Air conditioning is almost always the biggest load in a Lowcountry home, and it runs hard for a long stretch of the year. Summers here are not just hot; the air is saturated, so the AC works overtime pulling moisture out, not only cooling. Compressors cycle often and stay loaded. Two consequences follow:
- The AC startup surge dominates the math. A central compressor draws a heavy spike when it engages, and the generator must absorb that surge while still powering everything already running.
- You will genuinely run the AC during an outage. In cooler regions a backup generator might never touch the air conditioning. After Hugo, and in every sweltering August since, an outage with no AC is a real safety problem in the Lowcountry, so cooling is a primary load, not an afterthought. That pushes the required size up compared with a milder climate.
A capable installer sizes for the surge of your specific compressor and treats Charleston AC as sustained, primary load.
Whole-Home vs. Managed (Load-Shedding) Sizing
There are two ways to size, and the choice shapes the whole project.
Whole-home coverage means the generator is large enough to run essentially everything at once, exactly like the grid does. It is the simplest experience, since you never think about what is on, but it usually means a bigger, costlier unit.
Managed (load-shedding) coverage adds smart management, either load-shedding modules or a smart transfer switch that prioritizes circuits. If demand spikes, say two AC systems try to start together, the system briefly sheds a lower-priority load so the generator is never overwhelmed. That lets a smaller, more affordable unit comfortably protect a large home by managing peaks instead of brute-forcing them. For many Lowcountry homes, especially larger Mount Pleasant and Daniel Island houses with multiple HVAC systems, a smart managed setup is the best balance of cost, fuel use, and coverage.
How a Pro Runs the Standby Generator Sizing
A real sizing visit goes well past a square-footage rule of thumb. The installer:
- Inventories major loads: every AC compressor and air handler, well and septic pumps, water heater, range, dryer, and standout appliances.
- Captures starting surges, especially for compressors and pumps, since those spikes set the peak.
- Reviews the electrical panel: service size, breaker layout, and what realistically runs together.
- Chooses the coverage strategy: whole-home versus managed load-shedding, and which circuits get priority.
- Factors in the coastal climate, treating Charleston heat and humidity as sustained AC load.
- Sizes for the real peak with modest headroom, enough for the worst realistic moment without padding far beyond it.
Don’t Oversize
Bigger is not safer; it is usually just more expensive and can run worse. You pay upfront for capacity you never use, you burn more natural gas or propane during a multi-day Dominion outage, and a lightly loaded generator never reaches its efficient operating point, which is not ideal for long-term engine health. The goal is the right-sized unit, often a properly sized model paired with smart load management, that carries your home through the outages this coast actually delivers.
Next Steps
Once you have a rough size, the next calls are fuel and connection:
- Natural Gas vs. Propane, which fuel fits your property and how it affects runtime.
- Best Whole-House Generator, matching a specific unit to your home.
- Generac vs. Kohler vs. Cummins vs. Briggs, how the brands differ once you know your size.
Ready for a real load calculation? Run the sizing calculator for a ballpark, then see what we cover in Charleston and Mount Pleasant, and we will connect you with our vetted local installer.