“What size whole-house generator do I need?” is the first question almost every Greater Philadelphia homeowner asks once storm season knocks the power out for the third time. The honest answer is: it depends on how much of your home you want to keep running, and the only way to get an exact number is a professional load calculation. But you can get remarkably close on your own with a simple wattage worksheet. This guide from our Jenkintown electricians walks you through how to size a standby generator, the ranges most homes land in, and where a licensed pro comes in. When you’re ready for a firm answer, book a free generator sizing consultation and we’ll run the numbers for your specific panel and appliances.

What Size Whole-House Generator Do I Need? Start With What You Want to Run

Before you can answer “what size generator for whole house” coverage, decide on your goal. There are really two camps. The first is essential-circuits backup: you keep the lights, the fridge, the furnace blower, a sump pump, and a few outlets alive so the house stays safe and livable. The second is whole-home coverage: everything runs as if the grid never went down, including central air conditioning, electric ranges, and multiple large appliances at once. Your answer to “how big of a generator to run a whole house” changes dramatically depending on which of these you choose, and central AC is usually the single biggest swing factor.

How Many Watts to Run a Whole House? A Simple Sizing Worksheet

To estimate how many watts to run a whole house, add up the wattage of everything you want powered at the same time. The catch is that motor-driven appliances (air conditioners, well pumps, sump pumps, refrigerators) briefly draw a large surge when they switch on. That’s the difference between running watts (what a device uses continuously) and starting watts (the short spike at startup). Your generator has to handle the combined running load plus the largest starting surge happening at once.

As a rough guide, here are typical wattage ranges for common household items. Treat these as general estimates, not exact figures for your specific models:

• Central air conditioner (3–4 ton): typically ~3,500–5,000 running watts, with a much higher starting surge
• Well pump: typically ~1,000–2,000 running watts, plus a significant starting spike
• Sump pump: typically ~800–1,050 running watts, with a starting surge on top
• Refrigerator/freezer: typically ~150–800 running watts, spiking when the compressor kicks in
• Electric furnace blower: typically ~500–800 running watts
• Water heater (electric): typically ~3,000–4,500 running watts
• Lights, TV, Wi-Fi, phone chargers combined: typically ~a few hundred watts
• Electric range/oven: typically ~2,000–5,000 watts when in use

Add the running watts of everything you want on at once, then add the single largest starting surge. That total, with some headroom, points you toward a generator size. A professional load calculation refines this using your actual panel, appliance nameplates, and how your circuits are grouped, so you never oversize (wasting money) or undersize (risking overload).

Common Whole-House Generator Sizes for Philly-Area Homes

Most homeowners want to know where they’ll likely land before the site visit. As general ranges, many homes fit an air-cooled standby generator in the 14kW–26kW range. A smaller unit around 14kW–18kW often covers essential circuits and select comfort loads for a modest home. Larger homes wanting fuller coverage, especially with central AC, frequently move toward the 22kW–26kW end of the air-cooled tier. Homes that want true whole-home backup with multiple large appliances and central air running simultaneously may need a liquid-cooled unit beyond the air-cooled range. These are typical patterns, not promises; your load calculation determines the real number.

Generac and Kohler: Air-Cooled vs Liquid-Cooled Tiers

Standby generators from brands like Generac and Kohler generally fall into two tiers. Air-cooled models sit at the smaller-to-mid capacity end and are the most common choice for residential essential-circuit and partial whole-home coverage. Liquid-cooled models handle higher, sustained loads and are built for larger homes or true whole-home coverage with heavy simultaneous demand. Both tiers typically run on natural gas or propane and start automatically within seconds of an outage. The right tier follows from your sizing decision, not the other way around, which is why we recommend settling the load calculation first. You can learn more about our standby generator and whole-home power services to see how we approach each install.

What a Licensed Electrician Handles

A whole-house generator is not a plug-in appliance; it’s a permanent electrical system that ties into your home’s power. A licensed electrician handles the pieces that keep it safe, code-compliant, and reliable:

• Load calculation to determine the correct generator size for your home and priorities
• Automatic transfer switch installation, which safely disconnects you from the grid so the generator can never back-feed the utility line (a serious hazard to line crews)
• Permits and inspection so the work meets local code in your township
• PECO coordination for the utility-side requirements of tying a standby generator into your service
• Gas hookup coordination for natural gas or propane fuel supply, sized to the unit
• Panel evaluation to confirm your electrical panel can support the install; if it’s older or undersized, an upgrade may be part of the plan

If your panel is aging, it’s worth understanding the numbers before you add a generator on top of it; our guide on electrical panel upgrade cost covers what to expect.

Why Correct Sizing Matters for Safety and Reliability

Sizing isn’t just about convenience. An undersized generator can overload when a big motor surges, causing it to trip, shut down, or run hot, exactly when you need it most during a storm. An oversized unit means paying for capacity you never use and running the engine inefficiently. Getting the size right means your generator starts reliably, carries the loads you chose without strain, and lasts. That’s the whole point of a proper load calculation rather than a guess off a chart.

If you’re weighing options ahead of the next Greater Philadelphia storm season, don’t gamble on a rough guess. Our Jenkintown electricians will run a real load calculation, walk you through essential-circuit versus whole-home coverage, and recommend a right-sized unit and transfer switch for your home. Contact us for a free generator sizing consultation and get a number you can trust.