How to Size a Home Generator: A Practical Guide
Sizing a generator for your home is a two-step process: first, decide what you want to power during an outage; second, calculate the total wattage those appliances require. The calculator above handles step two automatically — just select your appliances and it does the math.
But before you open the calculator, it helps to understand what drives the numbers. There are really two types of home generator scenarios:
- Essential circuits only: Refrigerator, sump pump, lights, heat/AC in one room, medical equipment, phone charging. This typically requires 3,500–8,000 watts and can be handled by a mid-size portable generator.
- Whole-home power: Everything including central HVAC, water heater, kitchen appliances, and all lighting. This typically requires 10,000–22,000+ watts and requires a large portable or whole-house standby generator.
Most homeowners planning for emergency backup choose the “essentials only” approach for cost reasons — a 5,000–7,500 watt portable generator costs $600–$1,500, while a whole-house standby system costs $5,000–$15,000 installed. The essentials approach covers the most critical needs without the large investment.
The Most Power-Hungry Home Appliances
Understanding which appliances drive your generator requirement is key. In most homes, the largest loads are:
- Central air conditioning (3-ton): 3,000–4,000W running, 10,500–14,000W surge — the biggest constraint for most homes
- Electric dryer: 5,000–6,000W — completely impractical to run on a portable generator
- Electric water heater: 4,000–5,500W — often skipped; hot water in the tank lasts 2–4 hours without power
- Well pump (1 HP): 750W running, 2,100–2,400W surge — critical if you're on well water
- Sump pump: 800W running, 2,400–3,150W surge — critical during storms
- Refrigerator: 150–300W running, 600–1,200W surge — always a priority
The electric dryer and water heater are almost always dropped from emergency generator lists. Gas dryers and water heaters only need a small amount of electricity (for ignition and controls), making homes with gas appliances far easier to power with a smaller generator.
Generator Sizing Rules for Homeowners
Portable vs. Standby Generator for Home Use
The decision between a portable and standby generator is one of convenience vs. cost. Here's a straightforward comparison:
| Factor | Portable Generator | Standby Generator |
|---|---|---|
| Cost | $500–$2,500 | $5,000–$15,000+ installed |
| Installation | None (use extension cords or transfer switch) | Professional installation required |
| Startup | Manual (pull-start or electric start) | Automatic (within 10–30 seconds) |
| Fuel | Gasoline, propane, or dual-fuel | Natural gas or propane (permanent supply) |
| Size range | 1,000–12,500W typical | 7,500–20,000+ watts |
| Runtime | 8–12 hours per tank | Unlimited (on-demand fuel) |
| Best for | Occasional outages, flexibility | Frequent outages, medical needs, convenience |
What Generator Features Matter Most for Home Use
Once you've determined the wattage you need, look for these features:
- Electric start: Push-button starting is worth the extra cost for home backup use. Pull-starting a cold generator at 2am in a storm is no fun.
- Low-oil shutdown: Automatically shuts the generator off if oil level is too low, protecting the engine.
- GFCI outlets: Ground fault circuit interrupters protect against electrical shock — required for outdoor use.
- Fuel gauge: Know when you're running low before the generator stops unexpectedly.
- CO shutoff: Newer generators have carbon monoxide sensors that automatically shut off the engine if unsafe CO levels are detected.
- Inverter technology: If you need to power computers, TVs, or other electronics, look for a generator with inverter technology that produces “clean power” with stable voltage and frequency.
Don't Forget the Transfer Switch
A transfer switch is not optional — it's legally required in most jurisdictions and critically important for safety. Without a transfer switch, connecting a generator to your home's wiring (even through a dryer outlet or a “suicide cord”) sends electricity back onto utility lines, creating a deadly hazard for lineworkers and your neighbors.
A manual transfer switch costs $200–$600 installed by a licensed electrician and lets you choose which circuits receive power. An automatic transfer switch (ATS) costs $500–$1,500 installed and automatically switches your home to generator power within seconds of detecting an outage — ideal for standby generators.
Many homeowners use a transfer switch interlock kit ($75–$150) instead of a full transfer switch. This is a code-compliant metal bracket that physically prevents both your main breaker and generator breaker from being on simultaneously. It's DIY-friendly and allows manual selection of circuits.
For help sizing your whole-house standby generator, see our whole house generator size calculator.