How to Size a Standby Generator for Your Home
Choosing the wrong size standby generator is one of the most expensive mistakes a homeowner can make. Buy too small and the generator will be overloaded, run inefficiently, and potentially fail — possibly during the outage you bought it to handle. Buy too large and you’ve overpaid by thousands of dollars for capacity you’ll never use.
Proper generator sizing is a calculation — not a guess. This guide walks you through exactly how to determine the right generator size for your home.
Understanding Generator Sizing: kW and kVA
Generator capacity is measured in kilowatts (kW) — the amount of power the generator can produce continuously. Some specifications also reference kVA (kilovolt-amperes), which relates to apparent power. For residential purposes, focus on kW.
Important: generators have two power ratings:
- Rated/continuous power: What the generator can produce continuously — what you plan around
- Surge/starting power: A brief higher output (typically 10 seconds) that handles motor startup surges. Usually 10–25% higher than rated power.
Motor-driven appliances (AC compressors, refrigerators, well pumps, sump pumps) draw 2–3x their running wattage when starting. Your generator must handle these startup surges without tripping off.
Step 1: Decide What You Want to Power
Before calculating wattages, answer the fundamental question: what do you want to power during an outage?
Option A — Essential circuits only: Keep the basics running — lights, refrigerator, well pump if applicable, some outlets, phone charging. Smaller, less expensive generator.
Option B — Whole-home coverage: Power everything normally — all lights, all appliances, central AC/heat, all outlets. Larger generator, higher cost, maximum comfort.
Option C — Managed load coverage: Power most things but limit simultaneous high-draw appliances. Automatic load management systems (available on some modern generators) can handle this intelligently.
Step 2: List Your Critical Loads and Their Wattages
Go through your home and list every appliance you want to power. Find wattage on the label (usually on the back or bottom) or in the owner’s manual. Note both running watts and starting watts for motor-driven equipment.
Common Appliance Wattages
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Central AC (3 ton) | 3,500W | 7,500W |
| Central AC (4 ton) | 5,000W | 10,000W |
| Heat pump (3 ton) | 4,700W | 9,000W |
| Electric furnace | 10,000–20,000W | Same |
| Gas furnace (fan only) | 600–1,200W | 1,000–2,000W |
| Well pump (1 HP) | 750W | 2,000–3,000W |
| Well pump (1.5 HP) | 1,100W | 3,500–4,500W |
| Sump pump (1/2 HP) | 800W | 1,300W |
| Refrigerator | 150–400W | 800–1,200W |
| Freezer | 200–500W | 500–1,500W |
| Electric water heater | 4,500W | Same |
| Electric range (one burner) | 1,500–2,500W | Same |
| Microwave | 600–1,500W | Same |
| Dishwasher | 1,200–2,400W | 1,400–2,600W |
| Clothes washer | 500–1,000W | 1,500–2,000W |
| Electric dryer | 5,000–6,000W | Same |
| Lighting (LED, whole home) | 200–600W | Same |
| Television | 100–400W | Same |
| Computer/devices | 200–600W | Same |
| Garage door opener | 350–500W | 800–1,000W |
Step 3: Calculate Your Total Load
Add up the running watts of everything you want to power simultaneously. Then identify your largest motor-driven load — this determines the starting surge your generator must handle.
The rule: Your generator’s surge/starting capacity must exceed the running load of everything else PLUS the starting surge of the largest motor.
Example Calculation — Medium Home, Whole-Home Coverage
Running loads (simultaneous):
- Central AC (3 ton): 3,500W
- Refrigerator: 300W
- Lighting: 400W
- TV and devices: 300W
- Misc outlets: 500W
- Total running: 5,000W = 5 kW
Startup surge calculation:
- Everything else running: 1,500W
- AC startup surge: 7,500W
- Peak startup demand: 9,000W = 9 kW
Generator needed: Rated capacity of at least 5 kW continuous, with surge capacity of at least 9 kW. A 14 kW generator handles this comfortably with reserve capacity for additional loads.
Example Calculation — Large Home, All Loads
Running loads:
- Central AC (4 ton): 5,000W
- Electric water heater: 4,500W
- Refrigerator + freezer: 600W
- Lighting: 500W
- Washer: 800W
- TV and devices: 500W
- Well pump: 750W
- Total running: 12,650W = 12.65 kW
Peak startup demand:
- Everything minus AC: 7,650W
- AC startup surge: 10,000W
- Peak: 17,650W = 17.65 kW
Generator needed: Rated capacity of at least 13 kW, surge of at least 18 kW. A 20–22 kW generator is appropriate.
Quick Sizing Guide by Home Type
- Small home (under 1,500 sq ft), essential only: 7–10 kW
- Small home, whole-home: 10–14 kW
- Medium home (1,500–2,500 sq ft), whole-home: 14–20 kW
- Large home (2,500–4,000 sq ft), whole-home: 20–26 kW
- Very large home (4,000+ sq ft): 26 kW+
- Home with electric vehicle charging: Add 7,200W (Level 2 charger) to your calculation
- Home with electric range: Add 8,000–12,000W if you want to cook normally during outages
Special Considerations That Change the Calculation
Electric vs. Gas Appliances
Homes with natural gas cooking, water heating, and heating require significantly less generator capacity than all-electric homes. If your furnace, water heater, and range are gas, your generator only needs to handle HVAC fan, refrigerator, lights, and outlets — potentially a 10–14 kW unit for a medium home.
Well Pump
If your home uses a well pump for water, it must be included in your generator sizing. Well pumps have high startup surges and draw significant power — this often pushes sizing up one tier.
Medical Equipment
If anyone in the home uses medical equipment that requires continuous power (oxygen concentrator, home dialysis, powered wheelchair), that equipment’s power requirements must be guaranteed. Add its wattage and confirm the generator can handle it simultaneously with other loads.
Electric Vehicle
If you want to charge your EV during an outage, a Level 2 charger draws 7,200W (30A × 240V). This significantly increases generator requirements. Most homeowners skip EV charging during outages to preserve generator capacity for essential needs.
The Load Assessment — Why You Need a Professional
While this guide gives you the framework, the most accurate generator sizing comes from a professional load assessment. A licensed electrician or generator dealer will:
- Review your electrical panel to identify all circuits and loads
- Measure actual current draw of your appliances (more accurate than nameplate ratings)
- Calculate startup surge requirements precisely
- Account for your specific home’s configuration and usage patterns
- Recommend the right generator model and automatic transfer switch
Most reputable generator dealers offer load assessments for free as part of the quoting process. Always get at least two or three assessments — sizing recommendations can vary between dealers.
Oversizing vs. Undersizing — Which Is Worse?
Undersizing: Generator runs at or above capacity continuously, overheats, shortens lifespan, may trip on overload or fail. Could leave you without power during the outage you bought it for. Clearly worse.
Oversizing: Generator runs at low load, which causes “wet stacking” in diesel engines (less of a concern with gas generators), burns slightly more fuel per unit of output, and costs more upfront. Generally acceptable if modest — sizing up one tier from your calculated need provides useful reserve capacity.
The professional recommendation is typically to size 10–20% above your calculated peak load — providing a safety buffer for future load additions and preventing the generator from running at maximum capacity.
The Bottom Line
Proper generator sizing starts with knowing what you want to power, calculating your simultaneous running load, and ensuring the generator’s surge capacity handles your largest motor startup. For most medium homes wanting whole-home coverage, a 14–20 kW generator hits the sweet spot. Get a professional load assessment before purchasing — the cost is zero and the accuracy is worth it.