What Size Generator Do I Need for My Home?
Buying a generator without knowing what size you need is like buying a car without considering whether you need a compact sedan or an SUV. Generator sizing is a critical decision that determines whether your backup power system truly protects your home or leaves you vulnerable when it matters most.
Understanding the Two Power Numbers
Running Watts
The continuous power an appliance needs to operate. A refrigerator needs ~700W to stay cold; your HVAC needs ~3,500W to run. This is what the generator produces continuously.
Starting Watts
Electric motors need a brief surge of power to start often 1.5–2× the running load. Your AC might need 3,500W running but 6,000W+ to start. The generator must handle both.
The Sizing Formula
(Total Running Watts × 1.2) ÷ 1,000
The 1.2 multiplier adds a 20% safety buffer for surges, voltage drops, and future appliance additions.
Step-by-Step: What to Include
Always Include
- → Refrigerator & freezer
- → HVAC system (heating or cooling)
- → Electric water heater (if applicable)
- → Well pump & sump pump (if applicable)
- → Essential lighting
- → Internet modem & WiFi router
- → Medical equipment (CPAP, oxygen concentrator)
Usually Include
- → Microwave or stovetop (limited use)
- → Washer or dryer (one at a time)
- → Garage door opener
- → Television & home office equipment
Usually Exclude
- → Multiple large appliances simultaneously
- → Electric vehicle charging
- → Pool or hot tub systems
- → Lawn irrigation
Typical Appliance Wattages
| Appliance | Running Watts |
|---|---|
| Central AC (3-ton) | 3,500–4,000 W |
| Electric water heater | 4,000–5,500 W |
| Electric range / cooktop | 1,500–6,000 W |
| Well pump (1 hp) | 1,000–1,200 W |
| Sump pump (0.5 hp) | 800–1,000 W |
| Refrigerator | 600–800 W |
| Gas furnace blower | 800–1,000 W |
| Dishwasher | 1,800–2,400 W |
| Washer | 500–1,200 W |
| Electric dryer | 2,000–5,500 W |
| Microwave | 1,000–2,000 W |
| TV + electronics | 80–500 W |
| Internet / WiFi | 10–20 W |
| CPAP machine | 30–60 W |
| Oxygen concentrator | 300–600 W |
A Worked Example
Typical Kansas City Whole-Home Calculation
Quick Sizing by Home Size
Small Home
1–2 Bedrooms
10–15 kW
whole-home backup
Medium Home
3 Bedrooms
15–20 kW
whole-home backup
Large Home
4+ Bedrooms
20–25 kW
whole-home backup
Common Sizing Mistakes
Undersizing to save money
A 10 kW generator for an 18 kW home runs at max capacity constantly, fails to start large appliances, and creates a false sense of security.
Forgetting starting surge requirements
Running watts only tells half the story. The 1.2× multiplier is essential for motor startup headroom.
Sizing 'for now' only
Upgrading later is expensive. Plan for reasonable future additions EV charger, hot tub, pool equipment.
Dramatically oversizing
A 30 kW unit for a 15 kW home wastes money on purchase, installation, and fuel. 10–15% headroom is ideal.
The Final Recommendation for Kansas City Homes
For natural gas whole-home backup the optimal choice for most KC homeowners these sizes cover the vast majority of situations:
Not sure what size you need?
GridGuard provides free sizing consultations for Kansas City homeowners. We assess your panel, your load profile, and your budget then recommend the right KOHLER® system for your home.