Generator Sizing for Residential
Overview
Generator sizing is the #1 design decision. Undersized = generator can't handle load, trips off, customer angry. Oversized = wasted money, less efficient operation. This reference covers the math.
Two sizing approaches
Approach A: Whole-house
Generator supports ALL electrical loads. Larger generator + transfer switch handles entire main panel.
Pros:
- Customer can run anything they want
- Simpler ATS (transfers entire panel)
- More customer satisfaction
Cons:
- More expensive generator
- More expensive transfer switch
- More fuel consumption
Approach B: Essential circuits
Generator supports SELECTED critical circuits only. Sub-panel or transfer switch carries only chosen loads.
Pros:
- Smaller generator (cheaper)
- Lower fuel cost
- Easier installation
Cons:
- Customer can't run all appliances
- Some inconvenience
- Customer may forget which circuits are protected
For most residential: whole-house preferred (customer satisfaction).
Load calculation
For whole-house approach:
Step 1: List major loads
| Load | Typical wattage |
|---|---|
| Central AC (3-ton) | 3,500-4,500 W |
| Heat pump (3-ton) | 3,500-4,500 W |
| Electric furnace | 15,000-25,000 W |
| Gas/oil furnace blower | 600-1,200 W |
| Electric water heater | 4,500 W |
| Electric range / oven | 8,000-12,000 W |
| Electric dryer | 5,000 W |
| Washing machine | 500-1,000 W |
| Dishwasher | 1,200-1,500 W |
| Refrigerator (running) | 150-200 W |
| Refrigerator (startup) | 600-1,000 W |
| Freezer | 100-300 W |
| Microwave | 1,000-1,500 W |
| Lights | 100-500 W per room |
| TV | 100-300 W |
| Computer | 200-400 W |
| Garage door opener | 800-1,200 W |
| Well pump (1 HP) | 2,000-2,500 W |
| Sump pump | 800-1,500 W |
| Septic pump | 1,000 W |
Step 2: Identify simultaneous loads
Not all loads run simultaneously. Identify:
- Always-on: refrigerator, freezer, internet router, alarm
- Heating/cooling: AC OR furnace (not both in residential)
- Cooking: range cycle on/off as needed
- Other appliances: typically one at a time
Step 3: Calculate continuous + peak
- Continuous load = always-on + heating/cooling + lights
- Peak load = continuous + largest appliance starting up
Generator must handle peak. Continuous load determines fuel consumption.
Step 4: Generator sized for peak
Most generators rated for "starting watts" + "running watts":
- Starting watts: brief peak during motor startups
- Running watts: continuous capacity
Need: running watts ≥ continuous + appliance running watts; starting watts ≥ peak with largest motor starting.
Practical sizing rule
For a typical 2,000 sq ft home:
- Small (1-2 bedroom, gas heat): 8-10 kW
- Medium (3 bedroom, gas heat + AC): 14-17 kW
- Large (4 bedroom, electric heat): 20-26 kW
- Very large (5+ bedroom): 26-50 kW
| Home size | Generator |
|---|---|
| < 1,500 sq ft | 10-14 kW |
| 1,500-2,500 sq ft | 14-20 kW |
| 2,500-4,000 sq ft | 20-26 kW |
| 4,000+ sq ft | 26-50 kW |
These are starting points; verify with detailed load calculation.
Voltage considerations
Standard residential: 240V single-phase
- 240V supply: 120V derived for typical outlets, 240V for AC/dryer/range/water-heater
Generator must provide 240V split-phase (two 120V hot legs + neutral + ground).
Older + larger residences (rare): 240V 3-phase. Match generator output.
Fuel BTU per kW
Approximate fuel consumption:
| Generator size | Natural gas (CFH at full load) | Propane (gal/hr) | Diesel (gal/hr) |
|---|---|---|---|
| 10 kW | 130 | 1.3 | 0.7 |
| 14 kW | 180 | 1.8 | 1.0 |
| 17 kW | 220 | 2.2 | 1.2 |
| 22 kW | 280 | 2.8 | 1.5 |
| 26 kW | 330 | 3.3 | 1.8 |
Daily fuel consumption: roughly 15-30 gallons propane for 24-hour continuous run on a 22kW unit.
Considerations beyond size
Customer's outage profile:
- Short outages (under 4 hours): undersizing OK if essential loads only
- Long outages (24+ hours): right-sized matters; fuel runs out faster on smaller gen
Future expansion:
- Adding electric vehicle charging in next 5 years?
- Pool or spa coming?
- Home addition or remodel?
Size for future loads or risk replacement in 5-7 years.
Quality of grid:
- High-outage areas: prioritize larger; less frustration
- Low-outage areas: small unit may suffice
Customer preferences:
- Comfort: AC running through outage = larger
- Frugal: essential-only is acceptable
Sizing calculation example
Example home:
- 2,500 sq ft
- Gas furnace (small blower load)
- 3-ton AC
- Electric water heater (4.5 kW)
- Electric dryer (5 kW)
- Refrigerator + freezer + microwave
Continuous load: 1,500 W (lights, fridge, freezer, electronics) + 4,500 W (water heater) + 800 W (furnace blower) = 6,800 W
Peak with AC starting: 6,800 + 5,500 (AC inrush) = 12,300 W
Adequate generator: 14-17 kW. Choose 17 kW for headroom.
When to recommend bigger
- Customer wants AC + electric range simultaneously
- Customer has electric heat
- Customer has multiple AC units (multi-zone)
- Customer plans large addition
- Customer has well + septic pump (multiple motors)
When to recommend smaller:
- Essential-circuits approach
- Outage frequency rare
- Budget constrained
When customer wants smaller than calc shows
Some customers underestimate needs. Common conversation:
- "I'll just turn off AC when generator runs."
- Reality: customers don't remember; AC starts; generator trips
- Better: size correctly OR install essential-circuits with discipline
Connector sizes (cord, transfer switch)
Generator output cord sized to amperage rating:
- 30A 240V cord: typical portable
- 50A 240V cord: larger portable
- Hardwired (no cord): standby permanently installed
ATS amperage rating:
- 100A residential service: 100A ATS
- 200A residential service: 200A ATS
Cost vs benefit
| Generator | Equipment | Install | Total |
|---|---|---|---|
| 10 kW | Lowest of the range | Lowest of the range | Lowest of the range |
| 14 kW | A step up | A step up | A step up |
| 17 kW | Roughly 1.4x the 10 kW equipment cost | A modest additional step | Roughly 1.3x the 10 kW total |
| 22 kW | Roughly 1.7x the 10 kW equipment cost | A further step up | Roughly 1.5x the 10 kW total |
| 26 kW | Roughly double the 10 kW equipment cost | The highest install cost of the range | Roughly 1.7x the 10 kW total |
The single biggest sizing mistake: TRUSTING THE CUSTOMER'S "I'LL JUST TURN OFF AC" PROMISE. Customers don't remember to turn off loads during a 3-day outage. The AC kicks on, the generator trips, life is miserable. Always size for the customer's REALITY (everything running), not their best intentions. The 2-step-up generator costs a modest amount more; the customer satisfaction is worth 10x that.
References
- NFPA 70 + 110 + 37
- NEC Article 702 (Optional Standby Systems)
- Manufacturer sizing guides
- Manuall internal: Whole-House Standby Generator Installation SOP, Generator Fuel Options Reference