Backup Generator Sizing for Seattle Outages.

TL;DR. Most Seattle homeowners installing a backup generator land at 14 to 18 kW ($9,500-$13,000). That handles essentials plus most major appliances except simultaneous AC and electric range. Heat pump households need 20+ kW. Natural gas is the right fuel choice 90% of the time for homes already on PSE gas.

Direct answer, generator sizing by household type

Most Seattle homeowners going through their first install land at 14 to 18 kW, which handles essential circuits plus most major appliances minus simultaneous AC or electric range usage. That’s the sweet spot for cost vs capability.

The two ways to think about sizing

Method 1, what matters during an outage. You don’t need to power your entire house. You need to power what matters when the grid is down. Most Seattle households are fine with:

• Refrigerator and freezer (300 to 700 W)
• Furnace blower or heat pump (1,500 to 7,000 W depending on stage)
• Well pump or sump pump (1,000 to 2,000 W on startup, 500 to 1,000 W running)
• Kitchen lights and a couple of outlets (300 W)
• WiFi router and one TV (200 W)
• One bathroom (200 W)
• Garage door opener (intermittent, 800 W on startup)

Total essential load: 4,000 to 12,000 W (4 to 12 kW), depending on whether you’re running heat pump or just the furnace blower. A 9 to 11 kW generator handles this comfortably.

Method 2, simulate the worst plausible day. Cold day, dinner cooking, EV charging started. Heat pump running at full output (5,500 W on the cold day). Range or oven running (5,000 W). EV charger pulling Level 2 (7,700 W). Lights and appliances scattered (1,500 W). Total: 19,700 W.

If you want to power that day without thinking about it, you need 22+ kW.

Most homeowners pick somewhere in between. The middle ground (14 to 18 kW with transfer-switch load shedding to prevent simultaneous max loads) handles 95% of outage scenarios without overbuilding.

Real heat pump load on a generator

This is where most contractors get sizing wrong. Heat pump load is the dominant variable in modern Seattle homes.

Locked rotor amperage (LRA) is the startup spike when the compressor first engages. It lasts only 0.5 to 2 seconds, but the generator must handle it without bogging down. Hyperheat-style inverter compressors have lower LRA than older single-stage compressors, which is one reason modern heat pumps pair better with generators than 1990s equipment.

Natural gas vs propane

For a Seattle home already on natural gas, natural gas is the right answer 90 percent of the time:

• Unlimited runtime as long as utility gas is flowing
• No tank required (saves $1,500 to $4,000 of tank installation)
• Cheaper fuel cost per kWh of output

The exceptions:

• Off-grid or rural homes without natural gas service
• Earthquake-resilience focus: natural gas lines can fail in major earthquakes; propane tanks are independent
• Specific siting issues: gas meter location is too far from generator location for cost-effective tie-in

For most Eastside homes, the cheaper, simpler natural gas option wins.

Cost scenarios

Three typical scenarios at different scopes.

Scenario 1: craftsman, essentials-only

• 1,800 sq ft 1965 home with gas furnace and gas range
• Customer wants fridge, furnace blower, well pump, lights, WiFi, one bathroom
• Total essential load: 4,200 W
• Installed: Generac 11 kW with priority load center
• Typical range: $8,500 to $10,200

Scenario 2: modern, heat pump household

• 2,400 sq ft 2008 home with 4-ton heat pump, gas range
• Customer wants everything except simultaneous AC + electric appliances
• Installed: Generac 22 kW with smart load management
• Typical range: $12,400 to $14,800

Scenario 3: electrification household

• 3,200 sq ft 2014 home with 5-ton heat pump, induction range, 2 EV chargers
• Customer wants full home backup including charging one EV slowly
• Installed: Generac 26 kW with whole-home automatic transfer switch
• Typical range: $17,200 to $20,500

Automatic transfer switch (ATS) basics

The ATS is the brain of the system. When grid power drops, it:

1. Senses the outage (typically within 0.5 to 2 seconds)
2. Signals the generator to start
3. Waits for the generator to stabilize (10 to 15 seconds typical)
4. Transfers the home’s load from grid to generator
5. Holds until grid power returns and is stable for 60+ seconds
6. Transfers back to grid and signals generator to shut down

We install Service-Entrance Rated ATSs that meet NEC code and integrate with Generac, Kohler, and Briggs equipment.

Load shedding ATSs. A more sophisticated ATS can selectively shed loads when generator capacity is approached. For example: if the heat pump is running and the customer tries to start the dryer, the dryer is briefly held off until the heat pump completes its cycle. This is what lets a 14 kW generator effectively cover a 22 kW peak demand household.

Whole-home batteries vs generators

Different solution for different goals.

Whole-home batteries (Tesla Powerwall, Enphase, FranklinWH).

• Best for short outages (4 to 12 hours)
• Pair beautifully with solar (charges from solar during day)
• Quiet (no engine noise)
• Cost $14,000 to $32,000 installed for whole-home capacity
• Need to be recharged after outage ends

Generators.

• Best for multi-day outages
• Run indefinitely as long as gas is flowing
• Louder during operation (60 to 75 dB)
• Cost $7,500 to $20,000 installed
• No solar dependency

Combined approach. Some Seattle homes install both: battery for daily peak-shaving and short outages, generator for true multi-day events. The combined cost is high ($25,000 to $45,000), but it’s the most resilient option available.