How to Size a Whole House Standby Generator
Sizing a whole-house standby generator is different from sizing a portable generator. With portables, you pick and choose which circuits to power. With a standby system, the goal is typically full-house coverage — powering everything automatically when the grid goes down.
The sizing process involves a load analysis: calculating the total running watts of all circuits you want to power, accounting for motor surge requirements, and applying a safety margin. Most generator manufacturers and electricians use a standardized method that accounts for three load categories:
- Continuous loads (lighting, electronics, refrigeration) — add 100% of wattage
- Motor loads (HVAC, pumps, compressors) — add running watts + surge factor for the largest motor
- Resistive loads (water heater, electric range, dryer) — add 100% but consider if you really need these on generator power
The Role of the Automatic Transfer Switch
A whole-house standby system is only as reliable as its automatic transfer switch (ATS). This device monitors utility power and, within 10–30 seconds of detecting an outage, disconnects from the grid and connects your home to generator power. When grid power returns, it switches back and shuts down the generator.
ATS units are rated in amps — your ATS must be rated at least as high as your main service panel (typically 200 amps for modern homes). The ATS is typically installed between your utility meter and main panel by a licensed electrician. Cost for ATS installation ranges from $800–$2,500 depending on panel complexity and location.
Generac vs. Kohler vs. Briggs & Stratton: Standby Generator Brands
The three major standby generator brands for residential use each have distinct strengths:
- Generac: Market leader with the widest dealer network. Popular sizes: 10 kW, 14 kW, 16 kW, 20 kW, 22 kW, 26 kW. Their G-Force engines are proprietary and purpose-built for standby use. Best for most homeowners due to availability and service network.
- Kohler: Industrial-grade quality, premium price. Known for quieter operation and longer service intervals. Popular sizes: 14 kW, 20 kW, 38 kW. Excellent for homeowners who prioritize durability over upfront cost.
- Briggs & Stratton: Good mid-range option using reliable Vanguard commercial engines. Popular sizes: 10 kW, 12 kW, 15 kW, 20 kW. Often priced between Generac and Kohler.
- Cummins: Less common residentially but highly regarded. Known for long service life and reliability in commercial applications. Available in 13–20 kW residential configurations.
Installation: What to Expect
Installing a whole-house standby generator involves several contractors and steps:
- Site assessment: A generator dealer assesses your home's load, determines generator location (must be 5+ feet from windows/doors, per NFPA 37), and designs the system.
- Gas line extension: A licensed plumber extends your natural gas or propane line to the generator location. Gas must be properly sized for the generator's BTU demand.
- Electrical work: A licensed electrician installs the automatic transfer switch, runs conduit, and connects the generator to your panel.
- Generator placement: The unit is typically placed on a concrete pad near the electric meter. Minimum clearances must be maintained from windows, doors, and combustibles.
- Permit and inspection: Most jurisdictions require a permit and final inspection. Your installer should handle this.
- Commissioning: The installer tests the full system, sets up weekly exercise cycles, and registers your warranty.
Total timeline from order to complete installation: typically 2–6 weeks, depending on permits and contractor scheduling.
Ready to estimate your exact wattage? Use our main generator size calculator above, or check out our backup generator size calculator for emergency power planning.