Backup Generators, Properly Explained

1. Portable, inverter, standby — three classes

• Portable (gasoline or propane). $600–$2,800. Wheels, pull-start or electric-start, single-phase 120/240V output. Power critical loads via extension cords (limited) or an interlock kit + inlet box (the right way). Needs to be refueled every 8-12 hours and rolled outside before starting. Best for occasional short outages.
• Inverter portable. $1,200–$3,400. Same as above but with electronics that produce cleaner sine-wave output — safe for sensitive electronics (computers, modern HVAC controls). Honda EU series, Yamaha EF series. Quieter (51-60 dB vs 65-75 dB conventional).
• Air-cooled standby. $5,100–$8,200 equipment; $11,000–$17,500 installed. Permanent residential install, automatic transfer on outage detection, 10-26 kW. Generac Guardian, Kohler 14RESV-20RESV, Briggs & Stratton Symphony. Self-tests weekly. The mainstream choice for whole-home in 2026.
• Liquid-cooled standby. $14,500–$19,500 equipment; $20,000–$35,000+ installed. Heavy-duty commercial-grade. Generac Protector, Kohler 30RCL+. For 26 kW+ loads, multi-AC homes, or applications with very high uptime requirements.

2. Sizing — the math that prevents over-spending

The cardinal mistake is over-buying. A 22 kW unit costs $3,000–$5,000 more than a 14 kW unit, but the 14 kW unit can power everything a typical 1,800 sqft home actually needs. Here’s how to size correctly:

1. Critical-load survey. List loads in order of priority: refrigerator (700W running, 2,200W start), freezer (600W), well pump (1,500W running, 3,500W start), furnace blower (800W gas, 1,500W heat pump), lights (200W LED whole house), internet + computers (400W), microwave (1,500W), garage door (250W), water heater (4,500W electric or 200W gas). Total continuous: ~3,500W for essentials.
2. Add HVAC if you want central AC. 3-ton AC: 3,600W running + 8,000W start surge. 4-ton: 4,800W running + 11,000W start. The start surge sets minimum generator size.
3. Apply 20% headroom for safe margin.

Result: critical-loads-only = 7-10 kW. Whole-home with selective AC cycling = 14-18 kW. Whole-home with simultaneous everything = 22-26 kW. Generac and Kohler offer free online sizing tools that automate this. Don’t let a salesperson talk you up two sizes.

3. Fuel choice

• Natural gas. Hooked into your utility gas line. Unlimited runtime as long as the gas grid is intact. Cheapest fuel per hour ($1–$2/hr at 50% load for 18-22 kW). Caveat: very large generators may require gas line upsizing — many residential lines are sized for furnace + cooktop + water heater, not 22 kW continuous draw. Add $600–$2,800 for line extension/upsizing.
• Propane. Requires a tank. 500-gallon in-ground tank ($1,800–$4,200) holds about a week of standby runtime. Truck-delivered refill. Best for rural homes without natural gas service. Shelf-stable indefinitely.
• Diesel. Most efficient ($0.7 thermal/Wh vs 1.0 for gas), best for very large units (26+ kW), commercial-style. Drawback: emissions, smell, fuel storage (UL-listed double-wall tank for residential typically 200-500 gal).
• Gasoline. Portable only. Limited tank (5-7 gal = 8-10 hours runtime). Fuel degrades in 3-6 months without stabilizer. Avoid for permanent installs.

4. Transfer mechanisms — interlock vs ATS

Backfeeding through a regular outlet is illegal in every state, dangerous (kills utility lineworkers), and voids your generator warranty. Two legal options:

• Interlock kit + inlet box. $250–$700 installed. A UL-listed mechanical breaker plate physically prevents the main breaker and the generator backfeed breaker from being on simultaneously. You start the portable generator outside, plug it into the inlet box, flip the main off, flip the generator breaker on. Manual but cheap. Code: NEC 702.5, listed equipment only.
• Automatic transfer switch (ATS). $800–$4,200 installed. Senses utility loss, signals the generator to start, transfers load to generator in 10-30 seconds, transfers back when utility returns. Required for standby units. Two flavors: critical-load subpanel ATS (cheaper, only essential circuits) or whole-home service-entrance ATS (200A, the entire house transfers).

Common cost mistake: getting talked into a whole-home ATS when a critical-load subpanel ATS would do the same job for half the price. If you have an 18 kW generator and a 4-ton AC, the whole-home ATS lets you accidentally turn on everything and overload the generator. A critical-load panel forces discipline.

5. NEC 702 and permit compliance

Permanent generator installs fall under NEC 702 (Optional Standby Systems). Code requirements:

• 702.4 — capacity must equal or exceed connected load (the sizing rules above).
• 702.5 — transfer equipment must be approved and listed for the application. No improvised setups.
• 702.7 — signs identifying the generator location at the service equipment.
• 702.10 — outdoor weather protection.

Pad requirements (manufacturer-specific but commonly): 4-6" concrete or composite pad, minimum 36" clearance to combustible siding, 60" from windows/doors (carbon monoxide intake risk), 36" service access on operator side. Most jurisdictions also require 5-10 ft separation from gas meter and main electrical service.

Always pull a permit. Unpermitted installs (1) void manufacturer warranty, (2) fail home insurance claims after a fire, (3) cause closing problems when you sell.

6. Generator vs battery (the 2026 decision)

Battery storage has fundamentally changed the backup-power calculus. A 13.5 kWh Powerwall installs for $13,000–$17,000 with the 30% federal 25D credit knocking it down to $9,100–$11,900 net. A comparable 22 kW air-cooled standby generator installs for $13,000–$17,500 with no federal credit.

Battery wins on:

• Silence — generator is 65-75 dB at 25 ft; battery is silent.
• Federal 25D credit — 30% off battery, zero off generator.
• No fuel — battery doesn’t need gas-line work, propane tank, or fuel delivery.
• Solar integration — if you have or plan solar, battery + solar offers indefinite runtime in daylight.
• Daily TOU arbitrage — battery saves money every day; generator only during outages.
• Short outages (< 12 hours) — covers 90%+ of U.S. outage hours.

Generator wins on:

• Sustained multi-day outages — natural gas standby has effectively unlimited runtime.
• Heavy loads — 22 kW continuous output is hard for residential batteries to match.
• Hurricane regions where outages routinely last 5-14 days.
• Off-grid scenarios with no solar.

Recommendation: in the Northeast, Midwest, and West where outages are typically short, choose battery (with or without solar). In hurricane country (FL, NC, LA, TX coast, GA coast), and rural locations with sustained ice-storm risk (NH, VT, ME, the Plains), choose generator. Some homeowners in high-risk regions install both — solar + battery for daily and short outages, generator for sustained events.