Whole-home generator buying guide — sizing, fuel, transfer switch, install

Generator sizing follows similar principles to HVAC sizing: undersize and the system fails under load; oversize and you pay more upfront and burn more fuel. The right approach is a load calculation that totals actual electrical demand from the loads you want backed up.

The two basic configurations:

Whole-home backup. The generator powers everything in the home: HVAC, water heater, range, appliances, lighting, all outlets. Requires sizing for the largest realistic load combination — typically AC (or heat pump) plus refrigerator plus water heater plus lighting plus a few outlets. Most US homes need 18-26 kW for whole-home backup.

Critical-circuit backup. The generator powers a subset of essential circuits: refrigerator, freezer, sump pump, well pump, furnace blower, a few outlets, key lighting. Smaller generator (10-14 kW typically) and lower install cost. The transfer switch is configured to only feed the critical-circuits subpanel; the rest of the home loses power during outages. Common compromise for homeowners who want backup but don't need full whole-home capacity.

The sizing math: total the wattage of every load you want backed up at the same time, including motor-start surge for the largest motors (AC compressor, well pump, sump pump). Add 25% margin. The result is your minimum continuous-rating generator size. Most installers handle this calculation; ask for the written load list as part of the proposal.

The fuel source is one of the biggest decisions because it affects long-term operating cost, runtime during outages, and install complexity:

Natural gas. Direct connection to municipal gas line. Unlimited runtime (the gas keeps flowing during most outages because gas distribution is largely unaffected by electric outages). Lowest fuel cost per kWh. Most popular choice in markets with established gas service. Caveats: rare gas-distribution outages do happen (earthquake, major storm damage to gas infrastructure); the gas-line capacity from the meter to the generator must be sized for the additional load (sometimes requires utility upgrade).

Propane (LP). Refillable tanks (typically 250-500 gallon underground or above-ground). Cleaner-burning than diesel, easier to store than gasoline. Standard choice in markets without natural-gas service. Requires planned tank refill schedule; typical 500-gallon tank runs an 18 kW generator for 7-10 days at moderate load. Tank lease vs purchase is a separate decision; lease is more common.

Diesel. Higher energy density, longer storage life than gasoline, well-suited to large generators (30+ kW). Less common residentially; more common in commercial and standby applications. Diesel generators are louder, smokier, and require more maintenance than gas/propane.

Gasoline (portable only). Not suitable for whole-home permanent installation. Gasoline degrades during storage; stabilizers help but practical storage life is 6-12 months. Use for portable generators, not standby.

Dual-fuel. Some generators run on both natural gas and propane with a manual switch. Useful if you have natural gas but want propane backup for gas-line failures. Adds cost and complexity; valuable mainly in regions with documented gas-distribution reliability concerns.

The automatic transfer switch (ATS) is what makes a whole-home generator a true standby system rather than a manual generator. The ATS continuously monitors utility power; when it detects an outage, it disconnects the home from the utility, signals the generator to start, waits for stable generator output (typically 10-30 seconds), and then connects the home to the generator. When utility power returns, the ATS reverses the process, transitioning back to utility and signaling the generator to shut down.

Key ATS specifications:

Amperage. The ATS amp rating must match your home's electrical service. 200A service homes need a 200A ATS; 100A service may need an upgrade.

Location. Indoor (basement, garage) is most common; outdoor weatherproof models exist for installs without indoor space. Indoor ATS is easier to access for maintenance and troubleshooting.

Load management. Higher-end ATS units have load-shedding capability — they can selectively disconnect non-essential circuits (water heater, dryer, EV charger) during peak generator load to prevent overload. Useful for homes near the boundary between critical-circuit and whole-home capacity.

Surge protection. Some ATS units include surge protection between utility and home. Worth specifying in storm-prone markets where utility-side surges occur during outage transitions.

The ATS is the most failure-prone component of a whole-home generator system; it cycles every time there's an outage. Manufacturer warranties typically cover the ATS for 5-10 years; beyond warranty, expect to replace it once during the generator's lifespan.

A whole-home generator install typically takes 1-3 days depending on existing electrical service condition and gas/propane setup:

Day 0: site visit + permit prep. Installer measures, identifies setbacks, plans pad location, identifies gas-line route, estimates electrical work scope. Permit application submitted (typical 1-3 weeks for approval depending on jurisdiction).

Day 1: pad pour + pre-install. Concrete pad poured (4-6" thick, sized to generator footprint). Curing time is typically 2-3 days before generator can be set; some installers use precast pads or composite pads to skip the cure time.

Day 2: electrical + gas. ATS installed in or near the main electrical panel. Conduit run from generator pad to ATS. Gas line run from generator pad to gas meter or propane tank (this is often the longest part of the install — gas line trenching, sizing, pressure testing). Electrical permit inspection.

Day 3: generator set + commissioning. Generator placed on the pad. Final electrical and gas connections made. Generator started and load-tested. ATS tested by simulating utility outage. Final inspection. Homeowner training on basic operation, emergency stop, and maintenance schedule.

For more complex installs (panel upgrade required, long gas-line trench, structural work for the pad), expect 4-7 days. Concrete cure time is often the bottleneck.

Generators need scheduled maintenance to maintain manufacturer warranty and reliable operation:

Weekly self-test. Most modern generators run an automatic self-test (called "exercise") for 15-20 minutes once per week. This circulates oil, charges the battery, and confirms the generator starts. Set the exercise day for a time that won't disturb (commonly Saturday morning).

Annual service. Oil change, oil filter, air filter, spark plug replacement (every 2-3 years). Battery check and replacement when needed (3-5 year battery life typical). Visible inspection of all connections, mounting, and exhaust. Most installers offer annual service contracts; manufacturer warranty often requires documented annual service.

Load bank test (every 2-3 years). Generator runs at full rated load for 1-2 hours to verify it can sustain peak output. Identifies issues before they appear during a real outage. Some installers include this in annual service; others charge separately.

Fuel system. Natural-gas generators require minimal fuel-system maintenance. Propane generators need tank refill on schedule (don't let the tank go below 30% — generator may fail to start). Diesel generators need fuel polishing (water and contamination removal) every 1-2 years.

Battery. The starter battery is the most-replaced consumable. Failure to replace at end of life is the most common reason generators fail to start during outages. Manufacturer recommendations vary; expect to replace the battery every 3-5 years.