Buying a generator without sizing it properly means either running out of power mid-outage or paying for capacity you never use. This guide walks you through running watts, starting watts, fuel choices, and transfer switch requirements so you can choose the right generator for your specific needs.

Running Watts vs. Starting Watts: The Key Distinction

Every generator is rated in two numbers: running watts (also called rated watts) and starting watts (also called surge watts). Running watts is the continuous power output the generator can sustain indefinitely. Starting watts is the brief peak output available for a few seconds to start electric motors. The starting watts rating is typically 20–50% higher than the running watts rating.

Electric motors — found in air conditioners, refrigerators, well pumps, and sump pumps — require 2–3 times their running wattage during startup. If your total running load is 5,000 watts but your largest motor requires 3,000 watts to start, your generator must be capable of supplying 8,000 watts at the moment that motor kicks on. This is why you add only the single largest starting surge to your running total, not all starting surges simultaneously. Starting all motors at the same time would overload any reasonably sized generator, but in practice they stagger naturally over the first few minutes of operation.

How to Calculate Your Total Load

Start by listing every load you want to power during an outage. For each item, find the running wattage on the nameplate or manufacturer's spec sheet. If only amps and volts are listed, multiply them to get watts (W = V × A). For motor loads (AC, pumps, refrigerators), also note the starting wattage, which is usually listed separately or can be estimated at 2.5 times the running wattage.

Sum all running watts — this is your total continuous load. Then identify the single appliance with the highest starting surge and add just that surge (the difference between its starting and running wattage) to the running total. Apply a 10–15% safety margin on top of this sum to avoid running the generator at 100% capacity, which reduces generator lifespan and fuel efficiency. The result is your minimum required generator size. Round up to the next available commercial size (e.g., 7,500W, 10,000W, 12,500W).

Fuel Types: Gasoline, Propane, and Natural Gas

Gasoline is the most common fuel for portable generators. It is widely available and the generators themselves are less expensive upfront, but gasoline degrades within 30–90 days (even with stabilizer) and supply lines can be disrupted during extended emergencies when gas stations lose power. Most 7,500W gasoline generators consume 0.5–0.7 gallons per hour at 50% load, giving roughly 8–10 hours of runtime per tank.

Propane burns cleanly, stores indefinitely in sealed tanks, and is available in sizes from 20-lb cylinders to 500-gallon permanent tanks. Propane generators produce roughly 10–15% less power than the same generator running on gasoline because propane has a lower energy density per gallon. Natural gas standby generators are the most convenient long-term option — they connect permanently to the utility gas line and never run out of fuel — but they require a licensed plumber and electrician for installation and cost significantly more upfront than portable units.

Standby vs. Portable: Which Do You Need?

Portable generators (3,500–12,500W) are the right choice for occasional outages, construction sites, camping, or as a backup for specific critical loads. They cost $500–$3,000, require manual startup, and must be refueled. They should never be run inside or in attached garages due to carbon monoxide risk — always position them at least 20 feet from the house with the exhaust directed away from windows and doors.

Standby generators (7,500–20,000W or larger) start automatically within seconds of a utility failure, run on natural gas or propane, and can power the entire house including HVAC. They are professionally installed with a permanent transfer switch and typically cost $4,000–$15,000 installed. They make sense when you have medical equipment that requires uninterrupted power, experience outages more than 2–3 times per year, or have a sump pump protecting a finished basement. The automatic operation is particularly valuable for vacationers or second-home owners who cannot be present to manually start a portable unit during a storm.

Transfer Switches and Code Requirements

Connecting a generator to your home's electrical panel requires a transfer switch — a device that disconnects the utility feed and connects the generator simultaneously, preventing dangerous backfeed that can electrocute lineworkers restoring power. The NEC (National Electrical Code) requires a transfer switch for any generator connected to home wiring. Running extension cords directly from a portable generator doesn't require a transfer switch but limits you to what you can reach with a cord.

Manual transfer switches (a double-pole switch or subpanel) are the most common and affordable option ($200–$700 installed). Automatic transfer switches (ATS) are required for standby generators and cost $500–$2,000 installed. The transfer switch amperage must match or exceed the generator's maximum output current: a 7,500W/240V generator produces 31 amps, so a 30-amp transfer switch is the minimum. For a 12,500W generator, you need a 50-amp transfer switch. Always have a licensed electrician perform this installation — improper wiring is a fire and electrocution hazard.