Check off the appliances you need to power. Get your total wattage, recommended generator size, and estimated fuel consumption in seconds.
Check everything that needs to run at the same time during an outage. Surge (starting) watts are shown where applicable.
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Every generator has two wattage ratings: running watts (also called continuous watts) and surge watts (also called starting or peak watts). Running watts is the steady load the generator can sustain indefinitely. Surge watts is the brief burst of power the generator can deliver for a second or two to start a motor-driven appliance.
This distinction is critical because motors — refrigerators, well pumps, air conditioners, sump pumps — draw two to three times their running wattage at the moment the motor starts. A well pump that runs continuously at 900 watts may demand 2,100 watts for a half-second at startup. If your generator cannot deliver that peak, the motor will struggle to start, the generator will stall, and in the worst case the motor can be damaged by the sustained voltage drop. Always check surge wattage before buying.
The correct sizing formula: add up the running watts of everything you plan to run simultaneously, then substitute the surge wattage of your single largest motor load. That peak total is the minimum surge capacity your generator must meet. Most generators are rated honestly, but always read the fine print — some manufacturers advertise surge wattage as the headline figure rather than running wattage.
Inverter generators use a digital inverter circuit to produce clean, stable power — the kind that sensitive electronics like laptops, CPAP machines, and modern variable-speed appliances require. They throttle their engine speed to match the load, which makes them dramatically quieter (50–60 dB) and 20–40% more fuel-efficient than conventional units under light loads. The trade-off is cost: inverter generators are significantly more expensive per watt. For most outage scenarios involving electronics, phones, and smaller loads under 3,500 watts, an inverter generator is the better long-term choice.
Conventional generators run at a fixed engine speed (3,600 RPM) regardless of load, producing more noise and burning more fuel at partial loads. They excel at raw capacity and cost — a 7,500-watt conventional generator costs far less than a 7,500-watt inverter unit. For running well pumps, sump pumps, and power tools where total wattage matters more than power quality, conventional generators deliver better value.
Whole-house generator backup that runs your HVAC, electric water heater, stove, and all lights simultaneously typically requires 15,000–20,000 watts and a permanently installed automatic transfer switch — that is standby territory, not portable. Most homeowners are better served by identifying their critical load list: the refrigerator, a couple of lights, phone charging, and if necessary the well pump and sump pump. That list often totals 2,000–4,000 watts, which a mid-size portable generator handles comfortably. Add window AC or a space heater and you might need 5,000–7,500 watts. Running both AC and a well pump simultaneously pushes past 10,000 watts.
Connecting a generator directly to your home's electrical panel without a proper transfer switch creates a condition called backfeed — energizing utility lines that lineworkers believe are de-energized. This kills utility workers and is illegal in most jurisdictions. A manual or automatic transfer switch ($200–$800 installed) disconnects your home from the utility before the generator connects. Many homeowners use a portable generator with a heavy-duty extension cord run directly to specific appliances instead — this avoids backfeed and requires no transfer switch, but it means running cords rather than using wall outlets.
Running a generator at 100% of its rated output continuously stresses the engine, reduces its lifespan, and leaves no headroom for additional loads or unexpected surges. The 1.25 safety margin — sizing your generator to handle 125% of your total running watt load — ensures the engine runs at a comfortable 75–80% of capacity. This improves efficiency, reduces heat, extends engine life, and gives you a buffer when appliances cycle on simultaneously. In practical terms: if your total running load is 3,200 watts, look for a generator with at least 4,000 watts of running capacity. Always verify the surge (starting) rating separately — it must exceed your largest motor's starting demand.
Fuel consumption depends entirely on your load and how many hours per day you run the generator. At moderate loads (1,500–2,500 running watts), plan on 0.75–1.25 gallons per hour. Running the generator 8 hours per day for 3 days puts you at 18–30 gallons. Many homeowners cycle the generator — running it 4–6 hours in the morning to cool food and charge devices, then 4–6 hours in the evening — rather than running it continuously, which roughly halves consumption. Store gasoline in approved containers with fuel stabilizer and rotate it every 3–6 months. Never store more than 10 gallons in a residential attached garage.
Possibly, but you need to check both surge demands carefully. A 2-ton central AC unit surges to 7,500 watts at startup. A ½ HP well pump surges to 2,100 watts. If both happen to start simultaneously — which they can if the AC thermostat calls for cooling right as the pump cycles on — your generator needs to deliver 9,600 watts at peak surge, plus the running load of everything else. The practical approach: look for a generator with at least 10,000 running watts and 12,000+ surge watts. Alternatively, don't run AC and the well pump on the same circuit, and keep non-motor loads to a minimum while either is starting.
For outages lasting a few days and powering critical loads, a quality portable generator is entirely sufficient. Standby generators — permanently installed, propane or natural gas fueled, with automatic transfer switches — make sense when outages in your area are frequent and extended, when you or a household member depend on powered medical equipment, or when you want full-home backup with no manual intervention. Standby units start automatically within 10–30 seconds of a power failure and can run for days without refueling. The cost difference is significant: a capable portable generator runs $500–$2,000, while standby systems with installation typically cost $5,000–$15,000 or more.