Backup Generator Sizing

Key concepts

• Running watts (kW): The steady power a load needs while it is on. Refrigerators, lights, and fans are examples. Your generator must supply at least the sum of the running watts of everything you run at once.
• Surge watts (peak): Many motor-driven appliances (central AC, well pump, furnace blower) need a short burst of extra power to start. The generator must be able to deliver this peak without tripping.
• Transfer switch: A device that safely switches your backup panel or circuits from utility power to the generator. Automatic units switch when the grid fails; manual units require you to start the generator and throw the switch. Proper installation is required for safety and code compliance.
• Fuel type: Portable generators often run on gasoline. Standby units typically use propane (LP) or natural gas. Fuel choice affects runtime, storage, and whether the unit can run unattended for long periods.

Step-by-step sizing framework

1. List what you want to run. Write down each appliance or circuit and note its running watts and, if it has a motor, its surge or locked-rotor amps. Nameplate or manual data is best; online tables give typical ranges.
2. Add running watts. Sum the running watts of every load you might use at the same time. That total is your minimum continuous output. Generators are often rated in watts or kW (1 kW = 1,000 W).
3. Find the largest surge. Identify which single load has the biggest surge. Your generator's surge rating must meet or exceed that spike while still covering the rest of the running load.
4. Add a small buffer. A margin of 10–20% above your calculated peak avoids running the generator at full capacity and can extend life and reduce noise.

Example: If your essentials (fridge, lights, furnace blower, modem) add up to 2,000 W running and your furnace blower has a 1,500 W surge, you need a unit that can deliver at least 3,500 W surge and 2,000 W continuous. A 3,500–4,000 W portable or a 5 kW standby would typically cover this with room to spare.

Common mistakes

• Ignoring surge requirements and sizing only on running watts.
• Assuming every outlet in the house can be powered; most setups feed only selected circuits via a transfer switch.
• Running a portable generator indoors or near openings, which creates carbon monoxide risk.
• Forgetting runtime: small fuel tanks or high load mean refueling often during long outages.

Cost drivers (general)

Total cost depends on generator size, fuel type, transfer switch (manual vs automatic), and labor. Standby units with automatic transfer and professional installation cost more than a portable plus a manual interlock or transfer panel. Permits and any electrical panel upgrades add to the total. Get multiple quotes and confirm what is included.

Safety and permitting

Generators must be installed and connected in line with local codes and utility rules. Backfeeding the grid through a standard outlet is illegal and dangerous. Use a transfer switch or approved interlock so the generator cannot feed the utility. Place portable units outdoors, away from windows and vents. Verify requirements with a licensed electrician and your building department.

FAQ