Home Backup Generator Wattage Sizing Calculator
Size your backup generator capacity requirements for blackouts or emergency power. Toggle household appliances to calculate continuous running watts and starting surge peaks to ensure a safe electrical setup.
Critical Generator Sizing & Interlock Advisory
Power connections must comply with Article 702 of the National Electrical Code. Improper generator connection causes fire, exhaust carbon monoxide poisoning, and fatal utility accidents. Sizing estimations are for planning purposes. Always consult a licensed electrical contractor. Read our full Disclaimer.
Select Backup Appliances
Toggle devices you want to run during a power outage
Heating & Cooling
Kitchen
Pumps & Water
Laundry & Utility
Office & Entertainment
Required Generator Size
Calculations include a 15% safety operating margin
BACKFEEDING SAFETY WARNING
**Never connect a generator directly to a dryer or wall outlet (backfeeding).** This feeds high-voltage power backward through your electric meter, which can electrocute utility linemen working to restore power and cause catastrophic electrical fires. Always connect your generator via a professionally installed **transfer switch** or an **interlock panel kit**.
Recommended Running
954 W
Continuous operating limit
Recommended Starting
2,680 W
Motor startup surge limit
Suggested: Small Inverter Generator (1.5kW - 2.2kW)
A highly compact, quiet, and fuel-efficient unit suitable for tailgating or backing up simple appliances like light bulbs, routers, and laptops. Cannot support motors (like pumps or ACs).
Wattage Calculations
Conductor & Inlet Wire Sizing
Sizing the wire from your generator inlet box to your panel's transfer breaker? Size wire gauges safely.
Open Wire SizerUnderstanding Generator Wattage Sizing
When sizing a backup generator for your home, you must consider two types of wattages: **running (continuous) watts** and **starting (surge) watts**. Running watts are what the generator must produce continuously to keep an appliance operating. Starting watts represent the high, short-duration electrical load drawn by electric motors for a second or two when they first start up.
Because motors draw up to 3 times their running power during start-up, generators are rated with two numbers: the continuous operating capacity and the peak surge capacity. For instance, a generator labeled '7500 / 9500' can supply 7,500 running watts continuously and handle temporary starting surges up to 9,500 watts.
Connecting Your Generator Safely: Transfer Switches and Interlocks
For emergency backup power, there are only two legal and safe ways to connect a generator to your house's electrical panel:
- Manual Transfer Switch: A dedicated subpanel that isolates specific critical circuits (like refrigerator, furnace blower, sump pump) and switches them directly to generator power, preventing any connection to utility lines.
- Generator Interlock Kit: A mechanical sliding plate installed on your main service panel. It physically prevents the main breaker and the generator backfeed breaker from being turned on at the same time, providing a safe, flexible way to back up any circuit up to the generator's capacity limit.
Both methods require a professionally installed outlet box (inlet box) on the outside of your house and must be wired by a licensed electrician.
Frequently Asked Questions
What size generator do I need to run a whole house?▼
For general home backup, a portable generator between 5,000 and 8,000 running watts can run essentials like a well pump, refrigerator, lights, and furnace blower. If you want to power the entire house with central AC and an electric water heater without load managing, you typically need a whole-house standby generator rated at 15,000 to 22,000 running watts.
What is the difference between running watts and starting watts?▼
Running (continuous) watts are what an appliance consumes to operate under normal loads. Starting (surge) watts are the high, momentary currents required by electric motors (like those in refrigerators, AC compressors, and pumps) for 1–2 seconds to start spinning from a dead stop. A generator must have enough surge capacity to handle these motor starts.
Why shouldn't I stack starting surge wattages?▼
When calculating generator size, you add the single highest starting surge to the running sum rather than adding all starting surges. This is because electric motors only draw starting current for a second or two, and it is highly statistically improbable that all motors in your house will start at the exact same millisecond.
Can I connect my generator directly to an electrical outlet?▼
No! Connecting a generator directly to a home outlet, called 'backfeeding,' is illegal, extremely dangerous, and a severe fire hazard. It can send high-voltage electricity back out to the power lines, potentially electrocuting utility workers. You must use an approved manual transfer switch or interlock panel kit installed by a licensed electrician.