What Size Generator Do I Need to Run a House?
Calculate home backup sizing thresholds. Select portable vs standby generators.
The Quick Answer: General Sizing
Choosing the right size generator for a house depends entirely on what you want to power during an outage. Here is the general rule of thumb:
- 3,000 to 5,000 Watts: Basic essentials (refrigerator, some lights, phone chargers, router).
- 5,000 to 8,000 Watts: Essentials plus a well pump or gas furnace.
- 10,000 to 15,000 Watts: Most appliances, excluding a central AC or electric water heater.
- 20,000 to 26,000 Watts: Whole-house standby power, including central AC and electric range.
Understanding Starting Watts vs Running Watts
Every motor-driven appliance requires two types of power:
- Running Watts: The continuous power needed to keep the appliance running.
- Starting (Surge) Watts: The brief burst of power required to start an electric motor (often 2 to 3 times the running watts).
How to Calculate Your Required Generator Size
Step 1: List Your Essential Appliances
Write down everything you absolutely must run during a power outage.
Step 2: Find the Wattage for Each Item
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 700W | 2200W |
| Sump Pump (1/2 HP) | 1050W | 2150W |
| Lights & Chargers | 200W | 200W |
| Central AC (3 Ton) | 3500W | 10500W |
Step 3: Calculate the Total
Example for basic essentials (Fridge + Sump Pump + Lights):
Highest Starting Wattage is the Fridge (2200W).
Running Watts of everything else = 1050W (Sump) + 200W (Lights) = 1250W.
Total = 2200W + 1250W = 3450 Watts.
In this scenario, a 4,000W or 5,000W generator would be ideal to provide a safety margin.
Portable vs Standby Generators
Portable Generators
These run on gasoline, propane, or dual-fuel and range from 2,000W to 12,000W. They are manually started and require extension cords or a manual transfer switch.
Standby Generators
These are permanently installed outside your home, run on natural gas or liquid propane, and range from 10,000W to 26,000W. They automatically turn on within seconds of a power outage via an automatic transfer switch (ATS).