What Size Generator Do I Need for My House?
The right generator size comes down to one question: **what do you want to keep running during an outage?** Backing up a few essentials needs a fraction of the power required to run your whole home with central air conditioning.
As a quick guide: **essentials** (fridge, lights, outlets, internet, furnace fan) need about **5,000–7,500 watts**. **Most of the home** lands around **10,000–16,000 watts**. **Whole-home backup with central A/C** typically needs an **18–26 kW standby generator**. Here is how to size it properly.
Start with what you want to run
- Essentials only: fridge, some lights, outlets, internet, furnace blower, about 5–7.5 kW
- Most of the home: add a well pump, garage, and some A/C, about 10–16 kW
- Whole home with central A/C: about 18–26 kW standby
How to size it: running vs starting watts
Every appliance has a **running** wattage (what it uses continuously) and, if it has a motor, a higher **starting** (surge) wattage for the first moment it turns on. You size the generator to cover all your running loads plus the largest single surge.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Central A/C (3 ton) | 3,500 | 8,000 |
| Well pump (1 HP) | 1,000 | 3,000 |
| Sump pump | 800 | 2,000 |
| Furnace blower | 800 | 2,300 |
| Lights and outlets | 1,000 | 1,000 |
Portable vs standby
Pros
- + Standby: starts automatically on natural gas or propane
- + Standby: powers most or all of the home
- + Standby: no cords, no refueling
- + Portable: much lower up-front cost
- + Portable: fine for essentials
Cons
- – Standby: higher install cost
- – Portable: manual setup, refueling, and cords
- – Portable: limited capacity
Fuel and runtime
**Natural gas** is the most convenient for a standby unit, unlimited runtime with no refueling. **Propane** stores well and suits homes without a gas line. **Gasoline** is typical for portables but is not ideal for long outages. The right fuel depends on what is available at your home.
Get it sized right the first time
Oversizing wastes money, undersizing leaves you in the dark. A licensed electrician runs the load calculation and includes the transfer switch so your generator install is safe and code-compliant.
Frequently asked questions
Whole-home backup that includes central air conditioning typically needs an 18–26 kW standby generator. Homes without electric heat or large A/C can often manage on 14–18 kW. The exact size depends on your specific loads, which we calculate during a free assessment.
For most single-family homes, yes. A 22–24 kW standby generator can run central A/C, kitchen, laundry, and most circuits, often with load management to prioritize the biggest appliances. Very large homes or those with multiple A/C units may need more capacity or smart load shedding.
To back up essentials, refrigerator, some lights, outlets, internet, and a furnace fan, a 5,000–7,500 watt generator is usually enough. Add a well pump or window A/C and you'll want 8,000–10,000 watts.
Yes, for safe, permanent backup. A transfer switch isolates your home from the grid so your generator never back-feeds the utility lines (a serious hazard for line workers) and lets you power circuits safely. A standby generator uses an automatic transfer switch that starts it for you during an outage.
Yes, if it's sized for the A/C's starting surge. Central A/C draws a large momentary spike when the compressor starts, so the generator must handle that surge on top of your other loads. A 20 kW-plus standby generator, or one with load management, handles it comfortably.
If you have a natural gas line, it's the most convenient, with unlimited runtime and no refueling. Propane is a great option where gas isn't available and stores well. Gasoline suits portable units for occasional use but isn't ideal for long outages. We match the fuel to your home.
A portable generator is cheaper and fine for essentials, but you have to roll it out, refuel it, and run cords. A standby generator is permanently installed, starts automatically, runs on natural gas or propane, and powers far more of your home, the better choice for reliable, hands-off backup.
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