Generator Sizing Guide: How Many Watts Do You Really Need?
Learn how to size a home generator correctly. Count your starting and running watts to choose the best model.
Why Generator Sizing Matters
When the power grid fails, a home generator is your ultimate backup. But guessing the size of the generator you need can lead to costly mistakes. Buying a generator that is too small means it will stall or trip its breaker when motors kick on, potentially damaging sensitive electronics. Buying one that is too large wastes money on initial purchase cost, storage space, and excess fuel consumption.
The secret to correct generator sizing is the Total Surge Calculation. Let's walk through exactly how to size a generator for your specific needs, ensuring reliability during an emergency.
Step 1: Identify Essential Appliances
Start by listing the devices you absolutely need during an outage. Common essentials include:
- Refrigerator and/or chest freezer (to save food).
- Well pump or sump pump (for water access and flood prevention).
- Lighting (LEDs are highly recommended).
- A few electronics (routers, phone chargers, laptop).
- Heating or cooling (Space heaters, window AC, or furnace blower).
Step 2: Find the Running and Starting Watts
Look at the manufacturer nameplate on each device to find its wattage. Remember, any appliance with an inductive motor will have a "Surge" or "Starting" wattage.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Refrigerator | 700W | 2,200W |
| Sump Pump (1/2 HP) | 1,050W | 2,150W |
| Window AC (10k BTU) | 1,000W | 3,000W |
| Lights & Electronics | 200W | N/A (200W) |
Step 3: Calculate Your Generator Size
To determine the minimum generator capacity you require, use this formula:
Total Running Watts + Largest Single Starting Surge = Required Generator Size
We do not add all the starting surges together, because it is extremely unlikely all motors will start on the exact same millisecond. We only need enough buffer to start the largest motor while everything else is already running.
Worked Example: The Sizing Calculation
Using the table above, the total running watts = 700 + 1050 + 1000 + 200 = 2,950 Running Watts.
The largest surge difference comes from the Window AC. It requires 3,000W to start, which is a 2,000W surge over its 1,000W running baseline.
Generator Minimum Capacity = 2,950 (Total Running) + 2,000 (Largest Surge) = 4,950 Surge Watts.
Pro Tip: Add a 20% safety margin. A generator running at 100% capacity is loud and uses extreme amounts of fuel. For a calculated 4,950W load, a 6,000W or 6,500W generator is ideal.
Portable Generators vs Standby Generators
- Portable / Inverter Generators (2,000W - 8,000W): Excellent for running critical loads via extension cords or a manual transfer switch. Inverter models provide clean sine waves for electronics.
- Home Standby Generators (10,000W - 24,000W+): Wired directly into the house panel, running on natural gas or propane. They can power an entire house, including central AC and electric ranges.