What Size Generator Do I Need? A Simple Wattage Guide
By the Generator Hubs team · Updated · 7 min read
Work out the generator size you need in four steps: list your appliances, add running watts, add the largest starting surge, then add a safety margin.
Buying a generator that is too small means tripped breakers and stalled motors. Buying one that is far too big means paying more for the machine and for fuel. The right size comes from a short calculation you can do with a pen and the labels on your appliances.
Running watts and starting watts
Every generator lists two numbers. Running watts (also called rated watts) is the power it can supply continuously. Starting watts (peak or surge watts) is the short burst it can give for a few seconds when a motor starts.
Appliances with motors or compressors, such as refrigerators, well pumps, air conditioners and sump pumps, need two to three times their running power for a moment when they start. Lights, chargers, TVs and electronics need about the same power all the time.
Step 1: list what you want to power
Decide whether you want to keep the essentials running during an outage, or run most of the house. Write down each item. The exact figures are on the appliance label or in its manual. The ranges below are typical and only meant as a starting point.
| Refrigerator | about 700 running / 2,000+ starting |
|---|---|
| Sump pump (1/2 HP) | about 1,000 running / 2,000+ starting |
| Well pump (1 HP) | about 1,000 running / 3,000+ starting |
| Window AC (10,000 BTU) | about 1,200 running / 3,600 starting |
| Furnace fan (1/2 HP) | about 800 running / 2,300 starting |
| Microwave | about 1,000 running |
| Lights, TV, Wi-Fi, chargers | about 300–500 running in total |
Step 2: add the running watts
Add the running watts of everything that may be on at the same time. A typical essentials list (fridge, sump pump, furnace fan, lights and electronics) lands around 3,000 running watts.
Step 3: add the single largest starting surge
Motors rarely start at the exact same moment, so you only add the extra surge of the biggest one. If the well pump needs 3,000 starting watts and runs at 1,000, add the 2,000-watt difference to your total.
Step 4: add a margin
Add about 20 percent. Generators run cooler, quieter and longer when they are not at full load, and you leave room for one more appliance later.
Typical results
- Camping, RV, tools: 2,000–4,000 watts. An inverter generator is the usual choice for clean power and low noise.
- Home essentials: 5,000–8,000 watts with a portable generator.
- Most of the house, including central AC or a well pump: 10,000–15,000 watts or more. Look at tri fuel and large dual fuel models.
Not sure about your numbers? Call us at +1 408 320 0180 with your list and we will help you check it before you order.
Frequently asked questions
Can a 7,500-watt generator run a house?
It can run the essentials of most homes: refrigerator, sump pump, furnace fan, lights and electronics. Central air conditioning and electric water heaters usually need a larger unit.
Should I size on running or starting watts?
Size on running watts for the total load, then check that the starting watts cover the biggest motor you need to start.
