What Size Generator Do I Need?
A simple guide to choosing the right generator size based on your power needs.
Quick answer
A generator running a fridge, freezer, sump pump, lights, and phone chargers needs roughly 3,500–5,500W. Add central AC and you need 7,500–10,000W. For essentials only during a power outage, most households need 3,500–5,500W.
Running watts vs startup (surge) watts, the key distinction
Every electrical appliance has two wattage numbers that matter for generator sizing:
- Running watts (continuous watts): The power the appliance draws continuously while operating. This is the number used for most appliances on spec sheets and energy guides.
- Starting watts (surge watts / peak watts): The extra power required for a fraction of a second when a motor-driven appliance starts up. Refrigerator compressors, well pumps, sump pumps, air conditioners, and window units all have starting surges of 2–3× their running wattage. If your generator can't supply the surge, the appliance won't start, or the generator may stall or trip.
When sizing, add up all running watts for everything you'll run simultaneously, then add the startup surge of the single largest motor-driven appliance. That total is your minimum generator size.
Common appliance wattage reference
| Appliance | Running watts | Starting (surge) watts | Notes |
|---|---|---|---|
| Refrigerator (modern) | 100–400W | 600–1,200W | Varies widely by size and age |
| Chest freezer | 100–300W | 600–1,000W | Runs intermittently |
| Sump pump (1/2 HP) | 800W | 1,300–2,000W | Critical during storm events |
| Well pump (1/2 HP) | 900–1,200W | 2,000–3,000W | Higher surge for larger pumps |
| Central AC (2.5 ton) | 3,000–4,000W | 6,000–9,000W | Largest single surge in most homes |
| Window AC (10,000 BTU) | 900–1,200W | 2,000–3,500W | More practical on a generator |
| Electric water heater | 3,500–5,500W | No motor, same as running | Very high load, omit in most backup plans |
| Microwave (1,000W rated) | 1,000–1,500W | Same as running | No motor surge, runs intermittently |
| Electric dryer | 4,000–6,000W | Same as running | Very high load, omit in most plans |
| Washer | 500–1,200W | 1,500–2,500W | Lower priority in most outages |
| Space heater (electric) | 750–1,500W | Same as running | No motor surge |
| TV (50") | 60–120W | Same as running | Minimal draw |
| Laptop | 30–90W | Same as running | Minimal draw |
| Phone charger | 5–20W each | Same as running | Multiple chargers still modest |
| LED light bulb (60W equiv.) | 8–12W | Same as running | Very efficient, add up freely |
| CFL light bulb | 13–15W | Same as running | Slight in-rush, negligible in practice |
| CPAP machine (no heated humidifier) | 30–60W | Same as running | Low draw, high priority for some |
| Garage door opener | 350–500W | 700–1,500W | Brief use only |
Step-by-step sizing method
- Decide what you actually want to power. Don't try to run everything, prioritize essentials: refrigerator, freezer, sump pump (if you have one), one window AC unit or heat source, lights, and device charging. This is typically far less than a whole-home load.
- Find the running watts for each appliance. Check the label on the appliance, the owner's manual, or use the reference table above. Most labels show amps × volts = watts.
- Add all running watts together. This is your total continuous draw.
- Identify the single largest motor-driven appliance in your list (usually the sump pump, well pump, or AC unit). Note its starting surge wattage.
- Add the running total + the largest single startup surge. This is your minimum generator requirement.
- Add a 20% buffer. Running a generator continuously near 100% capacity shortens its life and reduces reliability. Aim for your load to be 75–80% of rated capacity at peak.
Real-world scenarios, what generator size you need
| Scenario | Key loads | Approx. running watts | Generator size needed |
|---|---|---|---|
| Basics only | Fridge, lights, phone charging, laptop | ~600–900W | 2,000W inverter works well |
| Essentials (no AC) | Fridge, freezer, sump pump, lights, TV, devices | ~2,000–3,000W | 3,500–5,000W |
| Essentials + window AC | Above + one 10,000 BTU window unit | ~3,200–4,500W | 5,500–7,000W |
| Essentials + well pump | Fridge, freezer, well pump, lights, devices | ~2,500–3,500W | 5,500–7,500W |
| Near-whole-home (no central AC) | Most loads except dryer and electric water heater | ~5,000–7,000W | 9,000–12,000W |
| Whole-home backup | Central AC, well pump, all major appliances | ~8,000–12,000W | Standby generator 12,000–20,000W |
Rated vs running watts on generator spec sheets
Generator spec sheets list two wattage figures. Rated/running watts is what the generator can sustain continuously. Surge/peak watts is the maximum for a brief period (typically a few seconds) to handle startup surges. Always size using the rated wattage, the surge rating is not available for sustained use.
Inverter vs conventional for your scenario
If your needs fall below 3,500W and you want quiet, fuel-efficient operation with clean power for electronics, an inverter generator is worth the premium. If you need more capacity, a conventional generator provides more watts per dollar. See the full comparison guide linked below.
Transfer switch and connection method
Never connect a portable generator directly to your home's electrical panel without a transfer switch, this creates a dangerous backfeed condition that can kill utility workers. Your options are: a properly installed transfer switch (manual or automatic) wired by a licensed electrician, or running heavy-duty extension cords from the generator to specific appliances. The extension cord approach is safe and common for basic backup use.
Simple takeaway
Add up the running watts of everything you want to power simultaneously, then add the startup surge of your largest motor. That total, plus a 20% buffer, is your minimum generator size. For most households running essentials only (no central AC), a 3,500–5,500W generator covers the job. If you have a well pump or want to run AC, move up to 7,500W. For whole-home backup, plan for 12,000W or a standby unit.