Sizing a backup power system is one of the most common (and most botched) decisions homeowners make when prepping for outages. Buy too small and you're rationing electricity by candlelight; buy too big and you've spent thousands on capacity you'll never touch. The right answer lies in understanding what you actually run during an outage—not what your house theoretically draws on a hot August afternoon.
TL;DR: Most homes need somewhere between 2,000 and 7,500 watts of backup power, depending on whether you're keeping essentials alive or running the whole house. The trick is auditing your real loads, separating must-haves from nice-to-haves, and matching both running watts and surge watts to your generator or battery system.
Step 1: Decide What "Backup" Means to You
Before you size anything, get clear on your goal. Backup power generally falls into three tiers, and each has wildly different wattage requirements.
Fridge, a few lights, phone charging, internet router. Roughly 1,000–2,000 running watts.
Add a window AC or space heater, microwave, TV, well pump. Roughly 3,500–5,000 running watts.
Central HVAC, electric range, dryer, multiple circuits running simultaneously. 7,500–20,000+ watts.
Step 2: Audit Your Actual Loads
Walk through your home and list every appliance you'd want running during an outage. Each device has a nameplate (usually on the back or bottom) listing either watts directly or volts and amps. If it lists volts and amps, multiply them: 120V × 5A = 600W.
You need two numbers for each device:
- Running watts: The steady draw while operating.
- Starting (surge) watts: The brief spike when motors kick on—typically 2–3× running watts for anything with a compressor or motor.
Typical Wattage Reference
| Appliance | Running / Surge Watts |
|---|---|
| Refrigerator | 150–800 / 1,200–2,400 |
| Chest freezer | 100–400 / 600–1,200 |
| Well pump (1/2 HP) | 1,000 / 2,100 |
| Sump pump (1/3 HP) | 800 / 1,300 |
| Window AC (10,000 BTU) | 1,200 / 1,800 |
| Central AC (3 ton) | 3,500 / 5,000–7,000 |
| Furnace blower | 800 / 2,300 |
| Microwave | 1,000–1,500 / 1,500 |
| Electric range (one burner) | 1,500–2,500 / — |
| LED lighting (whole house) | 100–300 / — |
| Wi-Fi router + modem | 20 / — |
| TV + streaming box | 150 / — |
| Laptop / phone charging | 50–100 / — |
Step 3: Add It Up the Right Way
Here's where people miscalculate. You don't simply add every surge wattage together—appliances rarely start simultaneously. Instead:
- Add up the running watts of everything you'll run at the same time.
- Identify the single largest surge in your list (usually the AC compressor, well pump, or fridge).
- Add that one surge value to the total running watts.
That sum is the minimum capacity your generator or battery system needs to handle without tripping.
Example: Fridge (200W run / 1,200W surge) + furnace blower (800W run / 2,300W surge) + lights and electronics (400W) = 1,400W running. Add the largest surge (2,300W from the blower starting) and you need a unit that can handle a brief peak of about 3,700W. A 3,500W running / 4,375W surge inverter generator covers this comfortably.
Step 4: Factor In Runtime and Fuel
Wattage isn't the whole story—how long can your system sustain that load? A 5,000W generator running at 50% load typically burns 0.5–0.75 gallons of gas per hour. For a 24-hour outage, that's 12–18 gallons. For a multi-day event, you need a fuel plan or a different power source entirely.
Cheapest per watt. Loud, requires fuel storage, manual start. Good for 8–24 hour outages.
Propane stores indefinitely. Slightly less power output but far better for extended outages.
Silent, indoor-safe, recharges from sun. Capacity measured in watt-hours, not just watts.
Permanently installed, auto-start, runs on natural gas or large propane tank. 10kW–24kW typical.
Step 5: Watts vs. Watt-Hours (for Battery Systems)
If you're considering a solar generator or home battery, you need to think in watt-hours (Wh)—the total energy stored—not just peak watts. A 2,000W battery with 2,000Wh capacity can deliver 2,000 watts for one hour, or 200 watts for ten hours.
To estimate daily energy need, multiply each appliance's wattage by hours of use:
- Fridge: 200W × 8 hrs (compressor cycles ~33% of the time) = 1,600 Wh
- Lights: 100W × 5 hrs = 500 Wh
- Electronics: 100W × 6 hrs = 600 Wh
- Furnace blower: 800W × 4 hrs = 3,200 Wh
Daily total: ~5,900 Wh. You'd need a battery system in the 5–10 kWh range to cover a full day without solar recharge.
Step 6: Don't Forget the Transfer Switch
For anything beyond running extension cords through a window, you need a way to safely connect your generator to your home's circuits. A manual transfer switch or interlock kit costs $300–$800 installed and lets you power hardwired loads like furnaces, well pumps, and lighting circuits without backfeeding the grid (which can kill utility workers and is illegal in most areas).
Never connect a generator to your home through a dryer outlet or any other "suicide cord" method. Use a properly installed transfer switch or interlock, and have an electrician do the work if you're not certain about the wiring.
Quick Sizing Recommendations
| Household Scenario | Recommended Size |
|---|---|
| Apartment or small home, essentials only | 1,500–2,500W inverter generator or 1–2 kWh battery |
| Suburban home with fridge, furnace, well pump | 5,000–7,500W generator |
| Larger home wanting central AC backup | 9,000–12,000W generator or 10kW standby |
| Whole-home including electric range/dryer | 14kW–24kW standby generator |
| Off-grid or extended outage planning | Solar + battery system, 10kWh+ with PV array |
Frequently Asked Questions
Can a 2,000W generator run a refrigerator?
Yes—a standard fridge draws 150–250 watts running but can surge to 1,200–2,000 watts on startup. A 2,000W inverter generator with a higher surge rating (typically 2,200–2,500W) handles it fine, with room left for lights and small electronics.
Will a portable generator run central air conditioning?
Only larger ones. A 3-ton central AC typically needs 5,000–7,000 surge watts just to start. You'll want at least a 7,500W (running) generator, and ideally a soft-start kit on the AC to reduce inrush current.
How do I know my generator's surge rating is real?
Quality generators specify both running and starting watts clearly. Cheaper units sometimes inflate the surge number or only sustain it for a fraction of a second. Stick with reputable brands and look for a surge duration spec when available.
Is a solar generator enough for a whole house?
For short outages with light loads, yes. For sustained whole-home backup, you'd need a substantial system—often 15–30 kWh of battery paired with a 5–10 kW solar array, plus an integrated inverter rated for your peak load. This typically costs $20,000–$50,000 installed.
Should I oversize my generator just in case?
A little headroom is smart—aim for 20–25% above your calculated peak. But significant oversizing means burning more fuel at light loads and reduced engine longevity, since generators run best at 40–80% of their rated capacity.
The Bottom Line
Most homeowners overestimate what they need and underestimate the importance of surge wattage. Walk through your home with a notepad, list what truly matters during an outage, do the math, and add 20% for breathing room. That number—not what the salesperson at the big-box store tells you—is the right size for your backup power system.