⚡ Generator Size Calculator
Click appliances to add them. Adjust quantities as needed.
How to Use This Calculator
Getting generator sizing right requires accounting for more than just the sum of your appliances' running wattages. Here is the method the calculator uses:
1. Identify Your Critical Load
Start by deciding which appliances must run during an outage. For most homeowners this means: refrigerator, one HVAC unit (or window A/C), furnace fan, sump pump or well pump, some lighting, and phone/laptop charging. Avoid adding non-essential high-draw items like electric dryers or water heaters unless you specifically need them during an emergency.
2. Account for Startup Surge Watts
Any appliance with an electric motor — air conditioners, pumps, refrigerator compressors — draws a brief power spike when starting. This surge can be 3–6× the running wattage and lasts less than a second. Your generator must handle this spike without stalling. The calculator identifies the single largest surge in your selection and adds its delta (surge minus running) to your total running load to arrive at peak load.
Common mistake: Selecting a generator based only on total running watts. A 5,000W generator is typically rated at 5,000W continuous — but its surge (peak) rating may be 6,250W. If your startup surge exceeds that peak rating, the generator will stall.
3. The 20% Buffer
The calculator's recommendation always adds 20% above the calculated peak load. This buffer accounts for appliance variation (older motors draw more), long-term generator wear, temperature derating (generators lose 3–5% output per 1,000 ft of elevation and output drops in heat), and allows headroom to add an appliance later.
4. Match to a Standard Size
Generators are sold in standard wattage increments. Once you have a recommended wattage, buy the nearest larger standard size. Our results recommend the nearest standard size equal to or above your calculated need.
Choosing the Right Generator Type
Portable Generator
- 1,800W – 12,000W
- Gasoline powered
- Manual start
- Must run outdoors
- Cost: $400 – $2,500
- Best for: occasional outages
Inverter Generator
- 1,000W – 7,500W
- Quieter, cleaner power
- Safe for electronics
- More fuel efficient
- Cost: $600 – $3,500
- Best for: sensitive electronics, camping
Home Standby Generator Auto-Start
- 7,000W – 20,000W+
- Natural gas or propane
- Starts automatically within seconds of outage
- Permanently plumbed and wired
- Cost: $3,000 – $15,000+ installed
- Best for: frequent outages, medical equipment, whole-home coverage
If your calculated need is under 5,000W and outages in your area are infrequent, a portable generator is usually the best value. For needs over 8,000W, or if you have medical equipment requiring uninterrupted power, a home standby generator is the right choice.
Frequently Asked Questions
A 2,000 sq ft home running essential circuits — refrigerator, central A/C (2 ton), furnace fan, lights, and phone chargers — typically needs 7,000–9,000W. Without central A/C (using a window unit instead), you can often get by with a 5,500–6,500W generator. Use the calculator above with your specific appliances for an accurate figure.
Yes, but your generator must handle the startup surge. A 2-ton central A/C runs at ~2,800W but surges to ~8,000W at startup. A generator with a 7,000W rated output typically has a 8,750W surge rating — just enough. A 3-ton unit surges to ~9,800W and needs a larger generator. Check your specific unit's surge wattage, cross-reference with your generator's peak/surge rating.
A minimum essential load (refrigerator, sump pump, lights, cell phone charging, and a few fans) typically totals around 2,000–3,500W running. With surge added and the 20% buffer, a 3,500W–5,000W generator handles most "essentials only" scenarios. If you have a sump pump and want a window A/C, budget for a 5,500W unit.
- List every appliance you want to power.
- Find each appliance's running watts (usually on a label or spec sheet).
- Sum all running watts — this is your running load.
- Find the appliance with the highest startup surge. Add (startup watts − running watts) to your running load total — this is your peak load.
- Multiply peak load × 1.2 (20% buffer) = minimum generator size.
- Buy the nearest standard generator size equal to or above that number.
Yes, but be aware that only one unit will be starting at any given moment — so the surge calculation uses only the largest single startup surge, not both. The running load includes both units simultaneously. Two 2-ton A/C units (2,800W each = 5,600W running) plus the largest surge (8,000W including 5,200W delta for one unit) = 10,800W peak. With 20% buffer that's ~13,000W. A 14,000W generator would handle this scenario.