Select your appliances, get an accurate wattage recommendation, estimate runtime, calculate fuel costs — all from one free toolset built for homeowners.
From initial sizing to cost planning — all pure math, no account required.
Select your appliances from a full list. Accounts for startup surge. Recommends the minimum generator wattage with a built-in 20% safety buffer.
→Enter your generator's wattage, tank size, and load percentage to see exactly how many hours — and days — your fuel supply will last.
→Plan your outage budget. Calculate daily and total gasoline cost based on your generator's size, hours of use, and local fuel prices.
→Running a generator extension cord can cause fires if the wire gauge is wrong. Enter load and length — get the safe minimum AWG instantly.
→ ReferenceFull table of running and startup watts for 40+ common household appliances and power tools. Essential reference when planning backup power.
→Choose every device you need to power during an outage. Set quantities for each item in your home.
The calculator sums running watts, identifies the highest startup surge, and applies a 20% safety buffer automatically.
See a specific wattage rating, the generator type that fits your needs, and tips for buying within that range.
The biggest mistake homeowners make is calculating only running watts and ignoring startup (surge) watts. When an electric motor starts — an air conditioner, refrigerator, sump pump, or well pump — it draws 3 to 6 times its running wattage for a fraction of a second.
A 5,000W generator cannot start a 5,000W A/C unit. A generator that can't handle the surge will stall, trip its breaker, or fail entirely — right when you need it most.
Our calculator accounts for surge wattage automatically, using real manufacturer data for every appliance on the list.
Run the Calculator →| Appliance | Running | Startup Surge |
|---|---|---|
| Central A/C (2 ton) | 2,800W | 8,000W |
| Well Pump (½ HP) | 1,000W | 2,100W |
| Sump Pump (½ HP) | 1,050W | 2,150W |
| Refrigerator | 150W | 600W |
| Air Compressor (1HP) | 1,500W | 4,500W |
Running watts (also called rated watts) is the continuous power an appliance needs while operating normally. Starting watts (surge watts) is the brief, higher power spike required to start motors — compressors, pumps, fans — from a dead stop. Most appliances with electric motors draw 3–6× their running wattage for a fraction of a second at startup. Your generator must handle both the combined running load and the worst-case startup surge.
Portable generators are lower cost ($500–$2,500), need to be manually started, and require storing and handling gasoline. They must be operated outdoors and connected via extension cords or a transfer switch. Best for occasional outages or under 10,000W needs.
Home standby generators (whole-home generators) start automatically, run on natural gas or propane, and connect directly to your home's electrical panel via an automatic transfer switch. They cost $3,000–$15,000+ installed. Best for frequent outages, larger homes, or anyone who needs full-home coverage without any manual intervention.
The appliance wattage figures are based on published manufacturer data and widely referenced industry standards. The calculator adds a 20% safety buffer on top of the calculated peak load, which is the standard recommendation in generator sizing guides. Real-world results can vary by appliance age, condition, and brand — always review the calculator output as a minimum starting point, not a guaranteed match. When in doubt, size up by one tier.
Yes, in virtually all cases. Connecting a portable generator directly to your home's wiring without a transfer switch (also called backfeeding) is illegal in most jurisdictions, dangerous to utility workers, and can destroy your generator when power is restored. A manual transfer switch ($200–$600 installed) lets you safely connect a portable generator to specific circuits. An automatic transfer switch (ATS) is required for home standby generators and costs $500–$1,500 plus installation.
Usually not — and you don't need to. A typical whole-house load during an outage is 8,000–15,000W, which requires a large and expensive generator. The better approach is to identify your essential circuits: refrigerator, a few lights, HVAC (or a window unit), sump pump, and phone charging. This critical load typically sits in the 3,000–7,000W range and can be handled by a mid-size portable generator at a fraction of the cost. Use our Generator Size Calculator to identify exactly what you need.