Whole Home Generator Calculator
Select your appliances to calculate the right generator size.
Common Appliance Wattage Reference
Running and starting watts for generator sizing
| Appliance | Running Watts | Starting Watts | Daily kWh | Priority |
|---|---|---|---|---|
| Central AC (3 ton) | 3,500 | 7,000 | 28–42 | High (summer) |
| Electric Furnace | 7,000–10,000 | 7,000–10,000 | 56–80 | High (winter) |
| Refrigerator | 150–400 | 400–1,200 | 1.5–4 | Essential |
| Well Pump (1/2 HP) | 750 | 1,500–2,200 | 2–4 | Essential |
| Electric Water Heater | 4,000–4,500 | 4,000–4,500 | 10–15 | Important |
| Sump Pump (1/3 HP) | 800 | 1,300–2,000 | 1–3 | Essential |
| Electric Range/Oven | 2,000–5,000 | 2,000–5,000 | 3–8 | Moderate |
How We Calculate This
This whole home generator calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
This calculator sizes a whole home generator by adding up the running and starting wattage of every appliance you need to power during an outage, then recommending a generator with adequate capacity and safety margin.
The basic rule:
- Running watts: the continuous power each appliance needs to operate
- Starting/surge watts: the extra power motors need to start up (typically 2-3x running watts for AC, refrigerators, pumps)
- Generator sizing should cover all running watts plus the largest single surge load simultaneously
- A 20% safety margin is added to prevent overloading and allow for efficiency losses
For standby generators, professional installation is required including a transfer switch, fuel line connection, concrete pad, and electrical permits. Total installed cost is typically $5,000-$15,000 for a whole home system. Portable generators are less expensive but require manual operation and cannot power hardwired appliances.
When Would You Use This Calculator?
This whole home generator calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
What size generator do I need for a whole house?
Most homes need 16-24 kW for full whole-home backup. A smaller home with gas heating and cooking may only need 12-16 kW. A large home with electric HVAC, hot tub, and EV charger may need 24-48 kW. The key is adding up your specific appliance wattages rather than guessing — this calculator does that for you.
What is the difference between running and starting watts?
Running watts is the continuous power an appliance uses during normal operation. Starting (surge) watts is the brief spike of power needed when a motor starts up — typically 2-3 times the running watts. Air conditioners, refrigerators, well pumps, and sump pumps all have significant starting surges. Your generator must handle the highest possible simultaneous surge.
Standby vs portable generator — which is better?
Standby generators (Generac, Kohler) are permanently installed, start automatically during outages, run on natural gas or propane (no refueling), and can power the whole home. Portable generators are cheaper ($1,000-3,000) but require manual start, gasoline refueling, and can only power what you plug in. For reliable whole-home backup, standby is strongly preferred.
How much does it cost to run a generator per day?
At 50% load: Natural gas generators cost roughly $30-60/day. Propane costs $40-80/day. Diesel costs $35-70/day. Gasoline portable generators cost $20-50/day but require frequent refueling. Actual costs depend on fuel prices, generator efficiency, and your load — running at lower loads uses less fuel per hour.