Add your appliances, calculate total load, and find the right generator & transfer switch size for your home.
How to Use This Generator Size Calculator for House
Start by clicking the quick-add preset buttons to instantly add common household appliances like refrigerators, HVAC units, and sump pumps. You can also type in any custom appliance with its running wattage and starting (surge) wattage. Adjust the safety margin and system voltage, then hit Calculate to see your minimum generator size in kilowatts, the recommended transfer switch amperage, and a full load breakdown.
Why This Matters
Sizing a generator incorrectly is one of the most costly mistakes homeowners make during a power outage. Undersizing means your generator overloads and shuts down — or worse, overheats and fails — right when you need it most. Oversizing wastes money: a 22 kW whole-home standby generator costs $5,000–$12,000 installed, while a properly sized 10–13 kW unit might serve a 2,000 sq ft home perfectly for $3,000–$6,000.
The critical number most calculators ignore is starting (surge) wattage. A 1.5-ton central AC unit might run at 1,800 W but surge to 5,400 W for the first 2–3 seconds when it kicks on. If your generator can't handle that surge, the unit won't start. Transfer switch sizing is equally important — a 50-amp transfer switch handles 12,000 W at 240V, while a 100-amp unit handles up to 24,000 W. Getting the transfer switch right ensures safe, legal disconnection from the utility grid, protecting utility workers and your equipment.
How It's Calculated
The calculator uses this formula:
Peak Load = Total Running Watts + Largest Single Starting Surge
Required Generator Capacity = Peak Load × Safety Margin
Minimum kW = Required Capacity ÷ 1,000
The transfer switch amperage is calculated as: Amps = Required Watts ÷ Voltage. We then round up to the nearest standard breaker size (30A, 40A, 50A, 60A, 100A, 125A, 150A, 200A). The recommended generator size rounds up to the nearest standard commercial size (3.5, 5, 7.5, 10, 12.5, 13, 14, 17.5, 20, 22, 25, 30 kW).
Tips & Common Mistakes
- Always use starting watts, not just running watts — motors (AC, refrigerators, well pumps) can surge 2–4× their running wattage for 1–3 seconds on startup. The largest surge determines your minimum generator floor.
- Don't run a generator at 100% load — sustained full load shortens engine life dramatically. The 20% safety margin is the minimum; 25–30% is better for HVAC-heavy homes.
- A transfer switch is not optional — plugging a generator directly into a wall outlet ("backfeeding") is illegal in most jurisdictions and can kill utility workers restoring power.
- Propane vs. natural gas generators lose 10–15% output on cold days — factor this in if you live in a cold climate by using a 25–30% safety margin.
- Check your main panel amperage — a 200A panel at 240V can theoretically handle 48,000 W, but your transfer switch only needs to cover the circuits you're actually backing up.
Frequently Asked Questions
What size generator do I need for a 2,000 sq ft house?
Most 2,000 sq ft homes with central AC, refrigerator, lights, and essential outlets need 10,000–15,000 watts (10–15 kW). If you want to run the whole home including electric range or dryer, plan for 20–25 kW. Use this calculator to get an exact number based on your specific appliances rather than square footage alone.
What is the difference between running watts and starting watts?
Running watts (also called rated watts) is the continuous power an appliance draws in normal operation. Starting watts (surge watts) is the brief spike of power needed to start a motor-driven appliance — typically 2–3 times higher, lasting only 1–3 seconds. Your generator must handle both: the continuous total of all running appliances, plus the starting surge of the largest single motor when it kicks on.
Do I need a transfer switch if I have a portable generator?
Technically no — you can run extension cords directly from a portable generator. But a transfer switch (manual or automatic) is strongly recommended for safety and convenience. It legally disconnects your home from the grid, protects utility workers, and lets you power hardwired appliances like your furnace, well pump, or sump pump directly. Many municipalities require one by code.
What size transfer switch do I need for a 7,500-watt generator?
A 7,500W generator at 240V draws about 31 amps. You'd want a 30-amp or 40-amp manual transfer switch for this size generator. However, many homeowners with 7,500W generators choose a 6-circuit or 8-circuit manual transfer switch rather than a whole-home panel, covering only critical loads like the fridge, furnace, lights, and sump pump.