What size generator do I need for my house?

Sum all essential appliances' running watts plus the highest starting watt surge. Typical homes: 5,000-7,500W portable for essentials (fridge, lights, sump pump, furnace fan), 10,000-15,000W for most circuits, 20,000-25,000W whole-house standby for everything including central AC and electric range. Our calculator adds your specific loads to give an exact recommendation.

What is the difference between running and starting watts?

Running watts = continuous power needed to operate. Starting watts = brief surge (2-6× running) when motors start. A refrigerator runs at 150W but needs 500-1,200W to start. You must size a generator for: total running watts + largest single starting surge. The starting surge only lasts 1-3 seconds but the generator must handle it.

How do I calculate generator kVA from kW?

kVA = kW ÷ Power Factor. For resistive loads (heaters, lights): PF ≈ 1, so kVA = kW. For motor loads: PF ≈ 0.8, so kVA = kW ÷ 0.8 (25% more). Example: 10kW of mixed loads at 0.85 PF = 10 ÷ 0.85 = 11.8 kVA. Generators are often rated in kVA, which is always ≥ kW.

What appliances can a 10,000 watt generator run?

A 10,000W generator can run: central AC (3,500W run + 4,500W start), refrigerator (150W), sump pump (800W), lights (500W), TV (200W), microwave (1,500W), and several outlets simultaneously — as long as total running watts stay under 10,000W and you don't start multiple motors at once. It cannot run everything in a large home simultaneously.

Do I need to account for altitude and temperature?

Yes. Generators lose approximately 3.5% of rated power per 1,000 feet above sea level, and about 1% per 10°F above 77°F (25°C). At 5,000 feet elevation: derate by ~17.5%. At 100°F ambient: derate by ~2.3%. If you're at high altitude or in extreme heat, size your generator larger to compensate.

Generator Sizing Calculator — Free kW, kVA & Fuel Consumption Tool | AllInOneTools

⚡ Electrical & Energy

Generator Sizing Calculator

Add your appliances to find the perfect generator size. Calculates running watts, starting surge, kW/kVA rating, and fuel consumption with 25% safety margin.

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Running Watts

Starting Watts

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Safety Margin

Power Factor

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Load Breakdown

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Fuel Consumption Estimate

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Frequently Asked Questions

What size generator for my house?▼

Sum running watts + largest starting surge. 5,000-7,500W for essentials, 10,000-15,000W for most circuits, 20,000-25,000W whole-house with central AC. Use our calculator to add your specific loads.

Running vs starting watts?▼

Running = continuous power. Starting = 2-6× surge when motors start (fridge, AC, pumps). Lasts 1-3 seconds. Generator must handle: total running + largest single starting surge.

kW vs kVA?▼

kVA = kW ÷ Power Factor. Resistive loads: PF≈1, kVA=kW. Motor loads: PF≈0.8, so 10kW = 12.5kVA. Generators rated in kVA are always ≥ kW rating.

How much fuel does a generator use?▼

At 50-75% load: 5,000W ≈ 0.75 gal/hr gasoline, 10,000W ≈ 1.0 gal/hr, 20kW standby ≈ 2-3 gal/hr propane. Consumption increases roughly linearly with load.

What can a 10,000W generator run?▼

Central AC (3,500W) + fridge (150W) + sump pump (800W) + lights (500W) + microwave (1,500W) + TV (200W) simultaneously. Don't start multiple motors at once.

Portable vs standby generator?▼

Portable (3-12kW, $500-$2K): manual start, gas, extension cords. Standby (10-50kW, $3-15K+): auto start, natural gas/propane, permanent install, automatic transfer switch. Standby for frequent outages.

Altitude and temperature derating?▼

Lose ~3.5% per 1,000ft above sea level, ~1% per 10°F above 77°F. At 5,000ft: derate 17.5%. At 100°F: derate 2.3%. Size larger if at elevation or extreme heat.