Generator Sizing Calculator

Calculate the right generator size for your home or business backup power needs. Select your critical loads including HVAC systems, appliances, lighting, and tools to determine the minimum generator capacity required. Includes surge load calculations, fuel consumption estimates, and recommendations for both portable and standby generators. Perfect for power outage preparation, off-grid applications, and commercial backup power planning.

What Size Generator Do I Need?

Size your generator to the total running watts of the essential loads you want to power at the same time, plus the starting watts of your single largest motor (well pump, sump pump, or AC compressor), then add 20-25% headroom so the unit never runs flat out. Motors briefly draw 2 to 4 times their running wattage at startup, so a generator sized only to running watts will stall the moment the sump pump kicks on.

Here is what typical setups need:

Essentials-only home backup (5,000-7,500W): Refrigerator, sump pump, furnace blower, lights, phone chargers, and a TV. This is the most common portable generator range for storm outages.
Most home circuits (7,500-12,000W): Everything above plus a well pump, microwave, window AC, and more general-use circuits — just not large central AC or electric heat.
Whole house with central AC (15,000-24,000W): A standby generator sized to your full load calculation, capable of starting a 3 to 5-ton AC compressor while everything else runs.
Job site tools (3,000-6,500W): Circular saw, drills, compressor, and work lights. Size to the largest tool's starting watts plus whatever runs alongside it.

Worked Example: Essentials-Only Backup

Scenario: Back up the refrigerator, sump pump, furnace blower, lights, and TV during a storm outage.

Running watts: Refrigerator 700W + Sump pump 800W + Furnace blower 800W + LED lights 300W + TV & electronics 300W = 2,900W
Largest starting surge: Sump pump adds about 1,200W extra at startup → 2,900 + 1,200 = ~4,100W peak
Add 25% headroom: 4,100 x 1.25 ≈ 5,125W → choose a 5,000-5,500W generator (e.g., a unit rated 5,000 running / 6,250 starting watts)

Result: A 5,000-5,500W portable generator covers all essential loads, handles the sump pump surge, and runs at a healthy partial load for better fuel economy and engine life.

Appliance Wattage Chart: Running vs Starting Watts

Use this chart to estimate your total load. Running watts are the continuous power needed; starting watts are the brief surge when motors kick on. Resistive loads like heaters, ranges, and lights have no starting surge. Use Ohm's Law(Watts = Volts × Amps) to convert nameplate amps to watts.

Appliance	Running Watts	Starting Watts
Refrigerator	700	2,200
Sump pump (1/2 HP)	800	1,300 – 2,000
Well pump (1/2 – 1 HP)	1,000	2,100 – 4,000
Window AC (10,000 BTU)	1,200	3,600
Central AC (3-ton)	3,500	8,000+
Central AC (5-ton)	5,000	10,000 – 15,000
Furnace blower (gas furnace)	800	2,350
Electric furnace	10,000 – 15,000	—
Microwave (1,000W)	1,000	—
Electric water heater	4,500	—
Electric range/oven	2,500 – 5,000	—
Electric dryer	4,000 – 5,000	—
Washing machine	500	1,150
EV charger (Level 2)	7,200 – 11,500	—
Lights (LED, whole house)	60 – 300	—
TV + electronics	200 – 500	—
Garage door opener	600	1,400

— = resistive load, no additional starting surge. Values are conservative sizing-chart figures; a new Energy Star refrigerator may run on as little as 150-400W, so check your appliance nameplates when possible.

Remember: You don't need to start all motors at once. Stagger startups — let each motor reach running speed before starting the next one. This reduces the peak surge your generator must handle. Use our whole house load calculator for a detailed NEC-compliant load analysis.

How to Calculate Generator Size: Step-by-Step

A properly sized generator keeps your essential loads running during outages without overloading the unit. Here is how to calculate the right size for your needs.

Step 1: List All Loads With Running Wattage

Write down every appliance you want to power during an outage with its running watts. Check nameplates or use typical values: refrigerator (150W), well pump (1,000W), furnace blower (500W), lights (60-100W each), and sump pump (800W).

Step 2: Identify Surge/Starting Watts

Motors and compressors need 2 to 3 times their running watts to start. A 1,000W well pump may need 2,000-3,000W to start. A refrigerator needing 150W to run may need 400-600W to start. Note the highest starting wattage for each motor load.

Step 3: Sum All Running Watts

Add up the running wattage of all loads that may run simultaneously. This gives your continuous load. For example: fridge (150W) + well pump (1,000W) + furnace blower (500W) + lights (300W) = 1,950W running.

Step 4: Add the Largest Surge Wattage

Take your total running watts and add the largest single surge from Step 2. If the well pump has the highest surge at 2,000W extra: 1,950 + 2,000 = 3,950W peak. This is the peak demand the generator must handle.

Step 5: Select With 20% Headroom

Multiply the peak demand by 1.20 for a safety margin: 3,950 x 1.20 = 4,740W. Select the next standard generator size: 5,000W. Running a generator at 80% or less extends its life and improves fuel efficiency.

Formula

Generator Size = (Total Running Watts + Largest Surge Watts) x 1.20

Where: Running Watts = Sum of all loads at steady state, Surge Watts = Highest single motor starting wattage, 1.20 = 20% safety margin

Worked Example

Scenario: Size a generator for a home with refrigerator, well pump, furnace blower, lights, and sump pump.

Step 1: Running watts: Fridge 150W + Well pump 1,000W + Furnace 500W + Lights 300W + Sump pump 800W = 2,750W
Step 2: Surge watts: Well pump 2,000W extra, Sump pump 1,200W extra, Fridge 400W extra
Step 3: Total running = 2,750W
Step 4: Peak = 2,750 + 2,000 (well pump surge) = 4,750W
Step 5: With headroom = 4,750 x 1.20 = 5,700W. Select a 6,500W generator

Result: A 6,500W generator handles all essential loads with adequate surge capacity and room for additional small loads.

Generator Size Guide by Home Service Size

Your electrical service size is a quick starting point for whole-house generator sizing. Here are typical recommendations based on panel amperage:

Service Size	Typical Home	Standby Generator	Portable Generator
100 Amp	Small home, no central AC	10-14 kW	5,000-7,500W
150 Amp	Medium home with central AC	16-20 kW	7,500-10,000W
200 Amp	Large home, AC + electric appliances	22-26 kW	10,000-12,000W
400 Amp	Very large / commercial	38-48 kW	N/A — use standby

These are estimates. Actual sizing depends on which loads you want to power during an outage. Use the calculator above for an accurate recommendation based on your specific loads.

Commercial Generator Sizing

Commercial generators are not sized from a wattage worksheet — they are sized from a full NEC load calculation. Start with an Article 220 calculation of the connected load, then classify the backed-up loads under Article 700 (emergency systems), Article 701 (legally required standby), or Article 702 (optional standby), since each has different sizing and transfer requirements. Run the numbers with our load calculation tool as a starting point.

Apply demand factors: Not every load runs simultaneously. NEC 220 demand factors for lighting, receptacles, and kitchen equipment can significantly reduce the calculated demand versus connected load.
Check motor starting kVA: Large motors, chillers, and compressors can demand several times their running kVA at startup and cause voltage dip that trips sensitive equipment. Verify the generator's surge capability with our motor starting calculator, and stagger or sequence large motor starts where possible.
Account for elevator and fire pump loads: Elevators impose high cyclic demand and regenerative loading; fire pumps must start and run at full capacity under NEC 695 even when everything else is online. These loads often dictate the generator size.
Size to an 80% continuous rating: Select a unit so the calculated demand sits at or below roughly 80% of the generator's standby rating. This preserves surge margin, extends engine life, and leaves room for load growth.

Typical results range from 30 kW for a small office to 2,000+ kW for hospitals and data centers. For any commercial or emergency-system installation, have a licensed engineer review and stamp the final sizing.

Generator Sizing Questions & Answers

What size generator do I need to run a house?

To run a whole house including central AC, most homes need a 15-24 kW (15,000-24,000W) standby generator. To back up essential circuits only — refrigerator, sump pump, furnace blower, lights, and a few outlets — a 5,000-7,500W portable is usually enough. A 7,500-12,000W unit covers most home circuits except central AC and electric heat. Add up the running watts of everything you want to power at once, add the starting surge of your largest motor, then add 20-25% headroom.

How do I size a standby generator?

Standby generators are sized in kW against your whole-house load calculation, not a simple appliance list. Run an NEC 220.82 load calculation to find your actual demand, or estimate by service size: a 100 amp panel typically needs 10-14 kW, a 150 amp panel 16-20 kW, and a 200 amp panel 22-26 kW. Choose a unit whose rating exceeds your calculated demand with 20-25% margin, and verify it can start your largest AC compressor — or add a load-shedding module or soft starter to manage the surge.

What size generator do I need for a 200 amp service?

A 200 amp service at 240V can theoretically draw 48,000 watts, but actual demand is much lower. Most homes with a 200 amp panel need a 22-26 kW standby generator to power the whole house, or a 10,000-12,000W portable if you only run essential circuits. The key is calculating your actual loads — most homes use 30-50% of their panel capacity at peak.

Standby vs portable generator — which should I get?

Standby generators (Generac, Cummins, Kohler) start automatically during outages, run on natural gas or propane, and power your whole panel through a transfer switch. They cost $5,000-$15,000 installed. Portable generators cost $500-$3,000, run on gasoline, and power selected circuits through extension cords or a manual transfer switch. If you lose power more than twice a year or have medical equipment needs, a standby unit pays for itself in convenience and safety.

Can I run my air conditioner on a generator?

Yes, but AC units draw heavy starting current. A 3-ton central AC unit draws about 3,500 watts running but needs 7,000-10,000 watts to start the compressor. You need a generator rated for the surge, not just the running watts. A 7,500W portable can usually handle a 2-ton AC. For a 3-ton or larger, you need 10,000W+ or a standby unit. Window units are easier — a 12,000 BTU window AC needs about 1,500W running.

How much fuel does a generator use?

A portable 5,000W generator uses about 0.75 gallons per hour at 50% load. A 10,000W portable uses 1.0-1.2 gallons per hour. Standby generators on natural gas use roughly 150-250 cubic feet per hour for a 20-22 kW unit. Propane consumption is about 2-3 gallons per hour for a similar-sized standby. Running costs range from $2-$5 per hour for gasoline portables to $1-$3 per hour for natural gas standby units.

Do I need a transfer switch?

Yes. NEC Article 702 requires a transfer switch for any generator connected to a home's electrical system. It prevents back-feeding power onto utility lines, which can electrocute line workers. Manual transfer switches cost $300-$800 installed and work with portable generators. Automatic transfer switches cost $500-$2,000 and are required for standby generators. Never use a suicide cord to connect a generator to an outlet — it is both illegal and lethal.

What is the difference between running watts and starting watts?

Running watts (rated watts) is the continuous power a generator can produce. Starting watts (surge watts) is the brief burst it can handle for motor startup — typically 10-20% higher than rated watts. Generators are sold with both numbers: a "7,500/9,500W" generator runs 7,500W continuously but handles 9,500W startup surges. Always size based on your total running watts plus the single largest motor surge.

How do I size a commercial generator?

Commercial sizing is more complex. Start with a complete load analysis per NEC 220 — include lighting, HVAC, elevators, fire pumps, and data center loads. Apply diversity factors since not everything runs simultaneously. Add 25% for growth. Factor in motor starting sequences: stagger large motor starts to reduce peak demand. Commercial generators range from 30 kW for small offices to 2,000+ kW for hospitals and data centers. Always work with an engineer for commercial installations.

How long can a generator run continuously?

Portable gasoline generators should be run no more than 12-18 hours at a time, then cooled down and refueled. Quality standby generators on natural gas can run for weeks continuously — they are designed for extended outages. Propane standby units are limited by tank size: a 500-gallon propane tank lasts about 5-7 days powering a typical 22 kW generator at 50% load. Schedule regular oil changes every 100-200 hours of runtime.

Generator Sizing Calculator

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What size generator do I need for my house?

What's the difference between running watts and starting watts?

Should I get a portable or standby generator?

What fuel type is best for my generator?

How long will my generator run on a tank of fuel?

Can I run my whole house on a generator?

Do I need a transfer switch for my generator?

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