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.
Select Your Loads
Essential
Running: 700W | Starting: 2200W
Running: 500W | Starting: 1500W
Running: 800W | Starting: 2400W
Running: 1000W | Starting: 3000W
HVAC
Running: 3500W | Starting: 10500W
Running: 4000W | Starting: 12000W
Running: 5000W | Starting: 15000W
Running: 1500W | Starting: 4500W
Running: 900W | Starting: 2700W
Running: 800W | Starting: 2400W
Running: 4000W | Starting: 12000W
Lighting
Running: 100W | Starting: 100W
Running: 200W | Starting: 200W
Electronics
Running: 150W | Starting: 150W
Running: 300W | Starting: 300W
Running: 50W | Starting: 50W
Kitchen
Running: 1000W | Starting: 1000W
Running: 900W | Starting: 900W
Running: 1800W | Starting: 1800W
Running: 700W | Starting: 2100W
Garage
Running: 550W | Starting: 1100W
Tools
Running: 1600W | Starting: 4800W
Running: 1800W | Starting: 5400W
Running: 6000W | Starting: 6000W
Other
Running: 4000W | Starting: 4000W
Running: 1200W | Starting: 3600W
Running: 3000W | Starting: 3000W
Custom Loads
Generator Settings
What size generator do I need for my house?
The size depends on which appliances you want to power during an outage. Essential loads (refrigerator, lights, furnace blower) typically need 5-7kW. Adding central air conditioning can require 15-20kW or more. Use this calculator to get an accurate estimate based on your specific needs.
What's the difference between running watts and starting watts?
Running watts is the continuous power an appliance uses during normal operation. Starting watts (surge watts) is the extra power needed when motors start up - typically 2-3 times the running watts. Your generator must handle the highest starting wattage of any single appliance plus the running watts of everything else.
Should I get a portable or standby generator?
Portable generators cost less upfront and offer flexibility but require manual startup and fuel management. Standby generators automatically start during outages and connect directly to your home's electrical system but cost more and require professional installation. Consider frequency of outages and convenience needs.
What fuel type is best for my generator?
Gasoline is widely available but has shorter shelf life. Propane stores longer and burns cleaner but provides less power per gallon. Diesel is very efficient and reliable but generators cost more. Natural gas is convenient for standby units with unlimited fuel supply but requires gas line installation.
How long will my generator run on a tank of fuel?
Runtime depends on generator size, fuel type, and load. A 5kW generator typically runs 8-12 hours on 5 gallons of gasoline at 50% load. Propane and natural gas provide different runtimes. The calculator provides estimates based on your specific setup and load requirements.
Can I run my whole house on a generator?
Yes, but it requires a large generator (15-25kW+) and proper installation with a transfer switch. Most people choose essential loads only to reduce generator size and fuel consumption. A qualified electrician can install a transfer switch to safely power selected circuits during outages.
Do I need a transfer switch for my generator?
Yes, a transfer switch is required by electrical code to safely connect a generator to your home's electrical system. It prevents dangerous backfeeding to utility lines and allows you to power hardwired appliances. Manual transfer switches cost less; automatic switches provide convenience for standby generators.
How much does it cost to run a generator?
Operating costs depend on fuel type, generator efficiency, and local fuel prices. Gasoline generators typically cost $2-8 per hour to operate depending on size and load. Propane and natural gas costs vary by region. The calculator provides cost estimates based on your local fuel prices and usage patterns.
Common Appliance Wattage Chart for Generator Sizing
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. Use Ohm's Law(Watts = Volts × Amps) to convert nameplate amps to watts.
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Central AC (3-ton) | 3,500 | 7,000 – 10,000 |
| Central AC (5-ton) | 5,000 | 10,000 – 15,000 |
| Window AC (12,000 BTU) | 1,500 | 2,200 |
| Electric furnace | 10,000 – 15,000 | Same |
| Gas furnace blower | 500 – 700 | 1,400 |
| Refrigerator | 150 – 400 | 600 – 1,200 |
| Freezer | 100 – 300 | 500 – 900 |
| Well pump (1/2 HP) | 1,000 | 2,000 – 3,000 |
| Sump pump (1/2 HP) | 800 | 1,300 – 2,500 |
| Electric water heater | 4,000 – 4,500 | Same |
| Electric range/oven | 2,500 – 5,000 | Same |
| Microwave (1,000W) | 1,500 | Same |
| Electric dryer | 4,000 – 5,000 | Same |
| Washing machine | 500 | 1,150 |
| EV charger (Level 2) | 7,200 – 11,500 | Same |
| LED lights (10 bulbs) | 100 – 150 | Same |
| Garage door opener | 600 | 1,400 |
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.
Generator Sizing Questions & Answers
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.
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