Understanding Generator Sizing
Choosing the right generator size is critical for reliable backup power. An undersized generator will overload, trip breakers, and potentially damage both the generator and connected equipment. An oversized generator wastes fuel and money. The key to proper sizing is understanding two numbers: running watts (the continuous power your appliances need) and starting watts (the temporary surge when motor-driven appliances kick on).
Every appliance with a motor—refrigerators, air conditioners, sump pumps, well pumps, furnace blowers—requires 2-3 times its running wattage for the first few seconds after starting. Your generator must be large enough to handle the total running watts of everything operating simultaneously, plus the starting surge of the largest motor starting at any given moment. This calculator simplifies that process by tracking both values for every appliance in your list.
How to Use This Calculator
Start by listing every appliance and electrical load you want to power. The calculator comes pre-loaded with common residential loads as a starting point. For each item, enter the running wattage (found on the nameplate or in the owner's manual), quantity, and starting multiplier (typically 2-3× for motor loads, 1× for resistive loads like heaters and lights). Mark essential items to see a separate emergency-only sizing recommendation.
Select your fuel type and enter your site elevation. The calculator automatically applies derating factors for altitude (generators lose power above 1,000 feet) and fuel type (propane and natural gas produce slightly less power than gasoline or diesel). The result shows the minimum generator size needed and rounds up to the nearest standard generator size available on the market.
Portable vs Standby Generators
Portable generators typically range from 3,000 to 12,000 watts and run on gasoline. They are affordable ($500-$2,000) and suitable for powering essential loads during short outages. Standby generators are permanently installed, connect to your home's electrical panel via an automatic transfer switch, and range from 7,500 to 50,000+ watts. They run on natural gas or propane and start automatically when power fails.
For most homes, a 7,500W portable generator covers essential loads (fridge, sump pump, furnace, lights, phone chargers). A 20,000-22,000W standby generator can power an entire average-sized home including central AC. Larger homes or homes with multiple high-draw appliances (electric water heater, EV charger, electric range) may need 30,000W or more. Use this calculator to determine your exact needs rather than guessing.
Altitude and Fuel Derating
Internal combustion engines produce less power at higher altitudes because the air is thinner and contains less oxygen. The standard derating rule is 3.5% power loss per 1,000 feet above 1,000 feet elevation. At 5,000 feet, a generator rated at 10,000W will only produce about 8,600W. If you live at altitude, this derating is critical for proper sizing—without it, your generator may not be able to start your largest motor loads.
Fuel type also affects output. Gasoline and diesel generators produce their full rated power. Propane (LP gas) generators typically produce about 10% less power than the same engine on gasoline. Natural gas generators produce about 15% less. Dual-fuel and tri-fuel generators are popular for flexibility, but always size based on the fuel you'll actually use during an outage.
Safety, Installation, and Maintenance
Generator safety starts with proper placement. Never run a generator indoors, in a garage, or in any enclosed space—carbon monoxide poisoning is the leading cause of generator-related deaths. Place portable generators at least 20 feet from your home with the exhaust pointing away from windows and doors. Install a battery-operated CO detector in your home whenever using a generator. For standby generators, a licensed electrician should install a transfer switch to prevent back-feeding electricity into utility lines, which can electrocute line workers.
Maintenance is essential for reliability when you need it most. For portable generators, run the engine for 30 minutes every month with a load connected, change the oil every 50-100 hours of use, replace spark plugs annually, and use fuel stabilizer if storing gasoline for more than 30 days. Standby generators typically have automatic self-test cycles and require professional servicing once or twice per year. Keep at least 24 hours of fuel on hand—for a 7,500W gasoline generator at 50% load, that means approximately 15-18 gallons of fresh fuel stored safely in approved containers.
Frequently Asked Questions
How do I determine what size generator I need?
Add up the running watts of all appliances you want to power simultaneously, then find the highest starting wattage among them. Your generator must handle the total running watts plus the largest single starting surge. This calculator does that math automatically and adds a safety margin.
What is the difference between running and starting watts?
Running watts (rated watts) is the continuous power an appliance needs. Starting watts (surge watts) is the extra burst needed when a motor-driven appliance first turns on—typically 2-3× the running watts. Generators must handle starting surges or they'll trip the breaker.
Does altitude affect generator performance?
Yes. Generators lose about 3.5% of their rated output for every 1,000 feet above 1,000 feet elevation. At 5,000 feet, expect about 14% less power. This calculator applies automatic altitude derating to ensure accurate sizing.
Which fuel type is best for a generator?
Gasoline generators are cheapest and most common. Diesel generators are most fuel-efficient and durable for heavy use. Propane is cleaner-burning and stores indefinitely. Natural gas provides unlimited fuel from your gas line but produces slightly less power per cubic foot.
Should I size my generator for essential loads only?
For emergency backup, sizing for essential loads (refrigerator, sump pump, furnace, lights, medical equipment) is practical and more affordable. For whole-house backup or jobsite use, size for all loads. Mark items as essential in this calculator to see both options.
How much fuel does a generator use?
Fuel consumption depends on generator size and load. A typical 7,500W gasoline generator uses about 0.5-0.75 gallons per hour at 50% load. Diesel generators use about 30% less fuel. This calculator estimates consumption based on your generator size and fuel type.
Privacy and methodology
This calculator runs entirely in your browser. No data is sent to any server. It sums running and starting watts for all appliances, applies altitude and fuel derating factors, and recommends the nearest standard generator size that meets or exceeds the adjusted requirement. Fuel consumption estimates use industry-average rates at 50% load. Always consult generator manufacturer specifications for final sizing decisions.