Generator Runtime Calculator

How Long Will Your Generator Run? Complete Fuel Planning Guide

When the power goes out, the first question every generator owner asks is: "How long will this last?" The answer depends on three critical factors: your generator's size, how much you're powering, and what type of fuel you're using. This guide breaks down generator runtime calculations and helps you plan fuel storage for any emergency scenario.

Understanding Generator Fuel Consumption

Generators don't consume fuel at a fixed rate. Your actual consumption depends heavily on the electrical load you're running. A 5,000-watt generator running a 1,000-watt load uses far less fuel than the same generator running a 4,000-watt load. This is why the load percentage matters so much in runtime calculations.

Most manufacturers rate fuel consumption at 50% load, which is the "sweet spot" for efficiency. Running below 25% load actually wastes fuel because the engine still has to overcome its own internal friction. Running above 75% load burns fuel rapidly and puts stress on the engine.

Fuel Consumption by Generator Size (at 50% Load)

The Runtime Formula

Generator runtime follows a basic formula:

But fuel consumption rate isn't constant. It scales with load percentage. Here's the relationship:

• 25% load: Approximately 40-50% of rated fuel consumption
• 50% load: The rated consumption (manufacturer's spec)
• 75% load: Approximately 130-150% of rated consumption
• 100% load: Approximately 180-220% of rated consumption

This is why running at lower loads dramatically extends runtime. Cutting your load in half doesn't just double your runtime—it can nearly triple it.

Gasoline vs. Propane vs. Diesel: Which Lasts Longer?

Each fuel type has different energy density and burns at different rates:

Gasoline

Gasoline contains about 120,000 BTU per gallon, making it energy-dense and widely available. However, gasoline degrades within 6-12 months even with stabilizer, and it's the most dangerous to store due to fume volatility. Most portable generators are gasoline-powered because of the lower upfront cost.

Propane

Propane contains about 91,500 BTU per gallon—roughly 25% less energy than gasoline. This means you'll burn through propane faster for the same power output. However, propane stores indefinitely, burns cleaner (extending engine life), and is safer to store. A standard 20-pound propane tank holds about 4.7 gallons.

Diesel

Diesel contains about 138,000 BTU per gallon—the highest of the three. Diesel generators are more fuel-efficient and the fuel lasts 12-24 months in proper storage. However, diesel generators cost more upfront and aren't available in small portable sizes.

How Much Fuel Should You Store?

Your fuel stockpile depends on your outage planning horizon and how many hours per day you'll run the generator. Most emergency planners recommend running the generator 8-12 hours per day during an outage, not 24/7.

The Cycling Strategy

Instead of running continuously, cycle your generator:

• Morning cycle (3-4 hours): Run fridge, make coffee, charge devices
• Afternoon break: Generator off, fridge stays cold for 4-6 hours
• Evening cycle (4-5 hours): Run fridge, cook dinner, lights, entertainment
• Night: Generator off unless running medical equipment

This approach cuts your daily fuel consumption nearly in half while keeping food safe and devices charged.

Fuel Storage Guidelines by Outage Duration

Add 20-30% to these numbers as a safety margin. You can always use excess fuel in vehicles or lawn equipment.

Maximizing Your Generator's Runtime

1. Reduce Your Load

Every watt you eliminate extends runtime. Unplug phantom loads, use LED lights instead of incandescent, and avoid running multiple high-draw appliances simultaneously. A space heater, hair dryer, and microwave together can overload a mid-size generator.

2. Maintain Your Generator

A poorly maintained generator burns more fuel. Change the oil every 50-100 hours of use. Clean or replace the air filter regularly. Use fresh fuel, not gas that's been sitting in a can for months. A well-tuned engine can be 15-20% more fuel-efficient.

3. Run at Optimal Load

Aim for 40-60% of your generator's rated capacity. This is the efficiency sweet spot. If you have a 5,000W generator, running 2,000-3,000W of load maximizes fuel efficiency while leaving headroom for surge loads.

4. Use an Inverter Generator for Light Loads

Inverter generators automatically throttle down when load decreases, saving significant fuel during low-demand periods. A 2,200W inverter generator at 25% load can run 10+ hours on a single gallon of gas.

Safe Fuel Storage Practices

Storing generator fuel requires attention to safety and fuel preservation:

Gasoline Storage

• Store in approved containers (red plastic or metal jerry cans)
• Keep in a detached shed or garage, never inside the home
• Add fuel stabilizer (Sta-Bil or similar) for storage over 30 days
• Rotate stock every 6 months by using in vehicles
• Maximum recommended home storage: 25 gallons

Propane Storage

• Store tanks upright outdoors or in a ventilated area
• Keep away from ignition sources and air intakes
• No expiration—propane stores indefinitely
• Check connections and valves annually
• Have a backup tank; you can't tell when one is empty

When the Fuel Runs Out

Have a backup plan for extended outages beyond your fuel supply:

• Battery backup: A portable power station can cover essentials when generator fuel is depleted
• Solar option: Solar panels can recharge battery systems for indefinite low-power operation
• Fuel procurement: Know which gas stations have backup power (truck stops, major chains)
• Community resources: Shelters, community centers, and some businesses offer charging during emergencies