Generator Runtime Guide

How Generator Runtime Works

Generator manufacturers publish runtime figures on a spec sheet, but they're almost always misleading. A label that says "12 hours at 50% load" with a 6-gallon tank means the generator burns approximately 0.5 gallons per hour at that load level. Run it at full load, and you might get 6–7 hours instead of 12.

The key relationship: fuel consumption increases non-linearly with load. Going from 50% to 100% load doesn't double fuel consumption — it typically triples or quadruples it in terms of gal/hr, depending on the engine design. This is why knowing your actual load percentage is critical for planning.

Typical Fuel Consumption by Generator Size

The following table shows approximate gasoline consumption rates for common portable generator sizes. These are real-world averages based on published manufacturer data. Actual consumption varies by brand, engine age, altitude, and temperature.

Example: A 7,500W generator with a 6-gallon tank running at 50% load (3,750W actual output) burns ~0.58 gal/hr and runs for approximately 10.3 hours per tank.

What Load Percentage Are You Actually Running?

Load percentage is calculated by dividing your total actual running watts by the generator's rated watts:

Load% = (Total Running Watts ÷ Generator Rated Watts) × 100

For example, if you're running a refrigerator (150W), a window AC (1,200W), 10 LED lights (60W total), and phone chargers (30W) on a 5,500W generator:

• Total running watts: 150 + 1,200 + 60 + 30 = 1,440W
• Load percentage: 1,440 ÷ 5,500 = 26%

At 26% load, that 5,500W generator burns roughly 0.28–0.30 gal/hr — extending runtime well beyond the 50% benchmark on the spec sheet.

Most homeowners running only essential loads end up at 25–40% of their generator's rated capacity. This is actually the most fuel-efficient operating zone for generator engines.

Factors That Reduce Runtime

Beyond load percentage, several real-world factors will reduce your actual runtime below spec sheet estimates:

1. Altitude

Generators lose approximately 3.5% of their rated power per 1,000 feet of elevation above sea level due to lower air density. At 5,000 feet, a 7,500W generator produces roughly 6,200W. This means you're running at a higher percentage of actual capacity than you think — and burning fuel accordingly.

2. Ambient Temperature

Hot weather reduces air density similarly to altitude. A generator operating in 100°F ambient temperature will produce less power and run less efficiently than at 70°F. Cold starts in very low temperatures consume extra fuel and put added stress on the engine.

3. Engine Age and Condition

An older engine with a dirty carburetor, worn piston rings, or a clogged air filter runs less efficiently. It takes more fuel to produce the same wattage. Regular maintenance (oil changes, air filter replacement, spark plug inspection) keeps fuel consumption close to spec.

4. Fuel Quality

Stale gasoline, gasoline with high ethanol content (E15 or higher), or gasoline mixed with water burns less efficiently and can damage the carburetor. Always use fresh fuel (under 30 days) or fuel treated with a stabilizer, and prefer ethanol-free gasoline if available in your area.

5. Cycling Loads (Startup Surges)

Every time a motor-driven appliance (refrigerator, AC, sump pump) cycles on, it draws a startup surge of 2–3× its running watts for 0.5–2 seconds. While this doesn't directly increase average fuel consumption much, it stresses the engine and can cause the governor to briefly hunt for load — slightly increasing average consumption over time.

How to Extend Generator Runtime During an Outage

When you're on day 3 of a multi-day outage and fuel is scarce, these strategies can significantly extend how long your generator lasts:

1. Stagger high-draw appliances

Never run the clothes dryer, space heater, and central AC at the same time. Identify your highest-wattage loads and run them sequentially rather than simultaneously. This may extend runtime by 30–50% depending on your load profile.

2. Shut off the generator overnight

Most essential loads — refrigerators, freezers, sump pumps — can coast for 6–8 hours without running the generator. A full refrigerator stays cold for 4–6 hours with the door closed; a full freezer for 24–48 hours. Shutting down at midnight and restarting in the morning can cut your daily fuel consumption nearly in half.

3. Reduce AC and heating load

HVAC is typically the single largest load on a generator. Raising the AC setpoint 4–5°F or lowering the heat setpoint 4–5°F can reduce HVAC energy consumption by 10–25% and meaningfully extend runtime.

4. Switch to LED lighting

If you're still using incandescent or halogen bulbs, switching to LEDs costs almost nothing and cuts lighting load by 75–90%. Across a 24-hour period, this can save meaningful kilowatt-hours.

5. Use a Kill-A-Watt to find phantom loads

Many devices draw power when they appear to be "off" — TVs, cable boxes, entertainment systems can draw 20–80W each in standby. Unplug or switch off power strips for anything you're not actively using.

6. Right-size your load for the generator

If you're running a 3,500W generator but only need the generator for a refrigerator, a few lights, and phone charging (under 500W total), consider whether a smaller, more fuel-efficient inverter generator would serve that part of your load better — with significantly better fuel economy.

How Much Fuel Do You Need for an Extended Outage?

Planning your fuel supply before an outage is critical, because gasoline becomes scarce very quickly after major storms. Here's a simple formula:

Fuel Needed = (Consumption Rate × Daily Hours) × Number of Days

Example: 7,500W generator at 50% load (0.58 gal/hr) × 16 hours/day × 5 days = 46.4 gallons

Most homeowners can legally store 25–30 gallons of treated gasoline in approved containers. For extended outages beyond 3–4 days, budget for fuel resupply or consider propane/natural gas standby alternatives.

Use our Fuel Cost Calculator to get a detailed breakdown for your specific generator, load, and outage duration.