How Long Will a Generator Run?
One of the most common questions homeowners ask during power outages is:
How long will my generator run on one tank of fuel?
The answer depends on:
- Fuel type (gas, diesel, propane)
- Tank size
- Generator load (watts used)
- Generator efficiency
- Runtime at 25%, 50%, or 100% load
A generator running at half load will last significantly longer than one running near full capacity.
Use the runtime calculator below to estimate your exact fuel duration.
Generator Runtime by Fuel Type
Gasoline Generator Runtime
Most portable gas generators:
- Run 6–12 hours at 50% load
- Consume 0.5–0.75 gallons per hour
- Burn more fuel at higher loads
Example:
8-gallon tank
0.6 gallons/hour at 50% load
≈ 13 hours runtime
Diesel Generator Runtime
Diesel generators are typically more fuel-efficient.
- 0.4–0.6 gallons per hour at 50% load
- Longer lifespan
- Better for extended outages
Diesel often runs 15–25% longer per gallon compared to gasoline.
Propane Generator Runtime
Propane runtime depends on tank size:
| Tank Size | Approx Runtime @ 50% Load |
|---|---|
| 20 lb tank | 6–8 hours |
| 40 lb tank | 12–16 hours |
| 100 lb tank | 30+ hours |
Propane burns cleaner but often slightly less efficiently than diesel.
Generator Runtime Chart (Estimated)
| Generator Size | 25% Load | 50% Load | 100% Load |
|---|---|---|---|
| 3,500W | 12–15 hrs | 8–10 hrs | 5–6 hrs |
| 5,000W | 10–14 hrs | 7–9 hrs | 4–5 hrs |
| 7,500W | 9–12 hrs | 6–8 hrs | 3–4 hrs |
| 10,000W | 8–10 hrs | 5–7 hrs | 2–3 hrs |
Actual runtime varies by brand and engine size.
Generator Runtime Calculator
Generator Runtime Calculator
What Affects Generator Runtime?
Load Level
Higher watt usage = shorter runtime.
Use:
👉 Appliance Wattage Chart & Calculator
👉 Generator Load Worksheet & Calculator
to calculate your load accurately.
Starting Watts
High motor startup loads increase fuel consumption spikes.
👉 Starting vs Running Watts Guide
👉 Starting Watts Calculator
Generator Efficiency
Inverter generators:
- More fuel efficient
- Adjust RPM to match load
Conventional generators:
- Run at constant speed
- Burn more fuel at low loads
How to Extend Generator Runtime
✔ Run only essential appliances
✔ Stagger motor startups
✔ Use LED lighting
✔ Keep generator at 50–70% load
✔ Perform regular maintenance
Frequently Asked Questions
How long will a 5,000 watt generator run?
Typically 7–9 hours at 50% load on a full tank (8–10 gallons), depending on engine efficiency.
How long will a generator run on 5 gallons of gas?
If burning 0.6 gallons per hour at 50% load:
5 ÷ 0.6 ≈ 8 hours.
Does load percentage affect runtime?
Yes. Lower load = lower fuel burn = longer runtime.
Is propane or gas better for long outages?
Propane stores longer. Diesel is most fuel-efficient. Gasoline is most common.
Can a generator run 24 hours straight?
Yes, but it requires:
- Proper ventilation
- Oil checks
- Refueling breaks
- Maintenance intervals
Link Back to Generator Fuel Hub
For complete backup power planning, visit:
This includes:
- Starting Watts Calculator
- Generator Load Worksheet
- Appliance Wattage Chart
- Extension Cord Gauge Chart
- Fuel Consumption Calculator
Final Thoughts
Generator runtime depends on fuel type, tank size, and how much power you’re using.
To avoid running out of fuel during an outage:
1️⃣ Calculate appliance load
2️⃣ Estimate starting surge
3️⃣ Use the runtime calculator
4️⃣ Add extra fuel margin
Planning ahead ensures your generator runs safely and efficiently when you need it most.