When evaluating a standby generator investment, many homeowners wonder: "How much will it cost to run?" Understanding fuel consumption helps you budget for outages, size propane tanks appropriately, and compare natural gas vs. propane economics.
Key Takeaways
- A 22kW natural gas generator uses approximately 250-350 cubic feet per hour at full load.
- Propane consumption for a similar generator is approximately 2-4 gallons per hour at full load.
- Actual consumption depends on electrical load -- generators running at 50% capacity use significantly less fuel.
- Budget for at least 3-5 days of fuel supply when planning for extended outage scenarios.
Fuel Planning: Generators rarely run at full load continuously. Typical residential usage averages 50-75% of rated capacity, which reduces fuel consumption proportionally. A 500-gallon propane tank can power most homes for several days.
Factors Affecting Fuel Consumption
Generator Size
Larger generators consume more fuel—but the relationship isn't linear. A 22kW generator doesn't use twice the fuel of an 11kW unit. Larger engines are often more efficient at partial loads.
Load Percentage
The most important factor. A generator running at 50% load uses much less fuel than one at 100%:
- 25% load: Approximately 50% of full-load consumption
- 50% load: Approximately 65-70% of full-load consumption
- 75% load: Approximately 85% of full-load consumption
- 100% load: Full fuel consumption rate
Outside Temperature
Temperature affects fuel consumption in multiple ways:
- Air-cooled generators work harder in hot weather
- Propane vaporizes less efficiently in extreme cold
- Higher AC loads in summer increase generator load
- Heating season may mean higher or lower loads depending on heat source
Natural Gas Consumption Rates
Natural gas consumption is measured in cubic feet per hour (CFH):
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Typical Consumption by Generator Size
| Generator Size | 50% Load (CFH) | 100% Load (CFH) |
|---|---|---|
| 10-14kW | 100-140 | 160-220 |
| 16-20kW | 140-180 | 220-280 |
| 22-26kW | 170-220 | 260-340 |
| 36-48kW | 250-350 | 400-550 |
Natural Gas Cost Calculation
Current Northern Virginia natural gas rates: approximately $1.20-1.50 per therm (100 cubic feet)
Example: 22kW generator at 50% load
- Consumption: ~190 CFH
- Cost per hour: 190 ÷ 100 × $1.35 = $2.57/hour
- Cost per day: $2.57 × 24 = $61.56/day
Propane Consumption Rates
Propane consumption is measured in gallons per hour (GPH):
Typical Consumption by Generator Size
| Generator Size | 50% Load (GPH) | 100% Load (GPH) |
|---|---|---|
| 10-14kW | 1.0-1.4 | 1.6-2.2 |
| 16-20kW | 1.4-1.8 | 2.2-2.8 |
| 22-26kW | 1.7-2.2 | 2.6-3.4 |
| 36-48kW | 2.5-3.5 | 4.0-5.5 |
Propane Cost Calculation
Current Northern Virginia propane rates: approximately $2.50-3.50 per gallon
Example: 22kW generator at 50% load
- Consumption: ~1.9 GPH
- Cost per hour: 1.9 × $3.00 = $5.70/hour
- Cost per day: $5.70 × 24 = $136.80/day
Propane Tank Sizing
For propane generators, tank size determines maximum runtime:
How Long Will Your Propane Last?
| Tank Size | Usable Gallons* | 20kW at 50% Load |
|---|---|---|
| 120 gallon | 96 | ~60 hours |
| 250 gallon | 200 | ~125 hours (5+ days) |
| 500 gallon | 400 | ~250 hours (10+ days) |
| 1000 gallon | 800 | ~500 hours (20+ days) |
*Tanks are typically only filled to 80% capacity for safety.
Tank Sizing Recommendations
- Minimum: 24 hours of operation at full load
- Recommended: 48-72 hours at expected average load
- Extended outages: 500+ gallon tank for multi-day events
Weekly Exercise Fuel Costs
Your generator runs briefly each week to stay ready:
- Typical exercise: 12-20 minutes
- Natural gas cost: $0.50-$1.50 per exercise
- Propane cost: $1.00-$3.00 per exercise
- Annual exercise cost: $25-$75 (natural gas) or $50-$150 (propane)
Real-World Outage Fuel Costs
Based on typical Northern Virginia home with 20kW generator:
Summer Outage (AC Running)
- Average load: 60-70% capacity
- Natural gas: $75-$100/day
- Propane: $150-$200/day
Winter Outage (Gas Heat, Electric Accessories)
- Average load: 30-40% capacity
- Natural gas: $40-$60/day
- Propane: $80-$120/day
Mild Weather Outage
- Average load: 20-30% capacity
- Natural gas: $25-$40/day
- Propane: $50-$80/day
Reducing Fuel Consumption
During Outages
- Turn off non-essential loads
- Raise AC thermostat a few degrees
- Delay running dryer or oven if possible
- Use LED lighting
System Design
- Properly size the generator (oversized = inefficient)
- Consider load management to run a smaller generator more efficiently
- Choose natural gas where available for lower fuel costs
Natural Gas vs. Propane: Cost Summary
For equal power output, propane typically costs 40-60% more than natural gas. Over 10-20 years of ownership:
- Exercise fuel difference: $300-$600
- Outage fuel difference: Depends on outage frequency/duration
- This cost difference should factor into your fuel type decision
Plan Your Fuel Budget
At AJ Long Electric, we help you understand all costs of generator ownership, including fuel consumption. We'll calculate expected costs based on your specific generator size, fuel type, and home's power needs.
Contact us for a complete cost analysis as part of your generator evaluation. We provide honest, detailed estimates so there are no surprises.
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Written by
Matt Long
Master Electrician
Our team of licensed electricians brings over 40 years of combined experience serving Northern Virginia. We're committed to providing expert electrical solutions with a focus on safety, quality, and customer satisfaction.
Reviewed by AJ Long Electric Master Electricians · VA License #2705031092 · View Credentials
