Quick Answer
A typical 20kW standby generator consumes 1.5-3.0 therms of natural gas per hour (50% load) or 1.5-2.5 gallons of propane per hour. Monthly fuel costs during outage conditions range from $50-$200 for natural gas and $100-$400 for propane, depending on utility rates and runtime duration.
Why Fuel Costs Matter
Fuel expenses represent the ongoing operating cost of standby generator ownership. Understanding consumption patterns helps you:
- Budget for extended outages
- Choose between natural gas and propane
- Size fuel storage appropriately
- Calculate total cost of ownership
- Evaluate payback periods accurately
Fuel Consumption by Generator Size
Natural Gas Consumption Rates
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 14kW | 0.9 therms/hr | 1.4 therms/hr | 1.9 therms/hr | 2.3 therms/hr |
| 20kW | 1.2 therms/hr | 1.8 therms/hr | 2.5 therms/hr | 3.1 therms/hr |
| 22kW | 1.3 therms/hr | 2.0 therms/hr | 2.7 therms/hr | 3.4 therms/hr |
| 30kW | 1.8 therms/hr | 2.7 therms/hr | 3.6 therms/hr | 4.5 therms/hr |
1 therm = 100,000 BTU. Check manufacturer specs for exact consumption data.
Propane Consumption Rates
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 14kW | 0.9 gal/hr | 1.4 gal/hr | 1.9 gal/hr | 2.3 gal/hr |
| 20kW | 1.2 gal/hr | 1.8 gal/hr | 2.5 gal/hr | 3.1 gal/hr |
| 22kW | 1.3 gal/hr | 2.0 gal/hr | 2.7 gal/hr | 3.4 gal/hr |
| 30kW | 1.8 gal/hr | 2.7 gal/hr | 3.6 gal/hr | 4.5 gal/hr |
Propane has ~91,500 BTU per gallon. Generator efficiency varies by brand and model.
Monthly Cost Scenarios
Natural Gas Cost Examples
Assuming $1.50 per therm (national average varies $1.00-$2.50):
| Runtime Scenario | Monthly Hours | Therms Used | Monthly Cost |
|---|---|---|---|
| Weekly 1-hour test | 4 hours | 7.2 therms | $10.80 |
| 2-day outage @ 50% | 48 hours | 86.4 therms | $129.60 |
| 5-day outage @ 50% | 120 hours | 216 therms | $324.00 |
| Weekly exercise + 1 outage | 52 hours | 93.6 therms | $140.40 |
Propane Cost Examples
Assuming $3.00 per gallon (national average varies $2.50-$4.50):
| Runtime Scenario | Monthly Hours | Gallons Used | Monthly Cost |
|---|---|---|---|
| Weekly 1-hour test | 4 hours | 7.2 gallons | $21.60 |
| 2-day outage @ 50% | 48 hours | 86.4 gallons | $259.20 |
| 5-day outage @ 50% | 120 hours | 216 gallons | $648.00 |
| Weekly exercise + 1 outage | 52 hours | 93.6 gallons | $280.80 |
Factors Affecting Fuel Consumption
Load Percentage
Your actual fuel usage depends on what’s running:
- 50% load (typical): Essential circuits, HVAC off or reduced
- 75% load: Full HVAC operation plus most household circuits
- 100% load: Everything on, rare in residential use
Temperature Impact
Extreme weather affects consumption:
- Cold weather: Longer runtime, higher heating demand
- Hot weather: AC compressors increase load dramatically
- Moderate weather: Minimal HVAC load reduces fuel use by 30-40%
Altitude Considerations
Generators lose efficiency at higher elevations:
- 2,000-3,000 feet: 10-15% derating, 10-15% more fuel for same output
- 5,000+ feet: 20-25% derating, requires propane for consistent performance
Fuel System Sizing
Propane Tank Runtime
| Tank Size | Usable Capacity (80%) | Runtime @ 50% Load (20kW) |
|---|---|---|
| 100 gallons | 80 gallons | ~44 hours |
| 250 gallons | 200 gallons | ~111 hours (~4.6 days) |
| 500 gallons | 400 gallons | ~222 hours (~9.3 days) |
| 1,000 gallons | 800 gallons | ~444 hours (~18.5 days) |
Always size for your longest expected outage plus 20% safety margin.
Natural Gas Line Sizing
Ensure adequate gas supply:
- Standard residential meter: 250-275 CFH capacity
- 20kW generator: Requires 150-180 CFH at full load
- Recommended upgrade: 325-400 CFH meter for simultaneous home + generator use
Annual Fuel Cost Planning
Budget Scenarios (20kW, Natural Gas)
| Outage Profile | Annual Runtime | Annual Therms | Annual Cost |
|---|---|---|---|
| Minimal (exercise only) | 52 hours | 94 therms | $141 |
| Typical (2-3 outages/year) | 150 hours | 270 therms | $405 |
| High outage area | 300 hours | 540 therms | $810 |
| Extended outage year | 500+ hours | 900+ therms | $1,350+ |
Budget Scenarios (20kW, Propane)
| Outage Profile | Annual Runtime | Annual Gallons | Annual Cost |
|---|---|---|---|
| Minimal (exercise only) | 52 hours | 94 gallons | $282 |
| Typical (2-3 outages/year) | 150 hours | 270 gallons | $810 |
| High outage area | 300 hours | 540 gallons | $1,620 |
| Extended outage year | 500+ hours | 900+ gallons | $2,700+ |
Fuel Cost Reduction Strategies
Natural Gas Optimization
- Time-of-use rates: Exercise generator during off-peak hours
- Load management: Stage circuit activation to reduce peak demand
- Annual tune-ups: Clean air filters and properly adjusted valves reduce consumption 5-10%
Propane Optimization
- Bulk purchase: Fill tank during summer when demand is lowest
- Automatic delivery contracts: Often priced 5-10% below will-call
- Tank ownership: Owning your tank (vs leasing) allows supplier shopping
Related Guides
For comprehensive cost analysis:
- Natural Gas vs Propane Generator Cost Calculator
- Generator Maintenance Annual Cost Estimator
- Whole House Generator Payback Calculator
- Standby Generator Size vs Cost Estimator
FAQ
How much does it cost to run a generator for a day?
A 20kW generator at 50% load costs approximately $27/day for natural gas or $54/day for propane at national average rates.
Does a generator use more fuel as it ages?
Properly maintained generators maintain consistent fuel efficiency. Neglected units with dirty air filters or fouled spark plugs may consume 10-20% more fuel.
Should I shut off my generator during short outages?
Frequent cycling (starting/stopping) causes more wear than continuous operation. Keep generators running for outages expected to last more than 2 hours.
How accurate are manufacturer fuel consumption ratings?
Manufacturer specs are tested under controlled conditions. Real-world usage varies 10-20% due to fuel quality, temperature, altitude, and load management.
Is natural gas or propane cheaper for generators?
Natural gas typically costs 40-50% less per equivalent energy output. However, propane offers independence from gas grid disruptions.
How do I calculate my specific fuel cost?
Track your generator runtime during the next outage and compare to your utility bill. Use the Home Standby Generator Cost & Outage Payback Simulator for personalized estimates.
Next Step
Use the Home Standby Generator Cost & Outage Payback Simulator to model your specific fuel costs based on local utility rates, your estimated outage patterns, and your preferred generator size. Compare annual fuel expenses against your potential outage losses to make an informed decision.