The key is that diesel price affects the operating-cost side, while age affects the purchase price and repair-risk side.
A simple way to think about it
For a diesel generator:
Total cost = purchase price + fuel + maintenance + repairs − resale value
If diesel is unusually expensive, fuel becomes a much larger part of the equation. Thattends to favor a generator that is more fuel-efficient, even if it costs more upfront.
For example, suppose:
| Used generator | New generator | |
| Purchase price | $8,000 | $14,000 |
| Fuel consumption | 0.9 gal/hr | 0.65 gal/hr |
| Diesel | $6/gal | $6/gal |
| Annual runtime | 500 hr | 500 hr |
| Annual fuel | 450 gal | 325 gal |
| Annual fuel cost | $2,700 | $1,950 |
The new machine saves about $750/year in fuel. The $6,000 purchase-price difference therefore takes roughly 8 years to recover from fuel savings alone.
But if you run them 2,000 hours/year, the fuel savings become about $3,000/year, changing the economics substantially.
There’s another important wrinkle
With an older used diesel, fuel efficiency isn’t the only concern. At high diesel prices, you should pay particular attention to:
- Actual fuel consumption at your expected load
- Engine hours and maintenance history
- Ability to get replacement parts
- Injector/turbo condition
- Cooling-system condition
- Generator-end condition
- Automatic transfer-switch compatibility
- Cost of a major engine repair
- Whether it can efficiently operate at your typical load
A cheap used generator that burns significantly more fuel can become surprisinglyexpensive if you run it frequently.
Conversely, if this is a backup generator that might run only 20–100 hours per year, fuel efficiency becomes relatively unimportant. In that situation, a well-maintained used unit can make more economic sense because you’re unlikely to accumulate enough operating hours for fuel savings to repay the new-generator premium.
The biggest question is your expected hours
As a rough rule:
- <100 hr/year: purchase price and reliability dominate.
- 100–500 hr/year: fuel efficiency starts becoming meaningful.
- 500–1,500 hr/year: fuel consumption can substantially change the decision.
- 1,500+ hr/year: treat it much more like a commercial equipment purchase;efficiency, maintenance intervals and lifetime operating cost become extremely important.
If you give me the new generator price, used-generator price, horsepower/kW, fuel consumption (or model numbers), and how many hours/year you expect to run it, I can calculate the 5- and 10-year total cost of ownership and show exactly what diesel price makes the new one financially preferable.