A tower site runs a 30 kW generator behind an 8 kW load. That’s not a sizing mistake. The engineer left room for motor starting current, for a colocation tenant that hasn’t moved in yet, for the one week a year when every piece of equipment draws at once. Evoltix sees this pattern across enough customer sites, through IntelliCore’s load data, to know it’s closer to normal than exception.
That gap between what a generator can produce and what a site actually pulls is called load factor, and it’s the reason an oversized generator costs more to own than its price tag suggests. Not because it fails. Because of what happens when it runs light.
Why Backup Generators End Up Oversized
Generator sizing isn’t guesswork, and it isn’t padding for padding’s sake. A few things reliably push the number up: starting current from HVAC compressors and motors, which can draw several times their running load for a few seconds; a margin for future growth, since nobody wants to swap a generator when the site adds capacity; and a safety factor built into the sizing process itself, particularly at sites where downtime isn’t an option.
None of that is wrong, and it explains a lot of oversized units. But not all of it. Some of the same margin gets added because nobody wants to run the numbers twice, because a bigger unit reads as the safer call, or because sizing small and having a site go down is the mistake nobody wants attached to their name. Legitimate engineering margin and a defensive guess look identical on a spec sheet. Either way, what almost nobody revisits is what that decision costs once the site is actually running.
The Load Factor Problem
Diesel generator sets are built to run loaded. Standby and prime rated units are designed to operate between 50 and 85 percent of nameplate capacity; continuous rated units, between 70 and 100 percent.
Source: Caterpillar Inc., “The Impact of Generator Set Underloading,” Electric Power Division.
A generator carrying 8 kW of a 30 kW rating is running at roughly 27 percent load, and that’s a common outcome once you look at real tower site duty cycles. Evoltix has published the full runtime and fuel cost breakdown separately (see Telecom Tower Generator Runtime: How to Cut Fuel and Maintenance Costs); the short version is that most backup generators run far fewer hours, and far lighter, than their nameplate rating implies. At a 27 percent load factor, the generator isn’t just underused. It’s operating below the range Caterpillar says diesel gensets are built for.
None of this is an argument for going smaller. A generator too small to cover its own starting current or its growth margin creates a different problem, nuisance trips or brownouts, not gradual wear. The fix isn’t a smaller generator. It’s less of the day to day load riding on the generator in the first place.
A generator sized for the worst day of the year doesn’t have to run like every day is that day.
Underloading Doesn’t Just Waste Fuel. It Wears the Engine Out.
Running light isn’t a neutral inefficiency. Caterpillar’s own engineering guidance is direct on this point: diesel generator sets operated below 30 percent of rated load for extended periods are prone to wet stacking, exhaust manifold slobber caused by unburned fuel and carbon that never gets hot enough to burn off cleanly.
Source: Caterpillar Inc., “The Impact of Generator Set Underloading,” Electric Power Division.
Visible slobber isn’t itself an emergency. Left unmanaged, though, the same underloading that causes it can lead to deposit buildup behind the piston rings, accelerated wear, and in extreme cases cylinder liner damage, all of which shows up later as unplanned maintenance and shortened engine life. Caterpillar’s own workaround, periodically loading the generator above 30 percent for about 30 minutes every four hours of light load running, is a maintenance discipline. It doesn’t fix the underlying mismatch between what the generator can do and what the site actually needs.
Knowing whether a specific site sits in that underloaded range starts with a load study, actually measuring what the site draws over time, rather than assuming the nameplate tells the whole story. That’s a different exercise than the original sizing decision, and it’s the one most sites skip.
A Battery-First ZPM Runs the Generator to a Job, Not a Schedule
Evoltix built the Zero-Glitch Power Module (ZPM), a power intelligence system with a battery-first architecture, around a different premise. The battery bank carries the site’s actual load, and the generator is called in by IntelliCore only when the battery’s state of charge drops to a set threshold, to recharge it, not to serve the load directly. That shift changes both how often the generator runs and how it runs while it’s on, since recharging a battery bank is a real, sustained load rather than the light, intermittent draw of a site’s day-to-day equipment.
This doesn’t require solar, and it doesn’t require replacing the generator. It works on an existing on-grid or off-grid site that already has a generator sized the way most are, with room to spare. One rural Alaskan WISP, running remote tower sites in some of the harshest conditions in North America, cut generator runtime by 91% after moving to battery-first operation with the ZPM. That’s the upper end of a broader range Evoltix has documented, 70 – 91%, across sites that have made the same shift. How much of that a specific site can expect depends on how mismatched its own generator already is relative to actual load, which is exactly the kind of thing a battery sized to real usage, not nameplate capacity, is built to address.
Is Your Site Carrying Excess Generator Capacity?
- You know your generator’s nameplate rating better than you know what it actually draws day to day
- The unit gets a full service interval on the maintenance schedule despite modest logged runtime
- You’ve seen exhaust residue, or a technician has mentioned that the generator runs light
- Fuel and maintenance line items feel high relative to how rarely the generator seems to actually run
If you want to see what a battery sized to your actual load, not your generator’s nameplate rating, looks like on paper, Evoltix’s Total Cost of Ownership Studio can model it against your numbers. Request a session.
Related Reading
Telecom Tower Generator Runtime: How to Cut Fuel and Maintenance Costs