Resources
Sizing, fuel, and the UPS question
The three decisions that determine whether a backup power system actually works: how big, what it burns, and whether batteries sit in front of it. Here is how we reason through each one.
Guide 01
How to size a standby generator
- 1. Inventory the must-run loads
- List every device that has to operate, with its running wattage from the nameplate. Skip anything you can live without for three days.
- 2. Add starting surge
- Motors such as well pumps, compressors, and furnace blowers draw roughly three to six times running current at startup. The largest motor's surge, not the sum of all of them, usually sets the peak.
- 3. Account for simultaneity
- Assume the worst realistic overlap: furnace calling while the well pump starts and the freezer compressor kicks in.
- 4. Add headroom
- Size 20–25% above calculated peak. Headroom protects voltage stability, reduces wet-stacking on diesel, and leaves room for future load.
- 5. Decide transfer scope
- Whole-panel transfer is simpler to live with; critical-branch transfer is cheaper and often smarter. Load management can bridge the two.
This is the reasoning, not a substitute for a load calculation. On any property with a deep well, multi-zone heating, or three-phase equipment, we measure rather than estimate.
Guide 02
Choosing a fuel
- Natural gas
- Unlimited runtime with no storage and no fuel degradation. It is the best option wherever a utility gas line reaches the property. Slightly lower power density than diesel.
- Propane
- Stores indefinitely without degrading, which makes it the standard rural Montana answer. Runtime is a function of tank size, so tank sizing is part of the design.
- Diesel
- Highest power density and the standard for large commercial gensets. Requires fuel polishing, water management, and testing, because stored diesel goes bad.
- Dual fuel (portable)
- Gasoline plus propane flexibility for portable units. Appropriate for job sites and outbuildings, not as a building's primary standby strategy.
Guide 03
UPS or generator: the honest answer is usually both
01
The transfer gap
An automatic transfer switch takes seconds to sense loss and transfer. That gap is invisible to a freezer and fatal to a server mid-write.
02
What a UPS is for
Instant, no-break power for minutes. It protects the transition and gives equipment time to either ride through or shut down cleanly.
03
What a generator is for
Duration. Once running, it carries the building for as long as fuel lasts: days on natural gas or a properly sized propane tank.
Want this applied to your building?
Send your load list and we'll run the calculation properly, including motor starting requirements and fuel recommendations.