Standby or portable
A standby generator is fixed in place, runs on natural gas, propane or diesel, and starts automatically. A portable generator has to be wheeled out, fueled and connected by hand, and must run outdoors well away from doors and windows because of carbon monoxide. Portables suit a small shop that can live with a manual start. Anything that must keep running with nobody on site needs a standby unit.
Transfer switches
The transfer switch connects the building to either the utility or the generator, never both at once.
- Automatic transfer switch (ATS). Senses the outage, starts the generator and switches the load over, typically within seconds. Switches back when utility power returns and lets the engine cool down.
- Manual transfer switch or interlock. Someone flips it. Cheaper, and common with portable generators.
- Whole-building or selected circuits. A service-rated switch can carry the whole building. A smaller switch feeding a critical-loads panel carries only what you choose, with a smaller generator.
Sizing
Size the generator for the loads it must carry, not for the building's full service. List each load with its running watts, then find the largest motor.
- Running load. Lighting, computers, refrigeration, point of sale, network gear, pumps and whatever HVAC you need to keep.
- Starting load. Motors in compressors, air conditioners, pumps and freezers draw several times their running current for a moment when they start. The generator has to absorb the largest of these on top of everything else running. Soft starters on large motors can shrink the generator you need.
- Three-phase or single-phase. The generator must match your electrical service. Many commercial buildings are three-phase.
- Headroom. Engines run best well below their maximum rating, and loads grow.
An installer should do a load calculation from your panels, or use a meter to record demand over time. Be wary of a quote that sizes the generator from square footage alone.
Fuel: natural gas, propane or diesel
| Fuel | Supply | Strength | Watch for |
|---|---|---|---|
| Natural gas | Utility pipe, no tank | No refueling; fuel does not go stale | Needs adequate gas pressure; supply can be cut in some disasters |
| Propane | Tank on site | Stores for years without degrading | Tank size sets run time; refills in a long outage |
| Diesel | Tank, often in the base | Strong under heavy and motor loads; common at larger sizes | Fuel ages and needs testing or treatment; tank and emissions rules |
Most small commercial standby units run on natural gas where a gas line is available. Diesel is common where there is no gas service, at larger sizes, and where on-site fuel is wanted in case the gas supply fails. Ask how long the unit runs at your expected load on a full tank, and who refuels it in a long outage.
Permits and rules
- Electrical permit for the transfer switch and connection, and usually a gas or fuel permit. The installation follows the National Electrical Code as adopted locally; optional standby systems are treated differently from emergency systems that a building code requires.
- Zoning and placement. Setbacks from property lines, doors, windows and air intakes, and clearances set by the manufacturer. Landlord approval if you lease.
- Noise ordinances, which often apply to the weekly test run as well.
- Air quality. Stationary engines fall under EPA emissions rules, and some states and air districts require permits and limit how many hours a unit may run outside an emergency.
- Fuel storage. Diesel and propane tanks have fire code and, at larger sizes, environmental requirements.
Your installer should pull the permits. Tax treatment of the purchase varies; ask your accountant rather than relying on a sales claim about deductions or credits.
A UPS still matters
Even a fast automatic transfer leaves a gap of several seconds. Computers, servers, phone systems and point-of-sale terminals drop in that gap unless they sit on a UPS. The UPS carries them through the switchover; the generator carries the long outage. Size it with the UPS sizing guide and compare models in the UPS register. Ask for a UPS that tolerates generator power, whose frequency can wander more than the utility's. If you run your own hardware, the servers guide covers shutdown planning.
The costs people miss
- Installation: the pad, gas line or tank, wiring, transfer switch and permits often rival the generator price.
- A gas meter or service upgrade if the existing gas supply is too small.
- A maintenance contract: oil, filters, batteries, coolant and load testing.
- Fuel for weekly test runs, and diesel testing or polishing.
- The starting battery, the most common reason a standby generator fails to start.
Before you buy
- List what must keep running, and for how long.
- Get a load calculation that accounts for the largest motor starting.
- Choose whole-building or critical circuits, and an automatic transfer switch.
- Pick a fuel you can count on for your longest likely outage.
- Confirm permits, placement, noise and air rules with the installer.
- Put computers and network gear on a UPS to cover the transfer.
- Budget for a maintenance contract and a weekly exercise run.
- Never connect a generator without a transfer switch or approved interlock.