In terms of operating principle, emergency and standby generators are similar, since both provide a backup power supply when the electric utility service is interrupted. However, the loads served by each type of system are different.
Basically, all equipment not classified as an emergency load is a standby load, and standby power is optional unless the code requests it explicitly. Therefore, there is no list of loads for an optional standby power system - the building owner is free to select them.
To determine which loads must be connected to emergency or standby power, first it is important to check the type of residential occupancy. In New York City, residential properties are classified into three occupancy groups:
| Group | Description |
| R-1 | Temporary residential space used for periods of less than one month (e.g. hotels, timeshares), student dormitories not classified under Group R-2, and congregate dwellings managed by the government or a non-profit organization |
| R-2 | Buildings or building portions that contain either sleeping units or more than two complete dwelling units, used for permanent residence. Apartment buildings are classified under Group R-2. |
| R-3 | Residential buildings with one or two dwelling units intended for long-term use (periods of at least one month) |
Once the occupancy group has been determined, the emergency and mandatory standby power requirements can be checked in the in the NYC Building Code.
The emergency and standby power requirements established by the NYC Building Code change by occupancy type, and the applicable requirements for residential buildings are summarized in the following table. Keep in mind that this table is just an overview, and should not be used to design emergency and standby power systems; reviewing the NYC Building Code is recommended for detailed requirements.
| Group | Emergency Power | Mandatory Standby Power |
| R-1 |
1) Exit signs and means of egress lighting 2) Elevator car lights 3) Emergency voice and alarm systems, including Auxiliary Radio Communication, even if installed optionally 4) Fire protection systems: Automatic fire detection, fire alarms, electrical fire pumps |
1) Ventilation systems for smoke venting or control 2) Stair pressurization systems 3) At least three elevators per building, with manual transfer to other elevators as needed 4) Fire command center power and lighting |
| R-2 |
1) Exit signs and means of egress lighting 2) Elevator controls, cab lights and ventilation. Additional loads for high-rise R-2: 3)Emergency voice communication systems 4)Electrical fire pumps, unless powered ahead of the main breaker |
1) Smoke control systems 2) Horizontal sliding doors 3) Smokeproof enclosures 4) Pressurized elevator shafts Additional loads for high-rise R-2: 5)Fire command center power and lighting 6)At least one elevator serving all floors, or one elevator per bank if different building areas are served by different banks. |
| R-3 |
1) Exit signs and means of egress lighting 2) Elevator controls, cab lights and ventilation. |
1) Smoke control systems 2) Elevators 3) Horizontal sliding doors 4) Smokeproof enclosures 5) Pressurized elevator shafts |
Optional standby power applies for all loads not listed in the table above. However, if the decision to install it is taken, the generator must be able to power the following loads in addition to the optional loads selected:
The purpose of this requirement is to provide redundancy, since these loads would normally be powered by emergency power systems or mandatory standby power systems. Sizing the optional power system with enough capacity to power them provides an extra safety measure.
Although the NYC Building Code provides no guideline, given the optional nature of these loads, there are many appliances and systems for which it is highly convenient to have a backup power supply:
Optional standby power exceeds code requirements and provides added value in project. Keep in mind that it may not always be feasible; for example, in a building that uses electric heating, the capacity and cost of a standby generator can be significant if that load is included.
If an optional standby generator is installed, the NYC Building Code allows the fuel supply to be shared with emergency generators, as well as common complementary equipment necessary for generator operation.