Lock Power Budget

The formula

Exactly what the calculator computes, in the order it computes it. Highlighted terms are your inputs.

// Sum each device's current by how it draws
continuous = Σ qty × mA   where it draws continuously
energized  = Σ qty × mA   where it draws only when unlocked

// Worst case: everything continuous, plus the share of locks unlocked together
peak       = continuous + energized × simultaneous%
supply     = peak × (1 + headroom)

// Standby, as in the Battery Standby calculator
battery Ah = continuous × standbyHours × 1.25

What it assumes

Each of these is a way the number can be wrong for your job. Read them before you buy hardware.

1. Enter datasheet currents, not the defaults. The starting rows are plausible examples, not values for any specific product. Lock current varies widely by model, holding force and voltage.
2. Fail-safe locks draw while locked; fail-secure locks draw while unlocked. A magnetic lock holds the door with current and releases when power is removed. A fail-secure strike is the reverse. Set each row accordingly.
3. Inrush is not modelled. Solenoid strikes and some motorized locks pull a short surge well above their holding current when they operate. The headroom covers ordinary inrush; check the supply manufacturer's guidance for motorized hardware.
4. Battery charging uses supply capacity. Many supplies divide their rating between the load and the battery charger. Check whether the datasheet rating is before or after charging current.
5. Voltage drop is separate. A supply with enough current can still deliver too few volts to the far door. Check long runs with the Voltage Drop calculator, especially at 12 VDC.
6. Life safety governs. Fail-safe locking on egress doors, fire alarm release and delayed egress are governed by NFPA 101, the IBC and the authority having jurisdiction. This calculator sizes power; it does not decide which hardware is permitted on a door.