Calculating a sample standby-energy requirement

What you should be able to do

Calculate a baseline charge and energy demand for a stated duty cycle, then identify the additional work needed to select a battery.

Define the exercise

A fictional system operates for 8 hours in standby at 0.40 A total, followed by 15 minutes in an active mode at 1.20 A total. The phases do not overlap. Each current includes all demand at the evaluated boundary for that phase, including system electronics.

Assume a constant 24 V at that boundary for the energy arithmetic. No converter losses or additional battery-side overhead are included. These are teaching assumptions, not a real battery discharge model. The example's 8-hour and 15-minute durations are not national code requirements.

Step 1 - Convert the time

15 minutes /60 minutes per hour = 0.25 h. Total elapsed exercise time = 8 + 0.25 = 8.25 h.

Step 2 - Calculate charge demand

Standby: 0.40 A x 8 h = 3.20 Ah. Active: 1.20 A x 0.25 h = 0.30 Ah. Total: 3.20 + 0.30 = 3.50 Ah.

The active current is the full current during that phase. Do not add the standby current to it again. If a source document instead gives an incremental active current, resolve that distinction before calculating.

Step 3 - Calculate modeled energy

Ampere-hours measure charge; watt-hours measure energy. They are not interchangeable units. At the assumed constant 24 V: Standby energy = 24 x 0.40 x 8 = 76.8 Wh. Active energy = 24 x 1.20 x 0.25 = 7.2 Wh. Total modeled energy = 76.8 + 7.2 = 84.0 Wh. Cross-check: 24 V x 3.50 Ah = 84 Wh.

If voltage and current vary, evaluate the appropriate time segments or use an appropriate energy model. Multiplying a battery's nominal voltage by Ah provides an approximation, not a guarantee of usable energy down to the system's cutoff voltage.

Use the correct measurement boundary

If the currents are device-side currents behind a converter, account for conversion losses and other battery-side demands using the actual equipment method. Do not add currents from different voltage rails as if they were all measured at the same battery voltage. State what every value includes so internal electronics are neither omitted nor counted twice.

Why 3.50 Ah is not the selected battery

The Altronix ACM-series worksheet shows a manufacturer method that separately accounts for internal and load currents, operating durations and a model-specific sizing factor. Its factor and capacity limits belong to that equipment and worksheet. Do not transplant them into every application.

Power-Sonic's PG-12V100 data sheet illustrates that rated capacity depends on discharge duration and endpoint voltage. A battery's label does not promise the same usable capacity at every discharge rate. Review the selected battery's discharge data, required cutoff, temperature and applicable aging or reserve requirements.

The final selection also needs charger compatibility, permitted capacity, recharge requirements, enclosure suitability and system approval. A physically larger battery does not automatically solve all these constraints.

Obtain real duration requirements

Use the actual system type, adopted rules, approved design and manufacturer instructions to determine standby and active/alarm durations. Fire alarm, intrusion, access control and other systems do not share one universal standby rule. This lesson teaches the calculation method without prescribing a nationwide duration.

Practice

1. Convert 30 minutes to hours.
2. Keep the 8-hour standby phase but extend the active phase to 30 minutes. What are the new charge and modeled energy totals?
3. If standby current increases to 0.50 A with the original 15-minute active phase unchanged, what are the totals?

Answers

Calculation record

System and evaluated power boundary: Source of required durations: Phase, total current and duration: Included internal and accessory loads: Charge demand per phase: Voltage/energy assumptions: Losses and unresolved battery-side demand: Applicable sizing method and factors: Discharge/cutoff/temperature evidence: Charger and capacity constraints: Reviewer and disposition:

Sources