Battery Standby

The formula

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

// Ampere-hours consumed in each state
standbyAh = standbyCurrent(A) × standbyHours
alarmAh   = alarmCurrent(A)   × (alarmMinutes ÷ 60)

calculatedLoad = standbyAh + alarmAh

// Derating for age, temperature, and end-of-life capacity
requiredAh = calculatedLoad × deratingFactor

// Round UP to a standard capacity — never down
battery = smallest standard size ≥ requiredAh
          4.5 · 7 · 9 · 12 · 18 · 26 · 35 · 40 · 55 · 75 · 100 Ah

// Recharge check, if a charger output is given
rechargeHours ≈ installedAh × 1.2 ÷ chargerCurrent

What it assumes

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

1. Constant current draw. Real loads vary — card reads, relay operations, and communications all cause momentary peaks. Sum the worst-case quiescent draw of every device, and include the panel's own consumption, which is routinely forgotten.
2. Sealed lead-acid chemistry. The standard sizes and the derating convention assume SLA/VRLA batteries, which is what almost all ESS panels use. Lithium chemistries behave differently and have their own sizing rules.
3. The derating factor is doing real work. A battery delivers its rated capacity when new, at 25 °C, at a slow discharge rate. It will not do so at four years old in an unheated closet in February. 1.20–1.25 is standard practice; use more in cold or hot environments, or where replacement intervals are long.
4. Peukert's effect is not modelled. Lead-acid capacity falls as discharge rate rises. A 7 Ah battery rated at the 20-hour rate delivers noticeably less than 7 Ah when discharged over 4 hours. For short, heavy standby periods, treat the result as optimistic and derate harder.
5. Check what your specification actually requires. Standby periods vary: many access control specifications call for 4 hours, NFPA 72 commonly requires 24 hours plus 5 minutes of alarm for fire alarm, and UL 2050 environments have their own requirements. This calculator does not know which applies to your job — enter the number your specification states.
6. Recharge time is a rough estimate. The 1.2 multiplier accounts for charging inefficiency. Real recharge depends on the charger's algorithm and temperature compensation. Many standards impose a maximum recharge time (NFPA 72 requires 48 hours), so check the result against your specification.
7. Avoid more than two parallel strings. Parallel batteries age unevenly, and a single weak cell will drag the bank down and mask the fault until it matters. Prefer a larger single battery, a larger cabinet, or distributing the load across additional power supplies.
8. Voltage drop is a separate problem. As the battery discharges its terminal voltage falls, and the devices at the end of the longest run see the least of it. Re-run the voltage drop calculator at end-of-discharge voltage, not at float.