2026-09-01

Telecom Battery Runtime: How to Calculate Backup Hours

Battery ESS48V Telecom BatteryEngineering

Battery runtime is the number that decides whether a site survives an outage, and it is also the number most often calculated wrongly. The error is almost always the same: dividing nameplate capacity by load and calling the result backup hours.

The basic formula

Runtime (hours) = usable energy (Wh) / site load (W)

Usable energy is not the number on the datasheet. It is:

Usable energy = nameplate energy x DoD x ageing factor x temperature factor

Each of those three factors is less than 1, and multiplied together they typically take 30-40% off the nameplate figure. Ignoring them is how a battery bank gets specified for eight hours and delivers five.

Worked example

Take a 48V 100Ah LiFePO4 module — nameplate energy 51.2V x 100Ah = 5,120 Wh. Assume the site is allowed to use 80% depth of discharge, the pack is partway through its life at 90% of original capacity, and it runs at a temperature where it delivers 95% of rated capacity.

Usable energy = 5,120 x 0.80 x 0.90 x 0.95 = 3,502 Wh.

At a site load of 500 W, that is 3,502 / 500 = 7.0 hours — against the 10.2 hours you would get by naively dividing 5,120 by 500.

Runtime at different loads

Backup hours for one 48V 100Ah LiFePO4 module at various site loads
Site load	Nameplate only	Realistic usable (80% DoD, 90% SoH, 95% temp)
100 W	51.2 h	35.0 h
200 W	25.6 h	17.5 h
300 W	17.1 h	11.7 h
500 W	10.2 h	7.0 h
750 W	6.8 h	4.7 h
1000 W	5.1 h	3.5 h

Figures are for a single 5.12 kWh module. Two modules in parallel double every row.

Where the losses actually come from

Depth of discharge — the share of capacity you allow the battery to use. LiFePO4 tolerates deep discharge far better than VRLA, but leaving headroom extends cycle life.
Ageing — capacity fades with cycling and calendar time. End of life for a telecom pack is usually defined at 70-80% of original capacity, so a pack near end of life holds noticeably less than a new one.
Temperature — cold reduces the capacity a cell can deliver. Hot does the opposite in the short term but shortens life, so the temperature factor and the ageing factor are linked over time.

Common mistakes

Sizing on nameplate energy — the single most common error, and it produces optimistic numbers that fail during the first real outage.
Ignoring end-of-life capacity — a bank sized to the requirement when new may fall short three years later. Size to the requirement at end of life, not at commissioning.
Forgetting the load is not constant — use the actual load profile, including any equipment that switches on during an outage, rather than an average.
Not accounting for conversion losses — where an inverter sits between battery and load, its efficiency belongs in the calculation.

Let the BMS do the measuring

Every one of these factors is an estimate until the battery reports itself. A smart BMS measures real state of charge and state of health, so runtime becomes a measured figure rather than a calculation on a spreadsheet — and it can be read remotely over SNMP instead of during a site visit.

For specification details on the module used in the table above, see the 48V LiFePO4 telecom battery series. More sizing background is in the 48V 100Ah specification and sizing guide.

FAQ

Why does my battery deliver less than the datasheet suggests?

Because the datasheet figure is nameplate energy at new condition. Real usable energy is reduced by depth-of-discharge limits, capacity fade from ageing, and temperature. Applying all three typically removes 30-40% of the headline number.

Should I size for today's load or future load?

Size for the load you expect over the battery's service life, including planned equipment additions. Adding capacity later means either paralleling more modules — if the bank and BMS allow it — or replacing the bank early.

How much margin should I leave?

Enough that the bank still meets the required runtime at end of life rather than at commissioning. Working from end-of-life capacity is the difference between a bank that meets spec for ten years and one that meets it for five.