These energy guidelines (for a generic battery) can help select the battery capacity required, based on the truck and the application. The examples below refer to a lead-acid battery:

Total energy (kWh) = battery voltage (v) x battery capacity (Ah)

E.g. 48v x 400Ah = 19200Wh or 19.2kWh

Usable energy (kWh) = total energy (kWh) x maximum DoD%

E.g. 19.2kWh x 80% = 15.4kWh

Power (kW) = energy used per unit time (kWh/h)

E.g. if you use all the energy in 1 hour, you would be consuming 15.4kWh/h

E.g. if you use all the energy in 5 hours, you would be consuming 3kWh/h

Energy consumption expressed as kWh/h

E.g. For a 3-wheel counterbalance truck ERP16VT, energy consumption on VDI 2198 test course is 4.2kWh/h.
A 48v 400Ah battery will last 3.7 hours

In reality trucks rarely use as much power as the VDI test course would suggest

A more accurate energy consumption can be measured using a battery logging device

Idle time (%) is the time when the truck is in use but energy from the battery is not used

E.g. includes paperwork, manual loading/unloading, picking, getting on and off the truck