How to size a battery pack
Most undersized packs are undersized on current, not on energy. Working through the five steps below in order avoids the two mistakes that account for most specification failures.
Step 1 — Build the load list
List every load with three figures: power in watts, hours per day, and whether it is continuous or intermittent. Intermittent loads matter more than they look, because they set the peak.
Add a diversity factor if loads do not all run simultaneously. In a residential system that is often 0.6–0.8; in an industrial system with a defined sequence it may be 1.0.
Step 2 — Convert to energy per day
Multiply each load's power by its hours, sum them, and you have watt-hours per day. This is the figure the battery has to supply between charges.
daily Wh = Σ (power_W × hours_per_day)
Step 3 — Choose the depth of discharge, then size the pack
required capacity_Wh = daily Wh ÷ depth of discharge
Use 80 % as a default and 90 % as a maximum for occasional events. This step is where the honest sizing happens: a pack sized to 100 % depth of discharge will not reach its rated cycle count, and the warranty position will reflect that.
Step 4 — Check the current, which is where packs actually fail
Energy sizing tells you how long the pack lasts. Current sizing tells you whether it works at all.
| Parameter | Where it comes from |
|---|---|
| Continuous current | Sum of simultaneous continuous loads ÷ voltage |
| Peak current | Largest motor's starting current, in amps |
| Peak duration | How long the start-up lasts, in seconds |
Then specify a BMS rated at roughly 1.3–1.5× the continuous current, with a peak rating that covers the starting current for its full duration.
This is the step most often skipped. A pack that meets its energy requirement and trips on a motor start has failed the application, whatever the capacity says.
Step 5 — Apply temperature derating and check the charge current
Capacity falls at low temperature and cycle life falls at high temperature. If the installation sees below 10 °C or above 35 °C for extended periods, derate the usable capacity by 10–20 % and re-check the sizing.
Finally, check charge current. If the daily energy has to be replaced in a limited window — a solar day, a shift break, a tariff window — the pack must accept enough current to do it. A pack that stores the energy but cannot be recharged in the available time is the wrong pack.
A worked example
An off-grid cabin: 2.4 kWh per day. A 48 V system, with a 2 kW inverter and a well pump drawing 1,800 W for four seconds on start-up.
- Energy: 2,400 Wh ÷ 0.8 = 3,000 Wh required
- Continuous current: 2,000 W ÷ 51.2 V ≈ 39 A, so a 100 A BMS gives ample headroom
- Peak current: 1,800 W ÷ 51.2 V ≈ 35 A continuous equivalent, but the starting current of an induction pump is typically four to six times running, so 140–210 A for four seconds
- Result: one 5.12 kWh pack is energy-adequate; the peak current is the parameter that decides the BMS specification
That last line is the whole point of doing the calculation in this order.