The rate at which a battery cell or pack delivers or absorbs electrical energy.
Abbreviated as W (Watt)
Where V is the voltage across the terminals (Volts) and I is the current flowing (Amps).
1 Watt = 1 Joule per second
Power is often confused with energy (Watt hour) — power is the rate, energy is the total. A 5kWh pack can deliver 5kW for 1 hour, or 1kW for 5 hours: same energy, very different power.
Power Under Load
A cell is not a perfect voltage source — it has an internal resistance (Rint). As current is drawn, the terminal voltage sags below the Open Circuit Voltage (VOCV):
So the power actually delivered to the load is:
This means power delivered to the load is always less than VOCV x I
The difference (I² x Rint) is lost as heat inside the cell. See Battery Power Demand Solution for how to solve for current given a target power demand.
C-Rate and Power Limits
Cell datasheets rarely quote a power limit directly — instead they give a max continuous and peak discharge/charge current as a C-rate (a multiple of the cell’s capacity in Ah). A 50Ah cell rated at 3C continuous can sustain 150A continuous. Multiply by the cell voltage at that point to get the continuous power limit.
Peak (pulse) current ratings are usually higher than continuous, for short durations (seconds), limited by thermal rise and voltage sag rather than the cell chemistry itself.
Specific Power and Power Density
Two of the Key Cell Metrics on any datasheet:
- Specific Power (W/kg) — power per unit mass
- Power Density (W/litre) — power per unit volume
These sit alongside specific energy (Wh/kg) and energy density (Wh/litre) on a Ragone Plot, which shows the trade-off: cells optimised for high power (thin electrodes, more current collector, lower internal resistance) generally sacrifice energy density, and vice versa. This is why a power-tool cell and an EV long-range cell of the same format can have very different internal builds.
Pack-Level Power
Building a pack from cells in series and parallel scales power capability accordingly — series adds voltage, parallel adds current capacity — but the achievable pack power is bounded by the weakest cell, the busbar/connector current ratings, and the pack’s thermal management, not just cell-level numbers multiplied up.
Worked Example
Using the Tesla Model Y 96S46P pack referenced elsewhere on this site (355.2V nominal, 230Ah):
If the pack can sustain 3C continuous discharge:
I = 3 x 230Ah = 690A
P = V x I = 355.2V x 690A ≈ 245kW
This lines up closely with the Model Y’s motor power rating — a useful sanity check when sizing busbars, contactors and fuses against a target power demand (see Continuous Current Rating).
See also: Electrical Formulas, Watt Hour, Battery Power Demand Solution, Open Circuit Voltage, Key Cell Metrics, Continuous Current Rating, Power vs Energy Density, Calculated Peak Power, Example Pack Sizing using Power Demand