Battery knowledge · Units and electrical basics
Energy
What a battery stores and delivers: the integral of power over time, in watt-hours or joules. It is capacity times voltage, which is why two cells with equal ampere-hours can hold different energy. [1]
- Unit
- Wh
- Relationship
- E = ∫ V·I dt
Example
Integrate a changing power trace
A load that draws 40 W for 15 min and then 20 W for 30 min uses 20 Wh in total. Add the area under each part of the power-versus-time trace.
Explore the graph
Voltage connects capacity in ampere-hours to nominal energy in watt-hours.
5 Ah × 12 V = 60 Wh nominal energy.
E_nominal = Q_nominal V_nominal; actual energy = ∫ VI dt
Illustrative model. Specific energy divides energy by mass; energy density divides it by volume.
Use in Lemonergy
Lemonergy Core equation
Estimate nominal energy
Lemonergy Core evaluates this relation.
Equation or tool to use
- nominal_energy_wh
Core equation
Inputs & settings
- Capacity (Ah)
- Nominal voltage (V)
This overview identifies the components to use. Follow the example in Lemonergy for their connections and settings.
- Open the lesson and inspect the equation.
- Change one input while holding the others fixed.
Example. A 5 Ah cell rated 3.6 V has 18 Wh nominal energy. Ten identical cells contain 180 Wh nominally.
Observe. Compare the result with the equation and units.
This is a rating estimate, not integrated delivered terminal energy.
Open the worked lesson →References & further reading
Technical references
- The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · 2026
Technical reference · Definitions of SI units and coherent derived units; 2026 revision.