Electrical measurements answer different questions. Charge tells us how much electric charge moved. Power tells us the rate of energy transfer. Energy tells us the accumulated amount delivered over a period.
The three equations
Q = integral I dt, measured in coulombs. P = V × I, measured in watts. E = integral V I dt, measured in joules; divide by 3,600 for watt-hours. Voltage and current must refer to the same operating point and time.
A worked example, not a device result
Suppose a load receives 10 microamps at 0.4 volts for one hour. Its power is 4 microwatts, its charge is 0.036 coulombs, and its delivered energy is 0.000004 watt-hours. The example assumes constant readings and excludes any auxiliary consumption.
Open-circuit voltage is measured without a useful load current. Short-circuit current is measured near zero terminal voltage. Multiplying these two separately measured extremes does not give actual delivered power.
Why test duration matters
A peak during a voltage or current sweep can help compare electrode behaviour. A practical application needs sustained energy, startup behaviour and recovery after darkness. Publish the time series, integration window and any periods of failure alongside the peak.
Evidence boundary: these are electrical definitions and an illustrative calculation, not measurements of MaxwellX hardware. NIST's traceability policy explains why a measurement claim also needs documented calibration and uncertainty. Policy text inspected; no NIST certification of MaxwellX is implied.