Quick answer: a DC energy logger multiplies a current reading by a voltage reading and integrates that product over time – and each of those steps adds uncertainty. Keep the setup on isolated, current-limited low-voltage sources and read the totals as estimates, not billing-grade measurements.
The four quantities, in order
Energy logging is a chain: sample voltage, sample current, multiply for power, then accumulate power over time. A mistake anywhere upstream propagates – a wrong current scale makes every energy total wrong, and a missing sample makes the integral drift low.
What the underlying components actually guarantee
On the classic UNO R3, analog reads use a 10-bit ADC and A0 through A5 are the usable analog inputs; the reference also notes roughly 100 microseconds per read on ATmega-based boards, which is a per-conversion figure, not a bandwidth guarantee for a whole application.
The ATmega328P datasheet confirms the 10-bit ADC with a single-ended result referenced to the selected VREF, recommends source impedance of about 10 kilohms or less, and explains sample-and-hold charging limits. It also stresses that absolute maximum ratings are stress limits, not operating conditions – a reminder not to run inputs at the rated ceiling.
The ACS712 is a Hall-effect linear current sensor with a ratiometric output. Its datasheet describes the sensor IC and its isolation characteristics; the breakout board you buy and the meter you assemble around it are separate hardware, so verify those against their own documentation before relying on them.
Diagnostic table
| Symptom | Likely cause | Check |
|---|---|---|
| Reading drifts at zero current | Sensor offset not corrected | Measure and subtract the no-load offset |
| Current scale wrong with supply | Ratiometric output tracks its supply | Confirm sensor supply and what the ADC uses as reference |
| Energy total reads low | Lost samples or irregular timing | Check whether integration assumes a fixed interval |
| Noisy or jumpy values | Source impedance too high | Keep analog source impedance near the datasheet guidance |
An ordered evaluation process
- Confirm the measurement is on an isolated, current-limited low-voltage source.
- Record the no-load current reading and treat it as an offset to subtract.
- Verify the ADC reference and the voltage divider ratio against your input range.
- Sample voltage and current close together so power reflects the same instant.
- Choose a fixed integration interval and count missed samples explicitly rather than assuming none.
- State the uncertainty you can actually defend; do not present the result as metering-grade.
If your interest is voltage measurement rather than energy, the Arduino LCD Voltmeter with 4 Channels guide covers the reading side.
Reader questions
Can this replace a utility or revenue meter?
No. It is a low-voltage DC logger with an uncalibrated chain. Use it for trends and relative comparisons, not billing.
Why does my current reading change when the supply voltage moves?
Ratiometric Hall sensors scale their output with supply, so changes in supply or reference shift the reading. Verify the reference arrangement.
How often should I sample?
Often enough that missed samples are a negligible part of the integral. The 100 microsecond figure is per conversion, not a full-application guarantee.
Can I use this to measure a mains circuit?
No. This reference stays on isolated, current-limited low-voltage DC sources; mains metering and revenue-grade claims are out of scope.
Documentation
- Arduino analogRead() reference (official reference-en source)
- Allegro ACS712 Hall-effect current sensor datasheet
- Microchip ATmega328P datasheet, 7810D automotive edition
Documentation note: the ADC, sensor and pin facts above come from the Arduino reference, the ACS712 datasheet and the ATmega328P datasheet. No calibration measurements or system-level accuracy testing are included, so treat readings as estimates for your own setup.
