Power Meters and Measuring Power
A device's label tells you what it should draw. A power meter tells you what it really draws. Power measurement answers everyday questions — how much does this appliance cost to run, is this charger delivering its rated output, how long will this battery last — and it reveals faults, because a motor with a failing bearing or an amplifier with a shorted transistor draws more power than it should.
Measuring DC Power
For DC, power is simply P = V × I. Measure the voltage across the load and the current through it at the same time, then multiply. Measure the voltage at the load itself, not at the supply, so that losses in the leads are not counted as power delivered to the load.
P = 12.6 × 2.4 = 30.2 W
If the load switches on and off rapidly (PWM-dimmed LEDs, a motor controller), a multimeter shows an average that may not be meaningful. A power meter that samples voltage and current together, or an oscilloscope with a current probe, is needed for an accurate figure.
AC Power: Real, Apparent and Power Factor
With AC, multiplying the RMS voltage by the RMS current gives the apparent power in volt-amperes (VA). For a purely resistive load such as a heater or an incandescent lamp, voltage and current are in step and the apparent power is also the real power in watts.
Motors, transformers and many electronic power supplies draw current that is out of step with the voltage, or in short peaks near the top of each cycle. Some of the current does no useful work. The ratio of real to apparent power is the power factor:
Apparent power = 230 × 0.40 = 92 VA. Power factor = 64 ÷ 92 ≈ 0.70.
Multiplying meter readings would have overstated the real power by 44%.
This is why a UPS or generator is rated in both VA and watts, and why the wiring must be sized for the full current even when the real power is lower.
True RMS Meters
A basic “average-responding” meter measures the average of the rectified waveform and scales it as if the signal were a pure sine wave. On a clean sine wave this gives the correct RMS value. On the spiky current drawn by a switching power supply, a dimmer or a variable-speed drive, it can read 30–50% low.
A true RMS meter calculates the real RMS value of whatever waveform is present. When measuring AC current or voltage on anything other than a resistive load, use a true RMS meter or a dedicated power meter.
Plug-In and USB Power Meters
Inexpensive meters make power measurement easy:
- Plug-in energy monitors sit between the wall socket and an appliance. They measure voltage and current together and display real power (W), apparent power, power factor and energy used (kWh). They are ideal for finding out what a fridge, computer or tool costs to run, and for measuring standby power.
- USB power meters plug in line with a USB cable and show voltage, current, power and the energy delivered (mAh and Wh). They reveal whether a charger and cable deliver the expected current and which USB-C power profile has been negotiated.
- DC panel and inline meters with a shunt or Hall-effect sensor show voltage, current and power in 12 V systems such as solar, camper and battery installations.
The dBm Scale
In datasheets, test equipment and laboratory work, power is often expressed in dBm — decibels relative to one milliwatt. The dBm scale makes it easy to compare signal levels across the enormous range from the noise floor of a sensitive instrument (−120 dBm) to a 1 kW amplifier (+60 dBm).
The conversion formulas are:
Watts to dBm: dBm = 10 × log10(PmW) = 10 × log10(PW × 1000)
dBm to watts: PmW = 10(dBm/10) → PW = PmW / 1000
PmW = 100 W × 1000 = 100,000 mW
dBm = 10 × log10(100,000) = 10 × 5 = +50 dBm
Common dBm Reference Points
| Power | dBm | Context |
|---|---|---|
| 1 kW | +60 dBm | Large audio PA amplifier or microwave oven magnetron |
| 100 W | +50 dBm | Hi-fi or car audio amplifier output |
| 10 W | +40 dBm | Desktop speaker amplifier |
| 1 W | +30 dBm | Small speaker or Bluetooth speaker amplifier |
| 100 mW | +20 dBm | Wi-Fi router output |
| 10 mW | +10 dBm | Bluetooth Class 1 module |
| 1 mW | 0 dBm | Reference level; typical signal generator output |
| 1 µW | −30 dBm | Small signal at a sensor input |
| 1 nW | −60 dBm | Received Wi-Fi signal near the router |
| 1 pW | −90 dBm | Weakest usable Wi-Fi signal |
Watts / dBm Calculators
Watts to dBm
Convert a power in watts to dBm (decibels relative to 1 milliwatt).
dBm to Watts
Convert a power in dBm to milliwatts and watts.
Frequently Asked Questions
Why does my plug-in meter show fewer watts than volts times amps?
Because the load's power factor is less than 1. Volts times amps gives the apparent power in VA; the meter's watts figure is the real power actually used. Motors, transformers and many electronic power supplies have a power factor between about 0.5 and 0.95.
Do I need a true RMS meter?
For DC, resistive AC loads and clean sine waves, an average-responding meter is fine. For AC current drawn by electronics, dimmers, variable-speed drives and anything with a distorted waveform, a true RMS meter is needed to read correctly.
How accurate are cheap plug-in energy monitors?
Typically within a few percent for loads above about 10 W, which is plenty for comparing appliances and estimating running costs. Accuracy falls off at very low power, so standby readings below a watt or two are only approximate.
Why use dBm instead of watts?
Because signal and power levels in electronics span an enormous range, from picowatts to kilowatts. On the dBm scale that range becomes −90 to +60, and gains and losses simply add and subtract. Instrument and wireless-module datasheets quote levels in dBm for exactly this reason.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.