When troubleshooting AC circuits, sizing a solar inverter, or evaluating a PC power supply, knowing what power is measured in is the difference between a system that runs reliably and one that constantly trips breakers. The direct answer: Real power is measured in Watts (W), apparent power in Volt-Amperes (VA), and reactive power in Volt-Amperes Reactive (VAR).
For purely resistive DC circuits, power is simply Watts (Volts × Amps). But in AC circuits, inductive and capacitive loads cause phase shifts between voltage and current waveforms. This creates a gap between the power doing actual work (Watts) and the total power the wiring and breakers must carry (VA). Assuming standard 60Hz North American mains (120V/240V nominal) and copper conductors, here is how to measure, interpret, and verify these values on the bench or jobsite.
The Physics of W, VA, and VAR
To understand why we need three different units, think of an AC circuit like a water pump driving a hydraulic ram. Real power (Watts) is the water actually hitting the ram and doing mechanical work. Reactive power (VAR) is the water sloshing back and forth in the accumulator bladder; it maintains system pressure and keeps the magnetic fields in motors energized, but it does zero net work. Apparent power (VA) is the total flow capacity your pipes must be sized to handle to accommodate both the working water and the sloshing water.
The ratio of Real Power to Apparent Power is the Power Factor (PF), expressed as a decimal between 0 and 1 (or 0% to 100%). A purely resistive load like a space heater has a PF of 1.0 (Watts = VA). A heavily inductive load like an unloaded induction motor might have a PF of 0.6, meaning the wiring must be sized for significantly more VA than the actual mechanical Watts being produced.
Meter Setup & Probe Placement for Power Measurements
A standard digital multimeter (like a Fluke 87V) cannot measure Watts or VA directly; it only measures V and A independently. To measure true AC power, you need a True RMS Power Clamp Meter (e.g., Fluke 345, Hioki PW3360) or a bench oscilloscope with math functions and differential voltage probes. Below is the setup for a dedicated True RMS power clamp meter.
Meter Setup Block
- Dial Position / Mode: Set to 'Power' (W/kW) or 'VA/kVA'. Ensure the meter is set to True RMS, not average-responding, especially if measuring non-linear loads like LED drivers or VFDs.
- Lead Jacks: Insert the black lead into COM and the red lead into the V/Ω jack for voltage measurement. Current is measured via the integrated clamp jaw (no inline shunt required for AC).
- Range: Set to Auto-ranging. If manual, set the voltage range to match your system (e.g., 600V) and the clamp range to the expected load (e.g., 60A for a 40A branch circuit).
Probe Placement & Test Sequence
- De-energize and Verify: If modifying wiring to expose a single conductor, turn off the breaker and verify dead with a non-contact voltage tester and a known-working DMM.
- Voltage Probes: Place the red probe on the Line (hot) terminal and the black probe on the Neutral terminal (for 120V) or Line 2 (for 240V split-phase).
- Current Clamp: Clamp the meter's jaw around only the Line (hot) conductor.
- Read and Record: Press the 'Hold' button. Note the W, VA, and PF readings simultaneously.
Expected Readings: Good vs. Bad Power Values
Knowing what a good reading looks like numerically prevents you from chasing ghosts. The table below outlines expected values for common 120V AC loads. If your readings deviate significantly from the 'Good' columns, you are likely dealing with failing components, harmonic distortion, or measurement errors.
| Load Type | Expected Real Power (W) | Expected Apparent Power (VA) | Power Factor (PF) | Bad Reading / Fault Indicator |
|---|---|---|---|---|
| Incandescent Bulb (100W) | 95 - 105 W | 95 - 105 VA | 0.99 - 1.00 | PF < 0.95 (Meter error or severe harmonic noise on line) |
| PC Power Supply (Under 50% Load) | 150 W | 170 - 215 VA | 0.70 - 0.90 | PF < 0.60 (Failing Active PFC circuit in the PSU) |
| Induction Motor (Loaded) | 750 W (1 HP) | 880 - 940 VA | 0.80 - 0.85 | Watts > VA (Impossible; indicates clamped wrong wire or meter calibration failure) |
| LED Driver (Dimmable) | 20 W | 25 - 35 VA | 0.60 - 0.80 | High VA, low W, high THD (Cheap non-PFC driver causing neutral overheating) |
Mistakes That Give Misleading Readings
- Using an Average-Responding Meter on Non-Linear Loads: If you measure a modern switching power supply with a cheap average-responding clamp meter, the current reading will be 10% to 30% lower than reality because the meter assumes a perfect sine wave. Always use True RMS.
- Ignoring Voltage Sag: Power is V × I × PF. If your branch circuit has undersized wire and the voltage sags from 120V to 112V under load, the current will spike to maintain the Wattage, artificially inflating your VA and I²R line losses.
- Measuring on Modified Sine Wave Inverters: Standard power meters will throw error codes or wildly inaccurate PF readings when fed a stepped-square wave from a budget off-grid inverter. You need a power analyzer with a high sampling rate (like a Yokogawa WT series) to capture the high-frequency harmonics.
Safety Categories (CAT Ratings) for Mains Power Testing
When measuring power at mains voltages, the primary hazard is not just the voltage, but the available fault current (let-through current) that can cause an arc flash if the meter fails internally. According to Fluke and IEC 61010 standards, your meter and test leads must carry the correct CAT rating for the specific test point.
- CAT III (1000V) or CAT IV (600V): Required for measuring power at branch circuit receptacles, hardwired appliances, and lighting ballasts. CAT III covers the branch circuit; CAT IV covers the service entrance and utility drop.
- CAT II: Only acceptable for testing plug-in appliances after they are unplugged and tested on an isolated bench supply, or on the load side of a cord longer than 10 feet from a CAT III receptacle.
Frequently Asked Questions About Power Measurement
What is power measured in on a standard digital multimeter?
It isn't. A standard digital multimeter (DMM) only measures Volts and Amps. To find power in a DC circuit, you measure V and A separately and multiply them (P = V × I). For AC circuits, multiplying the DMM's V and A readings only gives you Apparent Power (VA). To find Real Power (Watts) in AC, you must use a dedicated power meter that samples voltage and current simultaneously to calculate the phase angle difference.
Why is my power measured in VA higher than Watts?
This happens whenever the Power Factor (PF) is less than 1.0. Inductive loads (motors, transformers, solenoids) and capacitive loads (some LED drivers, long underground cables) draw reactive current that sloshes back and forth to maintain electromagnetic fields. This current does no real work (Watts), but it still heats up the wires and takes up capacity in your breakers, which is why the utility and your panel care about the higher VA number.
What is the difference between kW and kWh?
Kilowatts (kW) measure the rate of power flow at a specific instant—like the speedometer in a car. Kilowatt-hours (kWh) measure the total energy consumed over time—like the odometer. If you run a 1.5 kW space heater for exactly two hours, you have consumed 3.0 kWh of energy. Your utility bills you for kWh, not kW (though commercial facilities are also penalized for poor Power Factor and peak kW demand).
Can I measure 3-phase power with a single-phase clamp meter?
Yes, but it requires manual math and multiple passes. You must measure the Real Power (W) of Phase A, Phase B, and Phase C individually (clamping one hot wire at a time while referencing voltage to neutral or the other phase, depending on Wye or Delta configuration), then sum the three readings. For balanced 3-phase systems, you can measure one phase and multiply by 3, but for unbalanced loads (like commercial HVAC with single-phase control circuits tapped off one leg), you must use a 3-phase power analyzer with three simultaneous current clamps to get an accurate total.
