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Can a digital multimeter measure power, and what to use instead

On direct current you can multiply two separate readings and get a usable figure. On alternating current you cannot, because the phase relationship between voltage and current is invisible to a multimeter and that relationship is exactly what separates volt amperes from watts.

By the Akermin Editorial Team · 7 min read

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No. A handheld digital multimeter has no watts function, and the reason is structural rather than a cost saving. Power is the product of voltage and current at the same instant, and a multimeter has one measuring path that reads one quantity at a time. On direct current you can work around this by taking two readings and multiplying them, which is legitimate as long as the load is steady. On alternating current the workaround fails, because the product of two root mean square readings is apparent power in volt amperes, and converting that to real watts requires the phase relationship between the two, which a multimeter has no way of seeing.

One pair of terminals, one quantity at a time

Look at the front of any handheld meter. There is a rotary switch that selects a function and there are three or four jacks, of which you use two. Volts goes across the load, amps goes through it in series, and the internal signal conditioning for the two is completely different: a high impedance divider for voltage, a low value shunt for current. You physically rewire the circuit to move from one to the other.

That architecture is what blocks a watts function, not the processor behind it. Even a well specified bench instrument like the Fluke 8808A bench multimeter, whose bullets state five and a half digit resolution, a basic direct voltage accuracy of 0.015 percent, a dedicated leakage current measurement and four wire resistance, remains a one quantity instrument. More resolution does not create a second simultaneous input. A watts function is not stated in its listing bullets, and that is consistent with what a bench multimeter is.

If you are still working out which quantity to reach for in the first place, the walkthroughs for measuring direct voltage and measuring alternating voltage cover the terminal choices and the safety sequence for each.

The DC case where volts times amps is the whole answer

Direct current has no phase angle, so power really is the simple product. Measure the voltage across a load, move the leads, measure the current through it, multiply. A battery pack sitting at 12.6 volts delivering 1.85 amps is drawing 23.3 watts, and that number is as good as your two readings.

Three caveats keep it honest. The readings are taken at different moments, so a load that varies gives you a product of two unrelated instants. Putting the meter in series adds burden voltage, because the current shunt drops a small voltage of its own and slightly changes the operating point you are trying to characterise. And the series connection itself is the single most common way people destroy a meter and a fuse, by leaving it in the current jacks and then measuring voltage. A meter with a sealed, rated body helps with the consequences rather than the mistake, and the UNI-T UT196 solar multimeter states a CAT III 1000 V and CAT IV 600 V rating with an IP65 enclosure, which is the kind of specification that matters when the two readings are being taken on a live array.

Power factor and the number a multimeter cannot reach

On alternating current, instantaneous power is the instantaneous voltage multiplied by the instantaneous current, and real power is the average of that product over a cycle. For a sinusoidal supply this comes out as the root mean square voltage times the root mean square current times the cosine of the phase angle between them. Two root mean square readings give you the first two terms. The cosine term is the one you cannot get.

The size of the gap is not academic. Here is the same pair of readings interpreted at different power factors.

Readings Apparent power Power factor Real power Error if you call volt amperes watts
230 V and 2.0 A 460 VA 1.00 460 W None
230 V and 2.0 A 460 VA 0.90 414 W 11 percent high
230 V and 2.0 A 460 VA 0.70 322 W 43 percent high
230 V and 2.0 A 460 VA 0.45 207 W 122 percent high

A resistive heater sits at the top of that table and the multiplication is fine. An induction motor at light load, an older fluorescent fitting or an unassisted switch mode supply sit lower down, and the figure you calculated is simply not the figure on the bill. If the aim is to work out what an appliance actually consumes, the method in measuring appliance power consumption starts from an instrument that samples both quantities together instead.

What true RMS fixes, and what it does not

True root mean square conversion computes the heating value of the actual waveform rather than assuming a sine wave and scaling a rectified average. On a clean sine both approaches agree. On the chopped current drawn by a dimmer, a variable speed drive or a computer power supply, an averaging meter can read tens of percent low while a true RMS meter reads correctly. That is a real and important improvement, and it is why the Fluke 177 true RMS multimeter bullets lead with it alongside a six thousand count display, four updates per second and minimum, maximum and average memory.

It does not move you any closer to watts. True RMS is a magnitude correction. The phase information was discarded by the rectifier before any of the arithmetic happened, so no amount of accuracy in the magnitude recovers it. The same applies to the low pass filter and low impedance modes the UT196 lists: the filter strips the switching carrier off a drive output so you can read the fundamental, and the low impedance mode bleeds off induced ghost voltages. Both make the magnitude trustworthy. Neither produces a timing relationship. The guide to choosing a multimeter sets out which of these modes are worth having for different kinds of work.

Instruments that do report watts

Four categories handle power properly, and all of them share one trait: they sample voltage and current at the same instant.

  • Plug in energy monitors. The supply passes through the device, so both quantities are available continuously and the product is integrated over each cycle. Simple, and limited to single socket loads.
  • Power meters and power clamp meters. A current clamp plus a pair of voltage test leads, with the instrument computing real power, apparent power and power factor from the pair. This is the handheld answer for a circuit you cannot unplug.
  • Power analysers. Multi channel, three phase capable, and able to report harmonic content as well as power factor. The right tool when distortion is the question rather than consumption.
  • Dedicated direct current power testers. On a photovoltaic panel the useful figure is the maximum power point, which is a property of a curve rather than a single operating point. The LODESTAR LD673PV solar panel tester states that it measures maximum power, voltage at maximum power, open circuit voltage and short circuit current on a single panel up to 1000 W, with an open circuit voltage range of 12 to 80 V and short circuit current up to 35 A. It arrives at power by sweeping the panel rather than by multiplying two unrelated readings, and the bullets are explicit that it is for one panel at a time, not for series or parallel strings.

Getting a power figure out of two readings

If a multimeter is what you have, you can still produce something defensible as long as you label it correctly.

  • On a steady direct current load, measure voltage across the load and current through it, multiply, and report watts. Note the burden voltage if the current is large relative to the supply.
  • On alternating current, measure both, multiply, and report the result as volt amperes. Writing VA rather than W is the whole discipline here.
  • If you know the load type, state the power factor you assumed and show the arithmetic rather than hiding it. An assumption on the page can be corrected later; an unlabelled number cannot.
  • Where the load is resistive, a heater element or an incandescent lamp, treat power factor as one and the volt ampere figure becomes a watt figure directly.
  • If the figure is going to inform a decision about running costs or cable sizing, borrow or buy an instrument that samples both quantities together. The Veyfiy multimeter page is a useful illustration of what a general purpose handheld does and does not include at this end of the market.

The short version: a multimeter is an excellent instrument for finding out why something is not working and a poor one for finding out how much it consumes. Those are different questions and they want different hardware.

Frequently Asked Questions

Why can a multimeter not just multiply its own volts and amps readings?

Because it never has both at once. The rotary switch selects one function at a time and the input path is shared, so a voltage reading and a current reading are taken minutes apart through different terminals. Even with both numbers in hand, the product is volt amperes rather than watts on alternating current.

What exactly is power factor?

The ratio of real power in watts to apparent power in volt amperes. For a clean sinusoidal supply it equals the cosine of the phase angle between voltage and current. For a distorted current, such as the input of a switch mode supply, harmonic distortion reduces it further and the simple cosine no longer describes it.

Does a dual display meter solve this?

No. A dual display shows two derived quantities from the same input, such as volts and frequency together. It does not connect to two circuit points at once, so it cannot give you voltage and current simultaneously.

Is true RMS enough to get power on a distorted waveform?

It gets the magnitude right, which the older averaging circuits did not. It still says nothing about timing. True RMS gives you a correct root mean square current to feed into an apparent power calculation and no more than that.

What should I use if I want real watts from a wall socket?

A plug in energy monitor or a power meter with separate voltage leads and a current clamp. Both sample voltage and current together and integrate the product over each cycle, which is the only way to recover real power.