Measure first
What to measure before you buy anything
Most power purchases are made on an estimate that turns out to be wrong by a factor of two. These five instruments replace the estimate with a reading, and between them they cost less than the mistake.
By Jean-Benoit Buisson
The estimate that is always wrong
How much does the fridge use. How much is the van drawing when everything is off. How much did the solar actually make today.
Every one of those gets answered with a guess, and the guess sizes a battery, a panel or an inverter. A reading costs less than the second purchase.
1. The plug-in meter
Start here. No installation, no risk, and it answers the appliance question directly.
The figure that separates a useful one from a useless one is the lower end of its range. Standby draw sits between a third of a watt and a couple of watts, and a meter with a one-watt floor reports most of it as zero.
Resolution and accuracy are different things, and a good listing publishes both.
2. The DC clamp meter
For everything the plug meter cannot reach: a battery feed, a solar string, a parasitic drain.
Around a conductor rather than in series, so nothing gets disconnected. The three figures that matter are the DC range, the jaw opening — a battery cable with a lug is thicker than it looks — and whether there is a zero function.
Not one clamp meter in this catalogue publishes a safety category, which is the rating that says which circuits an instrument may go on.
3. The DIN rail meter
For a circuit rather than an appliance. A heat pump, a workshop, a charger, a solar feed.
If the reading has to divide a bill, MID certification is what gives it standing. If it only has to inform you, it does not matter at all, and the cheaper meter measures just as well.
A pulse output with a published rate is what gets the reading into software.
4. The USB power meter
Small, cheap, and it settles arguments about chargers, cables and power banks.
Four values at once — volts, amps, elapsed time, accumulated capacity — is what makes a charge legible while it happens. Running a power bank flat through one gives a real figure to set against the number printed on the case.
5. The battery monitor
A shunt in the negative lead, counting amp-hours in and out.
That is the only honest state-of-charge measurement. On lithium in particular, voltage is nearly flat from 20 % to 90 % and a voltmeter tells you almost nothing.
Install it once, and every other number in the system becomes checkable.
The order these get bought in
One and four on a whim. Two when something is wrong. Three and five when the system is big enough to have questions that last longer than an afternoon.
