Don is right.
Adding more information,
Different booster have different electrical designs. Digitrax used a "linear regulator method “ of regulating the track voltage and current limit where as NCE has a very efficient internal switching power supply to regulate the voltage and current limit**. The switching power supply keep the booster cool which is why there is no heatsink on NCE boosters.
So in addition to the high capacitance smoothing out the current to an average current as opposed to a peak current, the relationship of current input and current output can be even FARTHER apart depending on the booster’s internal circuit design.
In terms of some kind external measurement there is a closer relationship of current in versus current out with Digitrax. But it still not an accurate indication and some kind of scaling factor will have to applied.
**Excludes the latest NCE boosters that are provided with a NCE DC power supply specifically for them (PB5, DB5 & SB5). With these new designs now have the provided NCE power supply determine the booster's output track voltage and current.
You can get a crude indication of booster load by measuring amps on the input side of the booster. But there may not be a close correlation between input and booster output amp numbers. The primary booster rating limitation is supposedly on output current (ie – amperes) going to the track. Putting an ammeter on the input wires to the booster is an indication of Power (volts x amps or speed x torque) being consumed. But there usually is temporary energy storage/buffer capacitors inside the booster to accommodate booster output current to circulate. So it is possible for more amperes to circulate between the booster output and track than what appears as amperes on the input side of the booster. Especially true if there is ‘extra’ voltage available to power the booster. Try measuring both at once with a heavy booster load and tell us the results in your particular case.
We're using pretty much all Digitrax boosters in the club setup, and I'd like to have a constant, consistent reference to how hard a booster is working. I would like to know if I need to shift loads or recommend purchasing another booster.
Would putting a standard Ammeter & Voltmeter between the power supply and booster give me a pretty decent idea of what the booster is pulling? (Most of the digital meters I'm seeing are sold as a combo ammeter/voltmeter. Just the ammeter would probably be good enough.) Would I have to apply a corrective factor, or would a, say, 4A measurement from the power supply be pretty close to 4A booster output?