Review on bayite Mini Digital Current Voltage Amp Meter: Accurate & High-Precision Gauge with Hall Effect Sensor Transformer (5-120V, 100A) by Bill Russett

I would give 3 stars if it was $30 but at $16 it's a steal. The instructions are WRONG - it will not calibrate if the sensor is disconnected. It needs to be connected. Mine doesn't calibrate perfectly either, it reads 0.2 amps with no current. I use it to measure the power output of a generator on my boat. As long as you need to see the difference between 30 and 50 amps, not 31 and 31.1, this might work for you. Also note that the decimal for "reverse" flow is shown on the right, so make sure it's rotated the way you want it. EDIT: Just got back from a 4 day trip and found this very useful for monitoring a generator's performance. I've also noticed that it drifts a lot - it seems to fluctuate to 0.5-0.8 amps at 0 actual current and drop back to 0.2. So - reasonable but not particularly accurate - let's say 1% of 100 amps. I still feel like I got my $16 and a 100 analog ammeter reading wouldn't be more accurate. EDIT 2 - it seems to drift with temperature. Parameter 0 is actually 0 when cold and 0.8A when hot. Still $16 if you don't need less than 1 amp accuracy. I have shunt meters which are pretty much perfect but need to cut the wire and aren't usually good for positive side measurements like an alternator.