I was given one of these.
Told they are $3 on net.
Fits into cigarette fitting.

I have one of those I use in the car occasionally to get an accurate reading of the power at the accessory circuit. On a yacht, I prefer one of these permanently wired into the end of the circuit my navigation computer is on and is clearly visible from the cockpit.

www.ebay.com.au/itm/Digital-Green-LED-Display-Voltmeter-Voltage-Gauge-Panel-Meter-DC-12V-For-Car/263886213192?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

If the voltage drops down to about 12.2 I can then do something about it. It's interesting how many items come with a 5v USB charger in them now. I currently have two solar regs and two invertors with them.