a Fluke 337. It's a multi meter and a "True RMS" AC/DC clamp on amp meter. I use it to test the solar panel output, the alternator output, the draw of all of my loads just by clamping on of the wires. You cant beat it for convenience. You might be able to find one cheaper but Fluke is a really well made instrument. Don't buy one that's just an AC clamp on, get the DC as well.