Renewable energy
Multimeter
You will use a DMM constantly. A good one lasts a lifetime if you look after it.
Get a good digital multimeter
Buy yourself a decent digital multimeter (DMM). You will use it constantly checking battery voltage, panel output, pump wiring and whether something is actually live. If you are rusty on the theory, start with electronics basics (Ohm's law and why cable length matters in DC).
I picked up genuine Fluke meters much cheaper through China. Fluke sells them to Chinese students at a fraction of western prices, partly to combat copies and get people onto the real thing. My 12E+ was one of those. These days there are plenty of good Chinese DMMs too, so do not stress too much about brands. Just get something with decent DC voltage and current ranges and learn to use it.
Tim and I went through digital multimeters and batteries in this video we filmed up at Jarramali.