Probably the simplest option is the
Victron Smart Shunt for ~$130.. The whole unit resides in the battery box with no holes to drill or wires to run. The output is viewable on any phone/tablet/computer. Victron also
makes a version with a hardware display if that is important to you.
There are cheaper no-name options, but they have several drawbacks - they need to have the display installed and wired up, they are less accurate and most importantly, they only provide a snapshot of what is going on right now without any history. The Victron unit provides the current state of charge, but also statistics about the performance of your battery which are very useful for determining how your systems is working.
