How to accurate assess battery usage?

Welcome to the forum Cortttt,

The classic method is to use a Battery Monitor with an external shunt (high power precision resistor to measure current). Most BMS use the shunt to measure the Amps*Time in and out of the battery bank. If you take 10 amps for 2 hours out, that is 10*2=20 Amp*Hours out of the battery... If you recharge at 5 amps * 4 hours = 20 Amp*Hours back in.

For shunts--You want to make sure your shunt is rated (over) for the maximum current you plan on using.... I.e., if you have a 1,200 Watt AC inverter you run at 12 VDC:

• 1,200 Watts * 1/0.85 inverter eff * 1/10.5 volts battery cut off = 134.5 amps max continuous current

There are a lot out there--Many stand-alone, others that are integrated into Solar Charge Controllers or AC inverters.

I have not used any battery monitors--The links below are for beginning your search. Our host, NAWS does have engineers to answer any questions you may have. For stand alone versions:

https://www.solar-electric.com/search/?q=battery+monitor (full list from our host NAWS)

And a few highlighted units:
https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html
https://www.solar-electric.com/xantrex-84-2030-00-linklite-battery-monitor.html
https://www.solar-electric.com/xantrex-84-2031-00-linkpro-battery-monitor.html

And Amazon has a nice list of lower cost units:

https://www.amazon.com/s?k=battery+monitor&ref=nb_sb_noss_1

When you look for "energy monitors"--Make sure that they do what you want... I.e., show the battery bank being discharged (state of charge going down) and being recharged (state of charge going up). Many energy monitors are just measuring the flow through the meter and do not represent the battery bank state of charge.

Also note--Like any meter, these do have limits. And one of those is that they can "drift" and display inacurate battery state of charge. Many (some/most/all?) of these meter reset to 100% when the battery is fully charged (i.e, battery is held at 14.x volts for an hour or more).

Make sure you battery type is supported by the meter. Flooded Cell Lead Acid, AGM, Li Ion...

And for Li Ion, many/most installations have their own battery monitoring system (BMS)--Sometimes you can get state of charge and other information from those BMS units.

-Bill