If you want a meter that will give precise readings of your batteries condition, this is not the meter for you.
Like any meter that works solely off of battery voltage, it will give different reading at different load levels. Its simply because batteries read a lower voltage under load due to internal resistance.
For example, if it reads 80% at 8:00 pm with just a few CFL lights on, you likely have a good charge. If it reads 80% at 8:00 pm with the freezer running, you likely have a great charge. If it reads 40% at 8:00 pm with the freezer running, you likely have a good charge and it will go up into the 70%-80% range when the freezer shuts off. If it reads 40% at 8:00 pm with just a few CFL lights on, Its time to fire up the generator and recharge, before damage is done.
With my refrigerator, freezer and computer all running simultaneously I have seen readings as low as 20%. 20 minutes later, when the refrigerator, freezer and computer are turned off, it went back up to 90%. The reason was the heavy load and the internal resistance of the batteries. No worries there. But, an unexplained 30% means getting out the volt meter, the hygrometer and checking everything!
Once you learn how your particular battery bank works with your power usage patterns and particular charge controller and panels, this meter will help you spot variations from the norm. This wont do it for you like the $400 meters.
The most useful feature is the Time since the last full charge indicator. It keeps track of how long its been since the batteries have had a good full or equalizing charge. It resets the timer when it senses a full charge. A week later it goes to yellow and then after another week it goes red. It remembers when we tend to forget.
In short, its a quick and dirty way to keep track of your battery bank. Its best mounted someplace that you will see it often like over the kitchen light switch or over the TV. Batteries dont make any noise useful in keeping track of how they are doing and you cant see electricity. This is the cheapest way to keep tabs on the general, but not the detail, condition of your batteries.