At the right-hand side of the Energy Monitor display is an icon that represents the main battery. As well as providing a place for the arrows to point towards and away from, this icon gives you some idea of the usable energy in the battery. The picture at right shows the five states that this icon can take. Each is annotated with the name that I will use to refer to that icon state. Please note that "FULL" does not mean that the main battery is actually fully charged. Toyota prevent this from happening (so as to prolong the life of the battery). What it actually means, as I will describe in detail below, is that the state-of-charge of the main battery is in a range close to the highest permitted. The other icons must also be interpreted with a similar degree of latitude.
Before we go on, I must say that I have only seen the top four icon states in my car. You may only see the 3/4 and 1/2 states and this does not mean there is anything wrong with your car. In fact, some people, probably those that do little city driving, report only ever seeing 3/4. To see the FULL icon, I had to "force charge" the battery using a procedure I'm not going to disclose here. To see the 1/4 icon, I waited until the state-of-charge got pretty low and then drove the car in a manner which reduced the charge even further. Without these tricks, I would also only ever see 3/4 and 1/2. I think that to see the EMPTY state, you have to run out of fuel or drive hard up a very long, steep hill.
It is tempting to assume that the state of the battery icon really tells you how much charge there is in the main battery. This is not the case. It gives you a rough idea, but like so much else on the Energy Monitor display, it is mainly eye-candy. For example, if the icon is in the 3/4 state, the actual state-of-charge of the main battery could be anywhere in the range 55% to 74.5% of the manufacturer's rating, which covers almost half of the range permitted by Toyota.