As Weber pointed out, the WatchPower program is unlikely to use the REEP command; it's more likely to use the PF command, which I believe won't do the job of resetting an internal flash value that is used to calibrate the battery voltage measurement.
So you need to send a command to the inverter through the RS-232 port. For this, you will need a computer and a USB to RS232 adapter, like the one in this post. In order to talk to the serial port, you need a terminal emulator program, such as TeraTerm, RealTerm, or AccessPort. These are all free programs, you can find them readily by Google or other search. TeraTerm is a little harder to set up, so I'd suggest one of the others. Follow instructions to set up the correct serial port, and set the speed to 2400 bps (bits per second). You should be able to type junk at it (say abcenter) and it should respond with something like (NAKss (this is a Negative Acknowledge, i.e. it didn't understand or accept the command. The "ss" are more CRCs).
Nearly all commands have to end with a pair of valid Cyclic Redundancy Characters (CRCs). Weber has calculated the CRCs for the REEP command in the above post. The CRCs are eight bit characters, so they're not necessarily "printable", and as luck would have it, neither of them is printable. So that's why you have to send the command in hexadecimal form. Sending abcenter in hexadecimal is 0x71 0x72 0x73 0x0D. The "0x" (zero, not Oh) is a common way to specify a character in hex; it comes from the C programming language. 0x71 happens to be the ASCII code for character 'a'; 0x0D happens to be the hex code for the "enter" character that terminates all commands.
So once you are set up, all you need to do is to paste
0x52 0x45 0x45 0x50 0xC6 0xC2 0x0D
into the terminal program, and the inverter should reply with
The response is an open parenthesis (indicating the start of a response), and "ACK" for positive acknowledge (the inverter understood and has accepted the command). The "9" is the first CRC character; the inverter adds CRCs to its responses as well. The second CRC character happens to be a space character, so it's not visible.
Hopefully that makes more sense to someone who's not experienced with these things. Give it a go, and ask more detailed questions if necessary.
Unfortunately, this command may only fix one of the two internal flash values that are used to calibrate the battery voltage reading (one is an offset, the other is a scale factor). And of course, your problem may be hardware, not flash values. But it would seem worth a try.
[ Edit: fixed mangled keyboard formatting ]