For a whole bunch of reasons, I am not a fan of the Quantar revert board, but it does have two advantages: (1) being Motorola recommended, it is a "safe" choice (remember the saying, "No one ever got fired for buying IBM"?); and (2) it is relatively inexpensive (particularly if you already have the board and your SCM is programmed to recognize it). Note that the 3 dB reduction in PA power is hard coded into the SCM upon LOOP (loss of offsite power).
I have implemented an alternative solution that works flawlessly, provided you (or your client) has the courage to think outside the Motorola box. It involves 2 8G31DT gel cell batteries, a device known as the "Heart Interface," and associated wiring, breakers, outlets.
The Heart Interface is a box that combines a battery charger, an inverter, and a transfer switch. Prior to LOOP, the HI passes 110VAC to the regular Quantar power supply. On LOOP (which is defined as RMS voltage < 90 or >135, or freq <56 or >64), the HI switches to inverter mode in less than about 18 ms, which is so fast that the Quantar does not see loss of power and does not reboot. (Nor, for that matter, does a Windows PC reboot.)
Our 2x8G31DT solution was designed to carry the Quantar for not less than 2 hours, using twice the typical 90-5-5 duty cycle (on the theory that if the lighst have gone out, the phones will be ringing). The only time we actually tried to see how long it would carry the Quantar before reaching the 50% discharge state, we quit after about four or five hours because we got bored and ran out of time. Note that, since the Quantar continued to see 110VAC at all times, there was no reduction in output power.
One of the upsides of this solution is that, unlike the Motorola revert board, the HI will not kill the batteries by trickling fully charged batteries to death -- a demise that is only recognized once the lights have gone out. The charger function of the HI is a sophisticated three stage charger that can actually go to zero charge on a fully charged battery bank.
The downside of my solution is that you have to be willing to bet your reputation on it working, you have to take the time to understand how it works and how to assemble the components, and if you purchase the components at retail, it will cost more than the revert board.