Variable Speed Pumping - Replace just motor or entire pump? Pros and cons?

ok - pool stats up front:
23K fiberglass pool
Salt Chlorinated
Solar heated - panels on roof 15 feet above pump
3/4 HP whisperflow pump (eplus motor - single speed) - clean and clear cartrige filter

PG&E for pump is killing me on energy costs (I am sure chlorinator sucking juice to - but one thing at a time. ...)

Looking at variable speed pump conversion. Considering both replacing entire pump with a variable speed pump ($1000) as well as going with a variable speed retrofit replacement motor from Marathon or AO Smith ($500 - $700) . Trying to be cost conscious here. My existing Whisperflow
pump 'wetend ' is in excellent condition so considering both options. I have considered - but backed away from just changing to a two speed pump/ motor retrofit - not sure I am going to get the energy savings there and have heard that it won't work well in slow speed with 15 feet of head to circulate through solar panels (please tell me if this is a myth. I do not run any sort of system automation so for now - intelligence/timers to change speeds will be through onboard controller (variable speed pump / motor)
or if 2 speed is way to go - will more likely be in external timer device.

Questions I have...

- Is two speed suffifient to get decent (50%) energy savings by running full speed first couple hours - then low speed for 8 more hours? will pool solar panels and head to reach them prevent operation in low speed?

- if variable speed is way to go - will a variable speed motor retrofit of my whisperflow do the job? What will the disadvantages be in doing this Vs. just installing a complete variable speed pump?

- obviously a variable speed pump will spend a large portion of its operation in one of the lower speeds -

will a VS pump be able to run in a lower, energy efficient speed while circulating water through the solar panels? if yes - how will this be different that just running a 2 speed pump in low?

Do I need automation to close off the route to the solar panels at the actuator when its time to run in low speed?

...I am sure this have been covered - sorry for the newbie questions - but really need some expert advice.

Thanks!

Ken

1. VS pumps are FAR less reliable than 2 speed pumps. Repair costs on just the electronics is a large percentage (>70%) of the price of a new pump. They are a bit more reliable in California than Florida, since they are very susceptible to voltage surges caused by lightning.

2. VS pumps are only slightly more efficient than a *properly selected* 2 speed pump. But they are a 1-size fits all solution, and can be dynamically tuned to a given pool. This is a big advantage, since most pool dealers and builders cannot calculate flow dynamics on your pool.

3. In-floor vacuum systems, and roof-top solar systems typically are huge energy hogs. There's no fix that I know of for the in-floor systems. But, you will probably save money over 5 years by attaching a dedicated 1/2 HP pump to your solar system, that is switched by temperature. Then, you can 'turn down' your VS or 2-speed pump since you no longer have to provide the 20+ feet of head needed to push the water up to, and through, your panels. It's possible to do this with a VS pump, full automatic controller, and several electrical valves. But it's a lot easier to simply put an intermittently operated 1/2 HP pump in place.

4. Whether you can operate efficiently with a single VS pump, controllers and valving while servicing your roof top panels would require looking at your controller, valves, piping . . . and the pump curves of your selected pump. Most VS pumps don't produce the head at their lower speeds, that you'd likely need.

. . . I'll see if Mark wants to comment. He thinks more highly of VS pumps than I do. . . .