Thanks tphaggerty, good suggestions. I already run the pump with the basic Intermatic timer for the 5 peak sun hours---and that's enough. It's an indoor pool, it doesn't get much dirt.

With time-of-day metering, electricity at peak sun hours costs about 50c/kWh. The pump probably uses the full 3/4 hp, which is about 600W, so that's 3kWh/day and $45/month. There is enough sun here (Oakland, CA) to take advantage of solar heat May, June, July, August, September. You're right, maybe I shouldn't bother, but still. According to the literature, I can get 1/2 the flow for 1/4 of the power, so I would save around $150 each summer, plus about 1/2 of that in the winter.

Cleancloths: thanks for the comments. I thought of the siphon effect too. but then I wondered. There is a valve at the top of the panels that lets air in when the flow stops. I believe this is done so that the panels do not get scrunched by the vacuum produced by gravity pulling down the water. I would think this means that the water must still be under pressure when it gets to the top. Siphoning implies negative pressure, but the valve would not allow that (unless it allows SOME negative pressure---but can it be that smart?)