You can always benefit from a two speed pump. On low speed, flow rate is about 1/2 of high speed but energy use is 1/4. So for the same turnover, you can save about 50% in energy costs. Filtering also seems to work better on low speed.
However, there are a few downsides. The lower flow rate means the skimmers will not work as well nor a automatic cleaner if have one. But you compensate but running some at high speed and mostly at low speed.
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