Choosing a variable speed pump shouldn’t feel like a guessing game. We walk through a simple, field-tested way to size the right unit by matching your current single-speed horsepower to modern total horsepower (THP), accounting for plumbing size, distance, and voltage, and then using RPM control to hit your flow goals without wasting energy. No fluff—just practical rules you can apply the moment you step onto the equipment pad.

We start by translating old-school horsepower labels into today’s THP ratings and explain why you can safely choose equal or slightly higher THP and run it slower for quiet, efficient filtration. You’ll hear how pipe diameter, elbow count, elevation, and equipment distance shape total dynamic head, and why most backyard pools don’t need elaborate TDH calculations. We compare VS and VSF pumps, demystify how auto-flow control can handle minor resistance changes, and outline clear ranges: what to buy if you’re replacing a 1 HP, 1.5 HP, or 2 HP single-speed. We also unpack voltage realities on older 120 V systems so you don’t overbuy a pump that can’t deliver its full potential.

Chasing WEF numbers? We put them in perspective. The big savings come from programming your pump to lower daily speeds and only ramping up for spa jets, heaters, or cleaners. If your pump doesn’t report flow, we recommend adding a FlowVis meter so you can set precise gallons per minute and know your turnover isn’t guesswork. Finally, we cover brand matching—why sticking with Pentair, Hayward, or Jandy to match your existing automation can save you time, wiring headaches, and re-plumbing.

If you want a clear path to better circulation, lower noise, and smaller power bills, this conversation gives you the checklist: match THP to your old HP, size up rather than down, ignore small WEF differences, use a flow meter to set RPM, and keep your brand ecosystem consistent. Subscribe for more practical pool insights, share this with a friend who’s shopping for a pump, and leave a review to tell us what sizing questions you want answered next.

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