Why the savings are real: the cube law
Spin a pump at half speed and it moves half the water — but draws about an eighth of the power, because pump power scales with the cube of speed. Move the same daily gallons by running twice as long at half speed and you've cut energy to a quarter. Add that variable-speed motors themselves (permanent-magnet) waste less electricity than the induction motors in single-speed pumps, and 75–80% total savings isn't a brochure claim — it's arithmetic. A typical case:
| Single-speed | Variable-speed | |
|---|---|---|
| Setup | 1,900 W × 8 h/day | Same water at 50% speed × 16 h/day |
| Season energy (7 mo) | ≈ 3,240 kWh | ≈ 690 kWh |
| Cost @ $0.17/kWh | ≈ $550 | ≈ $117 |
| Saved per year | ≈ $433 — payback on a $1,100 pump in ~2.5 seasons | |
Your numbers above will differ with rate, hours and season — that's the point of the calculator.
New VS owners often keep their old schedule and just enjoy the lower bill. The bigger win is running longer and slower: 24 hours at 1,200 RPM can cost less than 6 hours of single-speed while giving continuous skimming, better filtration, and quieter operation than a refrigerator. Slow is the setting; the hours are nearly free.
Is a variable-speed pump worth it?
For most inground pools, the question has quietly answered itself: US Department of Energy efficiency rules (in force since July 2021) effectively require new pumps of about 1 HP and up to be variable-speed — so at replacement time it's less "whether" than "which one," and the calculator above tells you what the upgrade earns before your old pump even fails. Check your utility, too: pool-pump rebates of $100–300 are common and shorten the payback further.
The honest exceptions: a small above-ground pool with a modest dedicated pump, a short season with cheap power (the savings scale with both), or a working two-speed pump — the budget middle path that already captures much of the low-speed benefit. If your payback above reads longer than ~4 seasons, the upgrade is a comfort purchase, not a financial one — still fine, just name it honestly.
The HP on the label is shaft power, not what you're billed for. For the real number: check the data plate for amps and multiply by your voltage (usually 230 V), read the pump's own display if it's a newer model, or watch your utility meter with everything else off for ten minutes. Plug-in power meters generally can't help — most pool pumps are hard-wired 230 V.
Picking one
Sizing stays boring in the best way: match or modestly exceed your old pump's flow, favor a proven wet end, and check your automation compatibility if you have it. What separates models is real-world programming ease, quietness at low RPM, and the head curves at the speeds you'll actually run — the spec-sheet details our gear page compares. Whatever you choose, revisit the run time calculator after installation: your new best schedule is longer, slower, and cheaper than the one you have now.