Eric,
VS pumps should be called variable horsepower pumps as that is what they really are.
I run my 3 HP IntelliFlo at about 1200 RPM and it only uses 170 Watts of power. My pump is an older one, the new ones are even more energy efficient.
In general, the larger the VS pump, the more water it can move at a lower RPM. So, a 3 HP pump can move more water at a lower RPM than a 1.5 HP VS pump.
I run my pump 24/7, most of the time at 1200 RPM, for less than $20 bucks a month. If I had a newer pump it would be less than $10 a month.
Thanks,
Jim R.