Most heat pumps have electric back up or Auxiliary heat. Basically powerful electric heating elements that come in to play when the heat pump can't keep up. (They actually do three different things, but I won't label through all that here.) If you walked into your shop and turn the heat up more than about two degrees you will most likely bring on the Auxiliary heats and that will at least triple the cost of the heat. Heat pumps are one of the more efficient forms of heat (in the right climate), but the electric heaters are one of the least efficient. So if you let the shop cool down and then suddenly turn the heat up for an hour or two, you will get almost no benefit from the HP. Double check carefully with the manufacturer of the unit to see how it responds to sudden changes in the settings. Be careful, they can get a little mealy mouthed about this.