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Don't set back when your heat pump has electric resistance heaters for the second stage.

To better understand how a heat pump works in numbers (hypothetical)

Electric heater with the resistance heat coils (like the red glow in a toaster) will make 3412 BTU per kilowatt of electricity,

if one kilowatt of electricity costs 20 cents then you will get a little over 17,000 for one dollar

$1.00 = 17,000 BTU. Electric Resistance Aux Heat

If your heat pump has a COP (coefficient of performance) of 3.0 then one dollar will purchase three times as much heat from your heat pump.

$1.00 = 51,000 BTU Heat Pump Compressor Heat.

If you turn off the compressor for 6 of the 8 hours that you're at work and your house needs 51,000 BTU per hour to stay warm for those 8 hours (based on the outdoor temperature that day) you would save about 6 dollars that day. Then at 3 PM your clock thermostat turns your heat back on to be warm for you at 5 pm when you get home from work. The heat pump would need to operate for two hours non stop to maintain 64° that would cost you $2.00 for 51,000 BTUh for 2 hours.  (51,000 BTU will only maintain the temperature it will not increase the temperature on that particular day)

But you want 68° when you get home, so the auxiliary heat would also operate for some of that time in order to raise the temperature up to 68°.  If your 5 kW auxiliary electric resistance heat were to operate for 2 hours in order to make that home 4 degrees warmer then you will be using $5.00 per hour to get another 85,000 BTU per hour from a 5 kW auxiliary heater to increase the temperature.   That would cost you an additional $10.00 for the time the electric resistance heat is operating.  Add the $2.00 to the $10.00 and you get $12.00 to NOT heat your home for 8 hours. 

That is more than the $8.00 you would spend on heating your home with the heat pump alone.  Of course the 5KW heat may not operate the entire 2 hours of the recovery time from 3:00 to 5:00PM  But even if it only operated for one and a half hours you would still pay about $7.50 for that time.  If your electric resistance heat ran for only one hour during the 2 hour recovery time you would pay $5.00 for that time, then you would save about $1.00 for NOT heating your home.

For the heat pump setback to save you any real money, you need to leave it off for more than 24 hours before you get a benefit from turning it off. If you are going on vacation and you want to leave the thermostat at 55° so the pipes don't freeze, that is a good idea. no need to keep the home at 68° for a week or two. But for only 8 hours, or less, there is no savings.

As I said at the beginning, the numbers are hypothetical, but there are occasions where it may cost you more to set the thermostat back a few degrees for short time periods. I have actually seen this happen where a customer wanted to know why they were not saving $$$ by turning the thermostat down at 8:00 AM then turning it back up at 2:00 PM when the kids got home from school. Only 6 hours and 2 of those 6 were doing recovery from the lower temperature to the regular temperature. Yea I actually cost them more $$$ to not heat the house.   

Heat Pumps………………..Set them and forget them.

Steam ……………..a few degrees set back is fine but you won't notice a reduction in operating costs.

To which I will add… it depends somewhat on what's in your home. Plants? Pets? Musical instruments (particularly pianos)? Fine furniture? Anything more than a few degrees setback and they will be uncomfortable (the living things) or irretrievably damaged (the non-living).

But you will save money on energy…

Depends on the length of time of the temp set back and how much the temp set back is and the outdoor temp.

If you gone for a week absolutely set the temp back 10 degrees. I wouldn't go below 60 degrees. If you gone for 8 hours or less a setback is probably not worth it. When you set the temp back it is not just about air temp. The rugs, furnature, walls,floors and ceilings cool to the setback temperature and the temperature of all these items need to be raised back up to 70 degrees or whatever your normal temperature is.

With a short set back, it will take more fuel to raise the temp back up than what you will save,.

When you do a setback notice how long it takes for the system to recover.

That will give you a clue how to manage it.

Every system and building is different. There or no hard or fast rules.

I might add this to the owner of a heat pump. When you sat back the temperature more than one degree and your back up, or auxiliary heat is electric resistance, then your recovery will be using the more expensive electric resistance heat. Whenever the setpoint temperature is more than 2° above, the room temperature the electric resistance auxiliary heat will be engaged. You need to think about that in your energy, saving calculation. If your coefficient of performance on your heat pump is a three or higher then you will be using three times more electricity every minute you use the electric resistance heat.

Heat pump = set it and forget it

Similarly, if you set back the heat pump when you go to bed and then have it warm the house a few hours before you wake up it will be running at maximum capacity at the time of night when typically outdoor temperatures are lowest. The efficiency of a heat pump depends upon the outdoor temperature, so you're asking it to produce the most when it's least efficient.