Have you roasted chickens at 150C (302F) for only 1 hour before? That seems a bit low for only one hour in the oven. Most recipes advocate at least 350F (176C) for one hour, or until the internal temp is 165F (73.8C).
To roast a chicken at 60C (140F) for 4 hours is an interesting idea akin to smoking foods at low temp. The long slow cooking time causes the meat to be fall off the bone tender and perhaps juicy, but you’ll not get a crisp skin that way. It will probably look like it was steamed or boiled.
Chicken doesn’t need long and slow cook times like this, and is often better done fast (an hour or less) at higher temps to sear the bird and give it color. Still, there’s no harm in experimenting.
Personally, I’ve grown to dislike smoked chicken due to the texture. Long slow smokes impart good smoky flavor, but chicken cooked long and slow is a but mushy for my tastes. Even when I do Beer Can chickens, I up the temp and decrease coking time because I prefer the texture of the meat that is produced with higher heat and shorter cook times. Low and slow for chicken is rather pointless IMHO.
But, now that I have digresses into uncharted territory, lets look at your original question about energy use! It really depends on your oven, especially how well insulated it is. To get the oven to 60C will not take much time at all, so your initial start up time will be fast and use less energy. To keep the oven at that temp, it will need to cycle the elements on for a set amount of time to add more heat to the oven (heat will slowly dissipate during the cooking).
In theory, the shorter “on time” of the elements required to get to 60C and then maintain 60C for 4 hours should be about the same, but possibly slightly higher than running the oven at 150C for one hour. But again, it depends on how well your oven retains heat. I’d wager that a 4 hour cook at 60C will use slightly more electricity than a 1 hour cook at 150C, but now a substantial amount! Certainly no enough to make you notice it in your utility bill.
For maximum efficiency, do NOT open the oven during the four hours of cook time. This ensures the heat remains in the oven and the elements don’t have to turn on to maintain the heat as often (thus less electricity used).
I’m curious though as to why you’re concerned about this aspect?
After you have tried this, try cooking the bird at 500F (260C) for 15 minutes, then drop the temp to 350F (176C) for an additional 25 to 30 minutes, or until the internal temp is 165F (73.8)
