Our building of 130 apartments/20 floors has energy costs as the single biggest recurrent expenditure for the building by a long way so we went down the path of reducing power costs. We use both gas and electricity but I will describe our electricity experience and the benefits and pitfalls (gas is more challenging but can reap rich rewards with a focus on detail). We changed all building lighting to LEDS and installed garage sensors and immediately got a major cost saving which will pay itself off in about 3 years or less, this is a no brainer financially. We then installed solar panels which also have delivered at a slightly lower rate of return than LEDs but still way better than expected and we use all the power during the day so no return to the grid and the electricity saved is mainly at peak prices. Pitfalls are the obvious one of choosing the supplier which can wait for another post and one which was expected but still challenging which is roof anchor points and their use which we thought we managed very carefully but still had issues to resolve when it came to using the anchor points by abseilers.
Overall, if your roof is appropriate and if our experience is a guide then LEDs and solar panels are a good thing for an apartment block to consider and do the numbers on.