The power consumption of an old monitor is nearly the same when compared to the power consumption of an old tower PC. Similarly, the power consumption of a recent monitor is nearly the same when compared to the power consumption of a new tower PC. Ok, in some cases, the tower PC could consume bit more power within a factor of less than 2. I wouldn't claim that a computer uses "far more power" than a monitor, as claimed in the upvoted answer. Actually, today, when many prefer laptops, the big power user is the monitor and not the computer.
I'd say if your goal is energy saving, the computers should be turned off when not used for a certain amount of time. So, for example, during weekends, they should be turned off.
I happen to have three monitors (23 W per monitor) and a very modern low-power laptop computer (probably only few watts of electricity used) to which they're attached. I always turn off the monitors but don't turn off the computer as it uses so little power as a laptop.
Next time when renewing the computers, you could consider replacing the tower PCs with laptop computers attached with a lock to a secure object (so they won't be stolen). Most laptops have an ability to attach an external monitor. When selecting a laptop, place an emphasis on good battery life (which may mean either big battery or low power use) and low weight (which will filter out the ones having big battery).
Also, the choice of laptop allows them to be used as laptops as well, meaning if you require both fixed computers and laptops, the same computers can be used for both purposes. Less materials used, less damage for the environment. Less weight, less emissions during shipping the computers from China to wherever you happen to live.
Furthermore, during electricity breaks, laptops have far better characteristics. They can survive minor power breaks without filesystem damage, and in the case of major power breaks, they will typically shut down in a controlled manner (no filesystem damage) when there's only few percent of battery life left. About the only drawback of laptops is that when performing heavy computing, the fan will spin like crazy!