Power Consumed by Online UPS

Could be from charging the batteries. When battery saturates(fully charged), some charges created heat instead.

By the analogy of batteries the same to air reservoir or tank, compression always results to heat up - so does fully charged battery when online(unplugged)..

Yes they do, depending on what type of UPS you have the energy usage can vary....I would check with the manufacturer rep to see if yours is operating to spec, and see if any improvements can be made....or you could read the manual....

Yes. Nothing in life is free.

As noted earlier- UPS systems always draw more power than they serve.

Your best choice is to disconnect the INPUT power to the UPS during daytime (periods of high power demand) and back on during nighttime (when power rates are lower and no demand charges in place).

The batteries will always be there for back-up supply (although possibly at a slightly lower ride-through time, depending on how long since input power was cut) but the impact will be minimal.

Getting that parasitic load OFF the prime, On-Peak time slot will very likely save you a significant amount of money with VERY little risk.

Or,

put a cheap timer in the supply circuit.