Query slower with index on 14M rows table
You should play with it a bit, because it might not be clear beforehand what solution will produce the best results.

A few points to consider


It is very likely that an index on the date column will provide you with better selectivity, as it has a higher selectivity.
Composite indexes are usually a good idea, but please make sure to chose the order correctly date_time, meterid  vs. meterid, date_time. In most cases it makes more sense to leave columns with dense values (i.e dates, floats) to the end, as any column in the index following them is unlikely to have any effect. ( try meterid, date_time for an index.)
Subselects might force the optimizer to use a specific plan. Try converting it into a join if possible.