This is a very tricky thing to understand. Actually, the power consumption of your AC often depends much on your usage and star ratings rather than the tonnage.
If you have a 2 star rated 1 ton AC, it will consume more than a 1.5 ton 5 star rated AC. If you have kept your 1 ton AC at a hot place then also the AC will consume more power.
So, make sure you use the AC properly and buy a 3-5 star rated one. This will lead to less power consumption.