Assuming you get billed like I do, with usage tiers. I usually stay in the first two tiers of usage, which means I pay $0.088/kWh for the first 400kWh and $0.115/kWh for the next chunk. With double the usage you probably get into higher tiers and pay more per kWh.
So if you got the same system as me and it only offset 50% of your usage, it would save you from hitting those higher tiers so you would save at a higher rate.