Good link that describes the power issues: Tom's Electronics, IT and Photography: Solar powered Arduino and attiny projects
In sleep mode my attiny84 uses 0.004mA. When calculating it uses about 20mA, but it is sleeping most of the time. Running at 8MHz it will only execute code for about 1ms every 10 seconds, so it's average contribution to the power usage is 20/10000= 0.2mA.
As he mentions you could run an ATTiny for a year on a couple AA batteries (2200mah batteries / .2ma = 458 days) and even the smallest solar cell would power it "forever". Not sure how you get a solar cell in a bottle though 