Calculate battery life when using boost converter
The basic theory is that energy available = VIt = 18 (V) x 0.21 (A) x 1 (h) = 3.8 Wh and the power required is VI = 28 (V) x 0.02 (A) = 0.56 W.

A 3.8 Wh energy source will supply 0.56 W for 3.8 / 0.56 = 6.8 h.

Now multiply that by the efficiency of your system - maybe 80% (I didn't check) and you could expect about 5 h or so.