84cm = 14cm x 6 PowerBanks

35cm = 7cm x 5 PowerBanks

30cm = 2cm x 15 Power Banks

6x5x15 = 450 Power Banks

20Ah x 9v = 180Wh ----> 180Wh = 0,180kWh

0,180kWh x 0,11€ / kWh = 0,0198 €

0,0198€ x 450 PowerBanks = 8,91 €

Buying each Power Bank:

 15€ each ---> 15€ x 450 Power Banks = 6 750€

If we consider that a power bank has a useful life of 3 years, then:

6 750 € / 3 years = 2 250 €

2 250 € / 12 months = 187,5€ /month