Bank Australia is hoping to lower its cost-to-income ratio after announcing it had become the first domestic authorised deposit taker to source all of its electricity through renewable sources.While the bank has not disclosed the savings it expects to harvest from the shift, one of the mutual's business partners says it could slash its power costs by up to 47 per cent.Since 2015 Bank Australia has been reconfiguring its electricity sourcing through the installation of rooftop solar at its Melbourne headquarters and  call centre in eastern Victoria.It has also negotiated a ten year deal to take energy from windfarms operated by Pacific Hydro in western Victoria."Since  Bank Australia made the commitment to purchase 100 per cent renewable electricity in 2015, we've seen the costs involved continue to decline across the systems we've installed on our buildings  and the electricity we've purchased through our partners," said Bank Australia managing director, Damien Walsh."Our agreement with Pacific Hydro gives us price certainty over the next ten years."Bank Australia has been consulting with a range of business partners through a global business initiative known as RE100.The RE100 program is led by the The Climate Group, which has secured commitments from ten of the world's largest banks to migrate exclusively to renewable power by 2030.Jon Dee, the Australian head of RE100 estimates that companies can garner savings of between 13 per cent and 47 per cent through careful negotiation of agreements with renewable energy suppliers."Bank Australia has set a really positive example," Dee said."They've increased the use of wind and solar power and at the same time they'll save money on their energy bills. "They're showing other Australian companies that renewables are beneficial for the bottom line as well as the environment."Other Australian banks participating in the program include Commonwealth Bank and Westpac, which have committed to reaching the 100 per cent target by 2030 and 2025, respectively.Bank Australia joins a select group of banks to have already met the target.They include Credit Agricole(France), Danske Bank(Denmark) and Amalgamated Bank(US).The first bank to source all of its power through renewables was Spain's Bankia S.A. in 2013.Walsh said realisation of the target meant that his bank was avoiding 780 tonnes of carbon emissions a year."That's equivalent to taking 232 cars off the road," he said.