Charles Middleton said his organisation was aiming to carry out a piece of work next year to improve the standard of measuring social returns. He said more accurate measurement would be necessary to attract new finance.
"If we want to bring new organisations into the fold, we may need to provide them with more information," he said. "New funders may want more hard evidence about social returns."
He said that social return on investment would always be a difficult concept to measure in many areas of the sector. "With a lot of projects it's very unfair to create a complex measurement process to determine their social return," he said. "Some social returns are extremely difficult to pin down.
"No one has yet managed to come up with a definitive social reports and accounts measure."
He said it was virtually impossible for an investor to compare social returns in two different sectors, such as social housing and renewable energy, and make a judgement about which was providing more social good.