Evaluating the impact of a social enterprise is often difficult to do. For most sectors, there are no agreed-upon metrics to rely upon to tell others how successful you have been. But you don’t have to start from scratch. Here are some perspectives and suggestions that might be helpful:
REDF (formerly The Roberts Enterprise Development Fund) is recognized as a pioneer in the field of measuring the impact of social enterprises. A summary of their work on SROI (social return on investment) can be found here.
Also check out the Basic Guide to Outcomes-Based Evaluation for Nonprofit Organizations with Very Limited Resources from the Free Management Library.
Finally, be sure to take a look at the evaluation publications provided online by SVT Group.
While often a rather dry topic, evaluation or SRIO is well covered by these three resources. Good luck!