Take some time around the following questions:

1) Who will assess your solution?
(within your team? within your organization? Or will it be your users, your partners...?)

2) For your key targets, define the relevant indicators.
Both quantitatively and qualitatively, you can measure both adoption (number of users or customers, sales, perceptions, recommendations...) and impact (satisfaction, gains, improvements...).
Prioritize your indicators.

3) How to evaluate these indicators over time?

4) What results would be a success for you?

Keep in mind that measuring results is time-consuming, so choose your key criteria carefully!