Splitting utilities with Solar?

[Kezia Edmonson]

I installed solar few months ago; now looking to house hack large basement. Solar generates aprox $250/mo in credits; some months I generate more than I use, so bill is $0, other months bill varies b/c I use more than I generate (think winter), some months I use excess credits generated from other months (say May I generate more than I use, but June I use more than I generate), but it's still $0 b/c just using those excess credits. Avg across all months (from a year of generation) is $250/mo in credits. I want to get my investment back out of this solar however b/c obviously I had to pay a large cash chunk for the original investment. However, how would one suggest a tenant paying for electricity? Also, tenant would be using about 35% of space, me 65%. Would you set an approximate (pre-solar) flat rate cost they pay regardless or would you split any costs with them 35/65 (above the $250avg) and have them also contribute 35% of that $250 (so that it would be like a pre-solar cost percentage split)