The answer to your question depends on kind of solar project you are referring to:
If it is grid connected solar PV
plant feeding solar energy to grid and one have secured a PPA then I would say it will have to be debt financing with about 20-30% promoter investment in equity. The interest cost of debt will vary based on financial strength of the promoter and previous project implementation experience. The lower the cost of debt higher project IRRs one should expect. Further lower promoter equity investment also increases project IRRs. The cost of debt will also depend on the counter party which has issued the PPA.
But if one is setting up a rooftop solar project, operating lease financing could be the best option. The lease financing cost will again depend on the promoter's financial strength. And one can also avail of 100% financing with 0% upfront investment. Depending on the lease term, the cost of leasing can turn out to be lower than the energy cost paid by the promoter.
Both types of projects stated above are now being discussed by various utility companies around the world.
I hope this initiates more relevant discussion.