We installed solar in 2018 and used a 15 year loan to pay for it. For the solar credit amount that I claim this year, do I use the loan payments or the purchase price?

For purposes of the credit, costs are treated as being paid when the original installation of the item is completed, or, in the case of costs connected with the reconstruction of your home, when your original use of the reconstructed home begins.

If you cannot use all the credit this year, you can carry the unused portion of the credit to 2019.
Keep all the receipts because the solar energy tax credit covers everything from shipping the solar panels to the actual installation costs.