Protocol Math & Fees

Borrow APY

Solend charges an interest rate for borrowing assets. The borrow APY is set algorithmically, a function of utilization of the pool.
The following piecewise function is used:
Which produces the following graph:

Protocol fees

In addition to the borrow interest rate, there's a protocol origination fee. The protocol fee is charged upon origination of a loan. It’s split between a program fee and host fee. The program fee contributes to Solend's insurance fund. The host fee can be collected by any client host to incentivize decentralized hosting of clients. When Solend is the host, host fees also contribute to the insurance fund.
The fees for most vaults are set to 10 bips (0.1%). There's an 80/20 split between protocol and host fee. Check the in app or on the Main pool page for any deviant rates.
Type
Fee
Program fee
8 bips
Host fee
2 bips
A spread fee of 10% the borrow rate will be added in the future, to potentially replace the origination fee.

Solana network fees

There are two types of fees you pay on the Solana blockchain when transacting: transaction fees and rent fees.

Transaction fees

These fees compensate the validator network for the CPU/GPU and network resources necessary to process the state transaction. Transaction fees are extremely cheap (currently 0.000005 SOL).

Rent fees

These fees compensate the validator network for the memory resources that are required to store state in an account. Accounts which maintain a minimum balance equivalent to 2 years of rent payments are exempt for paying rent. When solend.fi is used, accounts are allocated on behalf of the user and funded with enough SOL to be rent-exempt. This amount can be recuperated later when the account is closed (though this isn't yet implemented on solend.fi). Rent fees can be expensive compared to transaction fees (~0.01 SOL), but are only paid the first time a user is interacting with Solend.