Arizona governs the interest rates in the state under Title 44, Chapter 9, Article 1 of Arizona Revised Statutes.  Section 44-1201 prescribes the rate of interest on any loan, indebtedness, judgment or other obligation at ten per cent per annum. Any different rate of interest may be agreed upon and contracted in writing under this Section.  Rate of interest on a judgment upon an agreement shall be the rate provided in the agreement as long as it is not in excess of the maximum permitted by law.

Section 44-1202 prohibits interest rates higher than the maximum permitted by law.  Any person who receives such increased payments will have to forfeit all interest.  Under Section 44-1203, all payments of money or property made on account of usurious interest shall be deemed to be payments made towards the principal.  All such payments may be pleaded as a counterclaim or set-off.  If such payments exceed the amount of the principal, Defendant/debtor may be given a favorable judgment for the excess with interest at the rate of ten per cent per annum.  This is provided under Section 44-1204.

A.R.S. § 44-1201

Rate of interest for loan or indebtedness; interest on judgments
A. Interest on any loan, indebtedness, judgment or other obligation shall be at the rate of ten per cent per annum, unless a different rate is contracted for in writing, in which event any rate of interest may be agreed to.
B. Interest on a judgment on a condemnation proceeding, including interest which is payable pursuant to section 12-1123, subsection B, shall be payable as follows:
1. If instituted by a city or town, at the rate prescribed by section 9-409.
2. If instituted by a county, at the rate prescribed by section 11-269.04.
3. If instituted by the department of transportation, at the rate prescribed by section 28-7101.
4. If instituted by a county flood control district, a power district or an agricultural improvement district, at the rate prescribed by section 48-3628.
C. A judgment given on an agreement bearing a higher rate not in excess of the maximum permitted by law shall bear the rate of interest provided in the agreement, and it shall be specified in the judgment.

A.R.S. § 44-1202

Forfeiture of all interest upon obligation involving interest exceeding the maximum amount set by contract
A person shall not directly or indirectly take or receive in money, goods or things in action, or in any other way, any greater sum or any greater value for the loan or forbearance of any money, goods or things in action, than the maximum permitted by law. Any person, contracting for, reserving or receiving, directly or indirectly, any greater sum of value shall, forfeit all interest.

A.R.S. § 44-1203

Application to principal of payments made upon interest contracted in excess of the maximum permitted by law; judgment in action to recover obligation involving usurious interest limited to amount due on principal
Where a rate of interest greater than the maximum permitted by law is contracted for, reserved or received, directly or indirectly, all payments of money or property made on account of such interest, or as inducements to contract for more than the maximum permitted by law, whether made in advance or not, shall be deemed payments made on account of the principal. In an action brought to recover the amount of the obligation the court shall give judgment for no greater amount than the balance determined to be due upon the principal, without interest, after deducting such payments.

A.R.S. § 44-1204

Payments of interest on obligation involving more than the maximum permitted by law on counterclaim in action brought on obligation; action to recover payments of interest in excess of principal on usurious obligation
In an action to recover on an obligation whereby there is contracted for, reserved or taken, a greater rate of interest than the maximum permitted by law, all payments made in money or property may be pleaded as a counterclaim or set-off. If such payments exceed the amount of the principal, judgment may be given in favor of the defendant for the excess, with interest at the rate of ten per cent per annum. If such payments exceed the amount of the principal of the debt or obligation, an action may be maintained to recover the excess.