Expanding later can cost you the rate you locked
The APS Resource Comparison Proxy is a rate rider available to partial requirements residential customers with qualified on-site solar generation, and customers lock in an initial RCP purchase rate for ten years. A material increase in capacity ends eligibility for that initial rate.
The usual approach to solar sizing is to start modestly and add panels later if you need them. Under this arrangement that approach carries a risk it does not carry elsewhere, because the addition may cost you the export rate you locked for a decade.
So the sensible sequence is reversed. Work out what your household is likely to need over the next several years first, then size once, rather than sizing for today and planning to revisit it.
Ask APS directly what counts as a material increase in capacity, and get the answer in writing before you finalise a design. That is a specific question with a specific answer, and it is the kind of thing that is much cheaper to ask now than to discover later.
Size for the household you are going to have
Think through the changes that would raise your consumption. An electric vehicle is the largest single one, and it can nearly change a household's electricity profile on its own. A pool pump, a casita, a home office, a heat pump replacing gas heating or an additional occupant all move the number too.
Tell your installer about anything you consider likely rather than only what is certain. Designing headroom into a system at the outset is straightforward. Adding it afterwards may not be, and under this arrangement it may be expensive in a way that has nothing to do with the cost of the panels.
Balance that against the other Arizona reality: exported solar is credited below the retail rate, so building a large system that exports heavily is not free money either. The goal is a system matched to the consumption you will actually have, not the largest one your roof allows.
Ask for models at two or three sizes, with the self-consumed share of production shown for each. Seeing where the return stops improving with size is the clearest way to make this decision.
The date you connect sets the rate you keep
Because customers lock in an initial RCP purchase rate for ten years, and the rate applied is the one in effect at interconnection, the timing of a project has consequences that outlast almost every other decision in it.
Ask APS what the current RCP purchase rate is, when the next change takes effect, and what rate a system interconnecting on your expected date would lock. Ask for it in writing, then hold your installer to a timeline that reflects it.
Ask your installer what happens if the schedule slips past a change: is the quote re-modelled, and who absorbs the difference. This is a foreseeable event in Arizona rather than bad luck, and a serious installer will have a position on it.
APS meters export energy on an instantaneous basis and provides monthly bill credits based on the purchase rate in the schedule. Instantaneous metering means energy your home is not using at that moment is exported and credited at the export rate rather than banked at retail value, which is another reason self-consumption matters more than size.
The state credit you claim yourself, and the federal one that ended
Arizona still has a state income tax credit of its own, claimed on Arizona Form 310, Credit for Solar Energy Devices. It is calculated by multiplying the cost of a solar energy device by 25 percent, capped at $1,000, and the form provides for tracking credit carryover across multiple years.
Ask a tax advisor how the cap and the carryover apply to your situation before you rely on a figure in a quote. A credit is only worth what you can actually use against tax owed, and an installer is not the right party to advise you on that.
Arizona also exempts solar equipment from state sales tax and excludes it from property tax, so the improvement does not raise your property tax bill the way a renovation of similar cost would. Neither arrives as a cheque, which is exactly why they get left out of people's own arithmetic.
The federal position has changed and much published material has not caught up. The 30 percent Residential Clean Energy Credit under Section 25D expired for property placed in service after December 31, 2025, so a cash or loan purchase now does not receive it. Section 48E survives at 30 percent for third-party owners under leases and power purchase agreements, so such a provider may claim it and reflect part of the value in the rate offered. Ask what they claim and what actually reaches you, and confirm with a tax advisor.