Start from twelve months of your own bills
Ask every installer to work from your last twelve months of electricity bills rather than from a typical household profile. A large household with several occupants does not resemble an average, and a design built on an average will be the wrong size in one direction or the other.
Look at the shape as well as the total. When does your consumption peak, how much does it vary between summer and winter, and are there loads such as a pool pump, heavy air conditioning or vehicle charging that dominate particular months.
New Jersey credits net metering over the course of a year, so summer surplus carries toward winter rather than being lost monthly. That suits a household whose consumption varies, but it makes the annual total the number to design against rather than any single month.
Ask what limit applies to how large a residential system you can install at your address, and ask your electric distribution company directly rather than relying on an installer's assumption. Get it in writing before a design is finalised.
Why a larger system is less penalised here
In most states, producing more than you consume is penalised because exports are credited below the retail rate. New Jersey is different: net metering credits at full retail over the year, and the Administratively Determined Incentive pays on generation with one SREC-II created per 1,000 kilowatt hours.
So the usual argument for keeping a system small does not apply with the same force. Generation is paid for whether or not you consume it, which is a large part of why New Jersey pays back faster than its sun hours alone would suggest.
That said, the incentive is guaranteed for a term of 15 years, its Qualification Life, and a system lasts considerably longer. Ask what a projection assumes for the years after that term ends, because a larger system built substantially on incentive income has more of its case sitting inside those fifteen years.
Ask for models at two or three sizes with both revenue streams shown separately for each. Seeing where the return stops improving with size, and what happens to each size after year fifteen, is the clearest way to make the decision.
Solar on a newer home, or offered by a builder
If solar comes as part of a new home or a builder package, apply the same scrutiny you would to a standalone quote rather than treating it as a fixture. Ask who manufactured the equipment, the model numbers, who installed it and who honours each warranty.
Ask specifically whether the system is owned, financed, or subject to a lease or power purchase agreement, because that determines what you own and what a future buyer would inherit. It also determines who receives the incentive income over the fifteen-year term, which in New Jersey is a substantial sum.
Confirm the registration and the metering. Ask for the registration confirmation showing when the fifteen years started, and confirm that a separate Solar Production Meter is installed, since New Jersey states that one meter cannot serve both net metering and incentive purposes.
And confirm the exemptions were handled. Ask whether Form ST-4 was issued to the seller at purchase, and whether Form CRES has been filed with your municipal tax assessor. Neither happens automatically, builder-installed or otherwise.
Two exemptions, and neither one happens by itself
New Jersey exempts solar energy equipment from state sales tax, but the exemption has a procedure and it happens at purchase. Under N.J.A.C. 18:24-26.4 the purchaser must issue to the seller an Exempt Use Certificate, Form ST-4, or other approved form, indicating on its face that the purchase qualifies for exemption as a solar energy system, with the installation property address inserted.
Ask your installer how that is handled and confirm the certificate was issued rather than assuming the price you were quoted already reflects it. It goes to the seller as part of the transaction, not to the state on a return later.
The property tax exemption is separate. Qualifying renewable energy systems are exempt from real property taxation under N.J.S.A. 54:4-3.113a to g, but Form CRES, the Certification of Renewable Energy System, must be filed with your local municipal tax assessor, and the system must be certified by the local construction code official.
The annual exemption is the difference between the total assessed value of the property before and after the system has been installed. Nobody files Form CRES for you by default, so ask whether your installer assists and put it on your own list either way. Requirements vary between municipalities, so a short call to your own assessor asking what they need is worth more than any general guidance.