Reasonable price $18K for 5.94 kW?

Could be higher. could be lower. Depends on particulars of the installation and, most importantly, your goals for wanting PV.

1.) Get educated about PV, how it works and options available for panels and the rest of the equipment. Buy (~ $20-25 At bookstores/Amazon or download a free but slightly outdated version of "Solar Power Your Home for Dummies".

2.) After the self education and more information, use all that to get more quotes.

3.) Buying on low initial $$ alone is a fool's errand. A few pennies/W spent upfront to get quality work from a quality local vendor is money well spent.

Take what you want of the above. Scrap the rest.

It is better than the quote I got for similar system in Northern California in late 2017. Do you have any more to compare it to? Run the numbers on PVWATTS to see if it will cover your consumtion..

Prices here in Northern Arizona are below $2.25/watt.

Turnkey prices have dropped a bit around here (SDG & E) since 2017. I'd SWAG ~ 15 % or so over 2 yrs. Some or a lot of that may well be due to the mandatory switch to T.O.U. for new (NEM 2.0) cust. as well as the associated time shift of T.O.U. rates that came along about that time that made PV about 20-25 % less cost effective than it was under the old T.O.U. times as measured by the $$ offset value of the electricity produced by a system under the old times vs. the new times while holding the rates constant.

Savvy potential PV cust. know the POCOs are doing all possible to gut NEM making PV less cost. The larger the number of savvy customers who know how the gutting is taking place, the greater the downward pressure on turnkey system prices.

Is your cost (18k for 5.94 Kw) before or after the federal rebate? I'm trying to compare my cost per watt, which is about the same before rebate; after rebate is considerably different, obviously.
I had this question reading other posts on the forum (I just joined) and I didn't see any specification whether people are quoting price before or after rebates.

The common way folks around here have referenced prices are per STC watt before any mfg. rebates/B.S. gimmicks that often amount to showing inflated prices and larger "discounts" or reductions from "list" or "retail" etc. prices from peddlers that no one with an eye and a balloon knot would ever pay in the first place, and that amount to little more than B.S. ways to muddy the waters and screw the solar ignorant.

Around here, it's been mostly understood it's also the price before any gov. tax credits or other tax breaks of any sort which can and often are different from one situation/application/locality/country to the next.

Doing it that way helps avoid confusion and tends to keep everyone on the same page. Besides, and at least as applicable in the U.S. anyway, most of us know how to multiply by 0.70 (or 0.74 for after fed. tax credit system prices for systems started after 01/01/2020).

OK; thanks J.P.M. That clarifies the basis of the numbers.