Echoing Prosperity somewhat, I lived on 12 volts for 11 years aboard a sailboat. You learn to compromise--or do without.
The typical 12 volt florescent is rated at eight (8) watts. They are as dim as you might imagine. While higher wattage 12 volt fixtures are available, this should give you some idea of how little power is considered "normal" in the business of living on 12 volts.
Even if your power requirement has halved to 160 watts, and you will use them for up to 12 hours per weekend, you still will need a good sized battery bank and solar array, and you will still need a regulator. The good news is that your array and battery bank will now cost about half as many thousands as before. The inverter will be a little less, while the regulator will probably be about the same.
Of course, if you still intend to run all the lights at some times, then you will still need the larger inverter.
I didn't mention it last time, but most inverters don't put out true AC. They put out a stepped waveform (modified sine wave) that approximates a true sine wave (like the power company). A lot of things run fine on modified sine wave power. Some things don't. I blew up a computer that didn't like it.
See
http://www.xantrex.com/support/docserve.asp?id=336
I don't know what AC florescents will do on a modified sinewave. They may work fine, or they may hum a little or a lot, or be less efficient. It is something you would have to determine before you proceeded. Hopefully, you wouldn't need to go to a true sinewave inverter, as they are much more expensive.
Regarding your existing deep cycle batteries. Assuming they are all 12 volt units, and you are installing a 12 volt solar array and a 12 volt to 120 volt inverter, then you would hook them in parallel. If you hooked them in series, you would need the rest of the equipment to work with 48 volts. It's available, but tends to be quite a bit higher priced. The solar panels themselves would not change, just how they were wired.
By the way, when paralleling batteries, they are supposed to be all the same size (capacity), age, and condition.
</font><font color="blue" class="small">( I haven't seen a UPS that connects to deep cycles,any suggestion on who sells them? )</font>
An UPS is an inverter, charger, and battery combination designed to provide
temporary AC power for a short period. For what you want to do, you need an inverter designed to provide power indefinately. Here's one that should work:
http://www.realgoods.com/renew/shop/product.cfm?dp=1702&ts=3027879&kw=inverter
But I still don't understand why you can't run AC to the garage. Normally you would go underground in conduit from your main house panel to a sub-panel in the garage. You would need to install the proper size double pole breaker in your house panel, run the correct wire in the conduit, and install an adequate sub-panel in the garage.
The sizing of all this would depend on exactly what you planned to do with the AC out there, beside powering 320 watts of light. But no matter what, I'm sure it would cost less than going the solar route, and there is a
lot of expertise here on TBN available to help you with any AC power work you plan to do.
SnowRidge