OK, so I understand getting a bigger panel, but as a solar n00b, does the charge controller convert the 24V from the panel to the 12V required by the house battery system or are you using a 24V house system?
"12v", "24v", etc. are what are called "nominal voltage". Those numbers basically indicate a range, not an exact voltage.
In order for power to flow from the solar to the battery, the solar has to have a have a voltage (you can think of voltage like pressure) higher than the battery. So most "12v" solar panels actually put out around 17v - 21v.
To charge a "24v" battery, you have to get the battery up to 28.8v, and so a "24v" solar panel will actually put out over 32v.
Thus, any PV module (solar panel) that puts out under 32v or so would be considered a "12v" panel since it's voltage is too low to be used on a "24v" system.
Only one type of solar charge controller does any voltage conversion - MPPT.
Normal charge controllers, and also the newer technology (but not as new as MPPT) "PWM" controllers just connect the solar panel straight through to the battery and then cut off when the battery gets up to a certain voltage. Unlike a "regular old charge controller", which just cuts off when the battery is full and then cuts in again when the battery voltage drops to a certain point, PWM will "pulse" many times a second, checking the battery voltage and giving it a little shot whenever needed to keep it topped off.
The solar panel voltage has to be somewhat higher than the battery voltage for the power to flow toward the battery, but the bigger the difference, the less the efficiency - so a say, 85w panel that puts out 17v will charge a "12v" battery more efficiently than an 85w panel that puts out 30v.
MPPT functions like a transmission - it can down convert the higher voltage from a solar panel to a lower voltage which can more efficiently charge a battery. The neat trick about that, is if you have the same watts, but lower the volts, the amps goes up. It's a bit like downshifting a tranny to get more torque.
However, MPPT controllers (at least the good ones) aren't cheap, and they only get you maybe a 10% (or less) bump in amps on average, so for a small system - usually not worth the expense.
In reality, most of my usage comes at night. I'm generally out and about during the day (hiking, moto-ing, whatever). I basically want to get a full charge on my two batteries during the day. I'm in Southern Cali and most trips are to the desert or beach. Open sun. I'm looking to have the panel removable for mtn trips but that's neither here nor there.
For a flat mount panel in SoCal, I'd figure on 5 hours (on average) of "good sun" per day. So, 85w * 5 hours = 425 watt*hours per day of harvest.
Divide that by the voltage, 425wh / 13v = 32.6 amp*hours per day.
So that's a decent guess as to your average harvest - 32ah/day. Now as long as your loads don't suck more than 32ah out of the battery in 24 hours, an 85w panel should be able to keep up with the demand.
You could get a couple more hours of harvest per day if you put the panel on a tilt mount and parked so it pointed at the sun, or detached it and set it facing the sun.
Also keep in mind when figuring out the mounting that -any- shade on a solar panel, even a small shadow, will cut the panel's output down a LOT...often to pretty near nothing.