> What do these numbers all mean?  Anyone
> want to give me a quick bit of advice on this?

Don:

[I'll reply on-list since others are thinking of solar.]

Add the watts of your panels and get a controller that
has a rating that is higher.  In your case, 75+40 = 115,
so your second choice of 127.5 watts is enough.  It
doesn't hurt to go higher, just the cost is a little
more.

There are several technologies in solar controllers.  A
while back there was only pulse-width.  When the battery
got fully charged, the controller shut off/on to give the
batteries a pulse that kept them full.  I am still running
one of these (Blue Sky) from ~8 years ago in a high mtn
cabin PV light system.  Works....

Then MPPT came along.  As you saw, you can get 21 volts out
of a 12 V panel--sometimes.  Older controllers just took
12V and ignored the rest.  MPPT controller take the full
voltage from the controller and convert it to 12V at a
higher amp (or wattage) rating than the panel, so you get
more power out of the panel.  Most MPPT controller claim
~30% more power.  So MPPT is your friend.

I bought a 10 amp MPPT from ebay recently for ~$25.  Here
is the same thing for $19.  I figure you can't go wrong to
that price.  I have not used mine enough to say if it is a
good controller or not, but it is working.

http://www.ebay.com/itm/10A-15A-20A-30A-MPPT-Solar-Panel-Regulator-Charge-Controller-12V-24V-Auto-Switch-/310639824059?pt=LH_DefaultDomain_0&var=&hash=item4853938cbb

A good site to learn is windsun.com, part of Northern Arizona
Wind & Sun.  Here's a page on MPPT:
http://www.solar-electric.com/whatismppt.html

I'd say that with 115 watts, which is ~8+ amps, you should
have plenty of solar capabilites to keep whatever you want
charged and running--unless you are using an inverter to
run one of Terry Key's window AC units.  ;-)

Richard