Using a 18v solar panel on a 12v system

Voc~21 volts and Vmp~18 vols is the nominal voltage (standard test conditions/marketing speck) for "12 volt panels" charging a 12 volt battery bank with a PWM solar charge controller (sounds like what you have).

Using a 400 Watt array (Imp~20-22 amps), you want to charge the battery bank at ~ 5% to 13% rate of charge... 5% is good for weekend/summer/sunny weather cabin. 10%+ is better for full time off grid system (9 months a year or more occupation)...

• 400 watts * 0.77 solar panel+controller derating * 1/14.5 volts charging * 1/0.05 rate of charge = 425 AH @ 12 volt battery bank maximum size for weekend/summer usage
• 400 watts * 0.77 solar panel+controller derating * 1/14.5 volts charging * 1/0.10 rate of charge = 212 AH @ 12 volt battery bank "nominal"/full time off grid
• 400 watts * 0.77 solar panel+controller derating * 1/14.5 volts charging * 1/0.13 rate of charge =163 AH @ 12 volt battery bank "typical minimum" suggested battery capacity

A pair of 6 volt @ ~200 AH "Golf Cart" batteries in series (for 12 volts @ 200 AH) would be a nice bank for this system... If mostly summer use (and/or you use a backup genset for bad/winter weather), 2x strings of these batteries (4x total) in series parallel would give you 12 volts @ ~400 AH.

If you want to "reliably" use the 500 Watt output of this Inverter on your battery bank, I would suggest (~250 watts per 100 AH @ 12 volts) a minimum battery bank capacity of:

• 500 Watt inverter * 1/250 Watts per 100 AH (at 12 volts) for flooded cell lead acid batteries = 200 AH @ 12 volt minimum FLA battery bank for this inverter.

Assuming you are in/around Mumbai India, fixed array, 400 Watts:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Mumbai
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 71° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
6.54	7.17	7.42	7.14	6.69	5.63
Jul	Aug	Sep	Oct	Nov	Dec
4.94	4.85	5.69	6.29	6.46	5.98

You have lots of sun, relatively speaking... The long term average for your "worst month" is ~4.85 hours of sun per day:

• 400 Watt array * 0.52 off grid system eff * 4.85 hours of sun average (August) = 1,009 Wh per average August day (some days/weeks more, some less depending on the weather patterns at that time).

Lots of guesses and assumptions... Please ask your questions and correct my guesses.

-Bill