More Batteries

In general, I do not recommend UPS systems for use as off grid inverters... Many are MSW (modified square/sine wave) which can be hard on motors/electronic power supplies (something like 80% of loads may work fine, 10% may fail in hours/days/weeks, and another 10%--Who knows). And they are usually designed for 30 minutes of use or so (just long enough to ride through a short power outage and/or shutdown a computer system).

A small inverter (only available in 12 volts) that could work well for your needs (plus has remote on/off and a "sleep mode") is the MorningStar 300 watt 12 volt (120 VAC 60 Hz and 230 VAC 50 Hz models available) PSW (pure sine wave). If you can find something similar locally (this inverter may be hard/expensive for your to get)--I would suggest over a UPS.

http://www.morningstarcorp.com/products/suresine/

So--Some basic math. I will use 12 volt battery bank, but a 24 volt battery bank will give the same results (i.e., 12 volt @ 400 AH battery bank is the "same" as a 24 volt @ 200 AH battery bank--just different series/parallel connections, and/or different choice of battery voltage/capacity).

300 Watts for 12 hours with 2 days of storage (poor sun) and 50% maximum discharge seems to be a good "optimum" bank design for use with lead acid batteries. Deep cycle storage batteries should have 5% to 13% rate of charge typically. With 5% good for weekend/seasonal use. And 10%+ recommended for full time off grid use (better battery life, less day to day load management vs available sun):

• 300 Watt load * 6 hours per day night usage * 1/0.85 AC inverter eff * 1/0.77 solar panel+controller deratings * 2 days storage * 1/0.50 maximum discharge * 1/12 volt battery bank = 916 AH @ 12 volt battery bank
• 916 AH * 12 volts / 24 volts = 458 AH @ 24 volt battery bank

Now--I would suggest that over 800 AH battery bank should be converted to the next higher voltage--I.e., 458 AH @ 24 volts in this case... Either can work, but the 24 volt would be a better choice (less current, smaller battery / DC bus wiring). But, you need to find the inverter and charge controller needs to meet your needs (features/cost/availability). Anyway--Continuing on the math...

Next need to size the solar array based on two different calculations. First is based on battery bank capacity (5% to 13% rate of charge, base on 20 Hour battery rated Amp*Hour capacity). Using the above calculation:

• 916 AH * 14.5 volts charging * 1/0.77 solar panel+controller deratings * 0.05 rate of charge = 862 Watt array minimum
  
• 916 AH * 14.5 volts charging * 1/0.77 solar panel+controller deratings * 0.10 rate of charge = 1,724 Watt array nominal
  
• 916 AH * 14.5 volts charging * 1/0.77 solar panel+controller deratings * 0.13 rate of charge = 2,242 Watt array "cost effective" maximum

And then there is sizing the array based on your daily loads. For your region:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Lahore
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 58° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.72	5.66	6.16	6.50	6.68	6.34
Jul	Aug	Sep	Oct	Nov	Dec
5.52	5.46	6.00	6.43	5.73	4.91

Based on 300 Watts * 6 hours per day or 1,800 WH per day and a minimum and 4.71 hours per day (January long term 20 year average):

• 1,800 Watt*Hours * 1/0.52 off grid system eff * 1/4.72 hours of sun per day (noontime equivalent full sun) = 733 Watt minimum array
  

Because you have a lot of sun in your area--Your minimum array should be at least 862 Watts minimum if you charge during the day (mostly) and run most of your power at night to ensure that the battery bank gets a minimum rate of charge.

If you will run more power during the day (say water pumping, laptop computer, etc.)--Then you may want to get towards 1,724 Watt array.

The above would be a "nominal" designed system with lead acid batteries for full time off grid. If the system is only used for weekends or a week or two at a time--You might be able to get away with a smaller battery bank (1/2 the AH ratings I calculated)...

-Bill