Ndungu after giving empirical data and then giving us something like "quite less" on the conclusive remarks I feel like looking for you and plucking hair off your head....
Assuming 5 hours of sun a day (its more but as the sun goes down or up power is not optimum)
1: You would get 30(watts) x 4 (panels) x 5 (hours) x 30 (days) ==> 18,000 Watt Hours in a Month. or 18 Kilowatt Hours...
2: At an exaggerated 20 bob per Kilowatt hour (its still far less even with taxes and all), you save Ksh 360 per month off your electricity bill.
Now to recover your costs, including NPV of money and all, it would take you __________ years to recoup your investments in Power savings.... (25 marks).
sigh <....>