How to calculate your own residential solar system.

There are two methods to calculate solar system for your residence or even for your business. Let us start with the simple way. First of all, you should have some idea about the consumption units of your house. It is simple mentioned in the electricity bill. Since I live in Sri Lanka, these calculations based on Sri Lankan Tariff methods. But any one can adopt this into your country tariff  categories.
Let's look at a example of average house with monthly consumption of 500 units.(ie 500kWh per month)
In sri lanka, domestic tariff is as follow.

If the consumption is between 0-60 kWh per month the following tariffs will be applicable

Monthly Consumption (1) kWh	Unit Charge (Rs./kWh)	Fixed Charge (Rs./month)
0-30	2.50	30.00
31-60	4.85	60.00

If the consumption is above 60 kWh per month the following tariffs will be applicable

Monthly Consumption (1) kWh	Unit charge (Rs/kWh)	Fixed charge (Rs/month)
0-60	7.85	N/A
61-90	10.00	90.00
91-120	27.75	480.00
121-180	32.00	480.00
>180	45.00	540.00

As you can see, the charge will be a considerable amount only after the 90 units. So we can use up to 90 units without a higher cost for electricity bill. So, if i can generate 410 units in each month and my monthly bill is 500 units, I spent a little amount of money for the bill. Considering the higher cost in solar panels and inverter, I decide to plan only for 410 units per month. Let's consider the first method.

Method 1.
Let's take number of solar panels as x.
let's consider a 320W poly-crystalline solar panel,
In this calculation we consider we only can have sunlight from 10.00 am to 2.00 pm (ie 4 hours per day) and we can only generate solar power in 25 days of a month for worst case scenario. In Sri Lanka, these values are greater than these and you can use them in accordance with your country.

Let us genetare a equation as follow for the  410kWh per month.

x * wattage of a solar panel * Number of hours per day * Total days per month = 410,000 Wh/month
x * 320w * 4h * 25days = 410,000 Wh/month
                                x  = 410,000/(320*4*25)
                                x  = 12.8125

This shows that we need 13 solar panels to achieve this target. Let's consider the Inverter capacity.
As we get, we need 13 numbers of 320W solar panels. That get,
320W * 13 = 4160W 

That is, we need 4.16 kW solar inverter. you can use any inverter greater than this value. 

Note: Even though we get the generation capacity of a solar panel as 320W, actual value will be less than this value due to the less efficiency, whether conditions(dust, mud), resistance of cables and many other factors. Actual value of generation is shown in the specification sheet of the panel. for this panel, it would be around 250W. So, considering the same equation we get the inverter power as,
250W * 13 = 3250W.
That mean even 3.5kW or 4 kW inverter will also be enough for the system. 

In this way we can design the solar system we need. As mentioned in the Note, the actual value may little less than the design value. so we can design it using actual wattage of the panel if you need. this is the basic method to calculate the system even for a person with less technical knowledge. I will explain an advance method in next post. please feel free to comment your ideas so that I can use it on my next post. Good day !!