Sizing a PV array

With regard sizing a grid-tie system, it is adequate to just consider the space you have available and the money you want to spend,  as you won’t be restricted by the capacity of the solar system. However, by calculating your power usage, you will be able to estimate the percentage of electricity you will save, and better understand how your electricity is being used.
 
You can estimate the amount of power you require by adding up all the power outputs of the electrical products in your home, Information regarding power consumption is generally indicated on a label fixed to the back of the appliance, if it is not, you may need to do some research on the web. The power rating is given in Watts, but for our purpose we would like to know the daily power use of the appliance, so multiply the power consumption with an estimate of the average number of hours the appliance is used per day, this gives us a figure in Watts hours. Some appliances, such as fridge / freezers, cycle on and off, so you will need to estimate the percentage of time the appliance is drawing current, this may vary depending on the circumstances, such as air temperature. Be aware also that start up power is often considerably higher than the constant power consumption, so add a margin, say 20% to allow for peak power. The combined total for all appliances should give you an indication of your total power use per day. 
 
let’s say you have the following appliances:

Consider also buying a buy a power monitor to attach to your electricity meter, from which you can read the total power you are consuming and you average daily consumption.
 
You will need to consider the average sun hours your panel will receive per day. This varies depending on the time of year and your location, so you will need to refer to local climate data to get an accurate forecast. For our purposes, lets take London as an example, during winter it is daylight for approximately 8 hours, during summer on the other hand, it is daylight for approximately 17 hours.  If you were to go entirely off-grid, you would need to work with the lower figure, but let's assume you just want to balence your use with your supply, then an average of 12.5 hours would be an appropriate starting point. Under current U.K. legislation, utility companies are obliged to pay for the electricity fed back into the grid by micro power generation customers, but still, on average, utility companies pay only a third to a half as much as they charge you, per KWh. However all that is set to change in April 2010, with a introduction of a feed in tariff, which is intended to guarantee a long term premium payment for electricity fed into the grid, dependant on the renewable technology employed. It is intended that the feed in tariff will be a major incentive to foster wide scale adaption of micro power generation, needed to achieve the current target of 20% EU renewable Energy by 2020.
 
We need to decide which panel we would like to use, we will choose a panel rated at 250 Watts. The rating however will be for a panel in bright direct sunlight, and in practice, cloud cover, sun angle and air quality will adversely affect the amount of light falling on the panel, and we need to make an allowance for this. Lets assume that over the course of the average day, the output is only 60% of the rated output
 
Out effective panels output is closer to 250 X 0.6 = 150W
In the space of the average day a single panel produces 150 x 12.5 = 1875 Wh
an allowance should be made for inverter inneficiencies, lets assume the efficiency of the inverter is 0.9. We will adjust the panel output to allow for this inefficiency like so 1875 x 0.9 = 1687
We can calculate the number of panels required in the array as follows
29140 / 1687 =  10.8 panels, We need 11 panels to balence our production with our power needs over the course of the year. That’s a lot of panels, time to cut down on your electricity consumption!
 
Follow on articles:
 
Mounting a PV array
Wiring a PV array