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PV Sizing Info

The first step to sizing a PV system is to determine the Watt hours (Wh) of electricity you use per day. You can determine the wattage of your appliances by looking at the device for a label that gives you watts consumed. If it does not tell you the watts, it will tell you the ampere draw and input voltage, and simply multiply these together to get the Watts it consumes.

Next, you need to determine how long each device runs every day. For some devices you can just use a timer. Another good way of determining Wh for AC devices is a nifty little product (one popular version is known as the KillaWatt) that plugs inline with the device to tell you how many hours a day it runs and how many watts it draws.

It is also good to keep in mind that every device that draws through an AC power inverter will have losses; typically 10-20% of the energy is lost through the inverter inefficiencies. So for those of you living off the grid, you should try to use as many DC devices as possible. To get a total on how much energy you use, make a chart like in fig A and fig B which separates AC and DC loads (if you plan on using both).

Solar sizing chart

Solar power chart

Now that you have determined the Wh you use, you need to do some calculations.

1. (Note: if you are doing a grid-tied solar system without a battery bank, skip ahead to step 3.) Since you will be you be using an inverter, you need to account for the extra energy it uses, so divide your total AC load by your inverter efficiency, and then add your DC watt hours (if you have any) to determine the total.

Example below:

Inverter 87% efficient
AC loads 525 Wh
525/0.87=603.44 or 604Wh
604 AC Wh + 478.5 DC Wh=1082.5 Wh or 1.1 kWh

2. With all battery banks, there are charging and discharging losses to take into account. This lowers the efficiency to typically 80%, so divide your result from step 1 by 0.80.

Example below:


3. With your total Wh determined, you can now calculate how many Watts of PV you will need. Find out how many full sun hours you get in your area in the worst case scenario such as winter (this information is available online), and then divide the Wh by these sun hours.

Example below:

1353.12Wh/5.5=246.02 watts, rounded up to the nearest tens place 250 Watts.

4. So now you need to decide how many PV panels you will need to fulfill the electrical needs of your home. In this case, I have chosen 60 watt amorphous thin film panels. These modules are produce 18.5 volts open circuit voltage and I will be using them on a 12 volt system. Since the panel voltage is higher than the system voltage, adequately charging the batteries will be a little easier. PV modules are available in many different voltages and can be arranged in different series/parallel configurations for to achieve the desired current and voltage. For our own system we need 250 Watts, so we divide that number by 60 (our module Watts) and round up to determine that we will need 5 modules, resulting in a 300 Watt PV array.



The above information is provided free as a guide and teaching tool.  If you need further assistance, please consider utilizing our consultation services.