Designing Grid Tie Systems

Recommended System Parameters

Key Components of a Grid Tie System (click to view enlarged image)

Choosing the size of your PV system (in KW) can be based on a number of factors. An important factor is your budget. Typical price of a Grid-Tie system can range from 6-10 $/Watt. In some locations, such as California, it may make financial sense to build a system that provides your peak-power requirements. If your location has a tiered pricing structure for your electricity consumption, you might want to consider generating your own power above a tier where the cost of electricity produced by your PV panels is equal to or less than the commercial rate of power. Start by estimating your annual usage and select a percentage of that to produce using your PV system. The Size of the PV system can be calculated by dividing the selected percentage of energy or energy consumption per day divided by the number of solar hours at your site. Enter that size of the system below before proceeding to the next step.

This system does not contain the battery backup pack and consequently does not need a Battery Charge controller either. This removes one of the key constraints in the design of the system i.e the DC side voltage which had to be chosen in the other systems based on available charge controllers and battery packs. This allows you to design a system with higher voltages than the other systems which in turn allows you to select smaller wires and other BOS components for your system.

Design the Series/Parallel connections of your PV Array. It is generally recommended to design the array with a DC side voltage of about 240 Volts and higher but you can choose any value from 48V and above for your system. You could choose a lower voltage but it is not recommended.

DC Nominal Voltage

The First Key Component we need to design and select are the PV Panels. We need to Select PV Panels that will generate enough power as defined above. There are essentially 3 different types of PV Panels, Crystalline Silicon, Amorphous Silicon and other Thin Film technology PV Panels. Crystalline Silicon panels are the oldest, most reliable and highest efficiency PV panels in the market today. The one drawback of Crystalline panles is that their properties are effected the most by temparature and their Power producing capacity decreases with increase in panel temparature. As a rule of thumb the PV Panel temperate is generally 25-40 degrees Centigrate above ambient temparature. Amorphous silicon and other thin-film technology panels are less efficient but have better temparature sensitivity and either do not loose power production capacity or sometimes improve their efficiency under higher temparatures. More information on PV Panels can be found under the section "PV Panels"

PV Panels

The First Key Component we need to design are the PV Panels. We need to Select PV Panels that will generate enough power to keep the loads we have defined in the previous section running for the defined Autonomous period. To read about the types of PV Panels and the factors influencing its design, please click here.

Invertor

The last key component we need to design and select is the invertor. Invertors convert DC electricity to AC Electricity which can be used to support AC Loads. Most of our common household equipment is designed to run on AC power. Invertors are mainly classified as Grid-Tie ( Support Islanding ) or Stand Alone Invertors. All Grid-Tie systems must use an approved Grid-Tie invertor before connecting the system to the main power lines. Selecting the right Invertor is a key decision in the overall system and the right Invertor depends on many factors, most notably the Run Watts and the Surge Watts. For this calculation we are going to assume a Surge load capacity at 1.5 * PV Power rating of the system.

BOS components needed for the system: DC side Fuse, AC side Fuse and wiring. All wiring calculations done below will be for a 2% loss of voltage and standard wire temparature of 75 degrees Farenheit. Also an important factor for determining wire sizes is the distance or length of the wire which you will need to calculate or estimate before using the calculations below.