Resources | Solar PV system design primer

 

This page provides an overview of residential solar PV system design.  It is intended for people who want to better understand the PV system design process  or might consider designing a solar PV system themselves.

The actual sizing of PV systems is outside the scope of this page.  Further reading and/or training are recommended for those seeking to design a PV system for their own use or pursue solar PV system design/installation as a career.

The general steps to design a solar PV system are:

1.  Inspecting the location

2.  Deciding what type of PV system to install

3.  Gathering electrical information

4.  Determining how much electricity you want your PV system to generate

5.  Sizing the system

 

1.         Inspecting property

This step is often referred to as site survey.  The purpose of this step is to find out whether a location is suitable for a PV system.  There are tools and computer software available to assist you, like Solar Pathfinder, http://www.solarpathfinder.com, or SunEye, http://www.solmetric.com.  Here are some items to look for:

•         Local zoning and subdivision ordinance provisions

You should check your township or city office. If you live in a subdivision, make sure there are no provisions in the ordinance for solar PV systems.

•          South facing roof or vacant lot space to mount solar panels

You need enough south facing roof (change to north if in Southern Hemisphere) or vacant log space where you can mount solar panels.  If you plan to mount the solar panels on the roof (roof mount), make sure the roof is structurally sound.  If you are not sure, hire a structure engineer to examine it.  If you plan to mount the solar panels on the vacant lot (pole or rack mount), make sure you know the ground type, like sandy, clay, etc.

•          Latitude of the location

Latitude, in general, determines how much sunlight the location receives during the day (“insolation”).  It also gives you an idea the best tilt angle for the solar panels if you would want to maximize your PV system performance.  Typically, the tilt angle for the solar panels is left at latitude for spring and autumn.  For example, if your latitude is 30 degrees, the tilt angle will be 30 degrees.  The tilt angle for the solar panels should be latitude minus 15 degrees in summer; whereas the tilt angle for the solar panels should be latitude plus 15 degrees in winter.

Roof mount solar panels may not require tilting since most roofs already have a pitch angle.  A 6/12 pitch roof is about 27 degrees, an 8/12 pitch roof is about 34 degrees and a 12/12 pitch roof is 45 degrees.

•          Shade around the property

Solar PV panels require direct sunlight to be effective.  Shade from trees, buildings, utility poles and other objects can hinder the performance of a PV system dramatically.  Shade is also affected by the location’s latitude (see above) and seasons.  Shade is longest during winter and shortest during summer in Northern Hemisphere.

2.         Deciding what type of PV system to install

After you have inspected your property, you can begin to determine what type of solar PV system to install.  The three general types to choose from are stand-alone, grid-interactive and grid-interactive with battery backup.

A stand-alone (or off-grid) PV system allows you to run all your electricity from your PV system without using utility power.  Generally, stand-alone PV systems are suitable for houses or structures where utility power is not readily available.  This type of system normally has other renewable energy sources or a generator as backup.

A grid-interactive system allows you to provide all or part of your electricity need from solar PV system.  If the PV system only provides part of your electricity need, the rest is provided by the utility company.  This is the most popular PV system type in urban areas.  One drawback of a grid-interactive PV system is that when the utility goes out, the PV system also goes out even if the solar panels are working.

A grid-interactive with battery backup PV system is similar to grid-interactive but has a battery bank attached to the system that acts as a backup power source when utility power is not available.  Typically, a subpanel connects critical appliances like refrigerator, water pumps and emergency lighting to the battery bank.

Of the three systems, grid-interactive PV systems are the simplest and least expensive; stand-alone PV systems offer more options at greater expense; and grid-interactive with battery backup PV systems are flexible yet are the most expensive and complex.  In addition, grid-interactive with battery backup PV systems are less efficient than pure grid-interactive systems.

3.         Gathering electrical information

After you have chosen the type of PV system to design, you need to gather electrical usage information for the location.  Different PV systems require different information:

•          Stand-alone: List of appliances with power consumption information

•          Grid-interactive: Annual electrical power usage

•          Appliance name

•          Voltage type, i.e. AC or DC (rare)

•          Quantity

•          Volt (e.g. 120V or 240V for AC and 12V or 24V for DC)

•          Amp (e.g. 0.5A, 6A or…)

•          Power = volt x amp

•          Amount of time used each day

•          Number of days used each week

Name	Qty	AC/DC	V	A	Power	Hr/Day	Day/Wk	Usage
PC	1	AC	120	0.60	72	8.00	7	576
CFL	10	AC	120	0.11	13	10.00	7	1300
Dishwasher	1	AC	120	5.20	624	1.50	2	267
Microwave	1	AC	120	8.30	1000	0.25	7	250
Fridge	1	AC	120	1.00	120	10.00	7	1200
Total								3593

4.         Determining how much electricity you want your PV system to generate

Once you know your average daily power usage, you need to determine how much power (as a percentage) you want your PV system to generate.  This step is mainly for grid-interactive and grid-interactive with battery backup PV systems.  For a stand-alone PV system, keep in mind that the system will need to generate 100% of electricity you need.

Using the example in Step 3, for a grid-interactive PV system with an average daily power usage of 16.44KWh requiring 50% of energy from a PV system, you will want to generate 8.22KWh.

5.         Sizing the system