Table of Contents
An Introduction to Solar Electricity
Modules and Array Mounting
The Independent Home as a Whole
Buying a PV System
Siting the PV System
Sizing a PV System
PV Home Wiring and Lighting
System Efficiency and Appliance Efficiency
Safe Safer Code
Maintenance of a PV System
Understanding the Electricity for Your PV System
Excerpt from Chapter 9
Sizing a PV System
A PV System must be large enough to produce enough watt-hrs to power a load. In the simplest situation one module that produces 50 watts will power one 50 watt light bulb for every hour that the sun shines. Most light bulbs are not used in the daytime but in the evening. Furthermore one light bulb is not matched to one module. Instead a number of modules charge a battery bank all day long and then this battery bank powers loads in the evening. If an array produces 1000 watt-hrs only 1000 watt-hrs are available to power the load. The specific load can be 1000 watts for one hour or 100 watts for ten hours. A battery bank is really a bank deposits equal withdrawals. The PV array steadily charges the batteries during the daylight while the load is withdrawn anytime whether in large wattages over a short time or in small wattages over a long time.
The first step in designing a PV system is to determine the daily usage in watt-hrs. Next an array is sized large enough to produce the same watt-hrs as the daily load and a battery bank is sized large enough to hold the daytime array output and return it at night to power the loads. For example a 100 watt-hr load is matched to a battery bank which stores 100 watt-hrs which is matched to a PV array which produces 100 watt-hrs. Unfortunately this example only works under the ideal condition of 100% efficiency. In reality the modules lose efficiency as they increase in temperature the batteries are only 80% efficient in their conversion of electrical energy to chemical energy to electrical energy and an inverter has a 15% loss of energy for any AC load.
In summer days are long and nights are short. This reverses in winter. When days are long the PV array receives more light and produces more electricity. When days are short less PV electricity is produced and the longer nights result in greater loads due to increased lighting demands. Clear weather is less common in winter further reducing the average insulation available.
In the appendix of this book you will find an appliance list to aid you in the wattage requirements of the most common appliances to be used in a remote site PV home. Also in the appendix you will find a blank worksheet for sizing your PV system. The first section of this worksheet helps you estimate you daily loads. Photocopy the blank worksheet for you own personal worksheets. In the following pages of this chapter " you will find the above mentioned PV system worksheet filled out with the hypothetical system we will design.
To accurately size a PV system for a remote site home" it is necessary first to determine the loads. Your loads will change throughout the year. If you will have no backup charging system and all loads are essential you should fill out the load section of the worksheet for the worst case month of winter when the least average sunlight per day is available and the greatest loads are needed.
If you have a backup charging system you may fill out the worksheet with load estimates for the average use of the year. During the worst part of the year additional loads can be supplied by the backup. It is often helpful to fill out the worksheet for several scenarios. After pricing a PV system for each case you will be able to make a well-informed decision.
Copyright permission by New England Solar Electric Inc.