Solar Power
Solar Power

Sizing PV-System For Domestic Use


By: Kwabena Osei

I always find it difficult to answer people the question: HOW MUCH DOES PV-SYSTEM COST? The cost of PV-systems depends on lots of factors. I hope my reasons for not being able to answer this question will be clarified after reading this article.


As I mentioned in my last article, a PV-system churns out electricity so long as the sun shines. This means the amount of electricity a PV-system produces depends on the area the system is to be installed. The photovoltaic battery (the solar panels) can continue to provide voltage and current as long as light continues to fall on the two materials. The longer the period the sun shines the more the amount of electricity one could get out of its PV-system.

PV modules, because of their electrical properties, produce direct rather than alternating current (ac). Direct current (dc) is electric current that flows in a single direction. Many simple devices, such as those that run on batteries, use direct current. Alternating current, in contrast, is electric current that reverses its direction at regular intervals. This is the type of electricity provided by utilities and required to run most modern appliances and electronic devices. Most appliances use direct current and manufactures of these appliances install transformers to convert the direct current into alternative current. The reason is that from a hydro- plant one needs to transport power at long distances and when electricity is being transported at these long distances they should be at high voltage and low current. But PV systems produces electricity at low voltage at high current, so the need form that conversion / compensation.

Storing electrical energy makes PV systems a reliable source of electric power day and night, rain or shine. PV systems with battery storage are being used all over the world to power lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools.


PV systems with batteries can be designed to power dc or ac equipment. People who want to run conventional ac equipment add a power conditioning device called an inverter between the batteries and the load. Although a small amount of energy is lost in converting dc to ac, an inverter makes PV-generated electricity behave like utility power to operate everyday ac appliances, lights, and even computers.

?Sizing? a PV-system means determining how much electric energy is required (e.g. in the household or powering a cabin) and how many solar modules are needed to generate this energy as well as the size of the batteries needed.

There are several methods used in sizing PV-systems. These methods are mostly based on either deterministic or statistical approach. They simply consist of estimating the average area solar radiation flux over the year and the number of sunless days likely to occur each month in that area. Knowing the consumption requirements, the number of modules and the storage capacity can be determined.


To determine the sizes of a plant, one should have knowledge of the solar radiation of the area the system is going to be installed. This gives an idea of the amount of solar energy available to be converted into electricity. Radiation data for a particular site are normally given in the form of global radiation on a horizontal surface. It is therefore necessary to translate this information into the radiation received by the panel at an inclined orientation.


A solar system must provide enough energy to replace that being consumed daily by the loads (lights, appliances, equipment), plus some to compensate for energy used by the system itself. Over-sizing the system has a very negative effect on the prices of the solar electricity. Under-sizing a stand-alone, on the other hand, reduces the supply reliability.


To do your system sizing, here are the two main things you will need to know:


1. How much electricity will you be using
You calculate this for every load and then add the results together. Electrical use is figured in terms of ‘Watt-hours’. This is the appliance’s power rating (Watts) multiplied by the average amount of time (hours) it operates daily.

In addition to the electricity used by appliances, the system itself also consumes some power. For example: you do not get back all the energy you put into a battery; an inverter uses some energy to convert electricity from DC to AC; and voltage is lost as electricity travels through wires. A factor to allow for these is including in the sizing instructions.


2. How much electricity will be produced by a solar module
You estimate this by multiplying a module’s power rating (Watts) by the ‘Area Factor’ from the map. This gives you the typical Watt-hours produced per day by a solar module at an average location in the area.

Use the sizing instructions as a guide in calculating these figures. Follow the directions and you will get a reasonable estimate of how many solar modules and batteries you will need.

Please keep in mind that every solar installation is unique and is affected by many factors, such as:


Local weather patterns: these can vary greatly even within a small geographic area.

Installation: even brief shadowing affects a module’s daily output and modules that are not oriented properly toward the sun will produce less power.

Seasonal changes: this estimate is based on a yearly average. The energy produced in winter will be less than average and in summer it will be more than average.


The number of modules will vary based on these factors. You may also desire back-up battery capacity and may want to anticipate future needs by having additional solar power available now.

Energy Conservation
As you review your energy needs, remember the importance of using energy-efficient equipment and appliances. For instance, using a 20W fluorescent light for three hours a day, rather than a 75W incandescent light, will give you the same amount of light and save you over 60,000Wh of energy during the course of a single year. This is generating capacity you do not have to buy or power you can use for other purposes.


Energy accumulation
Solar systems produce energy whenever there is sunlight. A 50W module may produce 1,500Wh of energy in a week. If you are powering a cabin that you only visit on weekends, just a couple of modules may give you 3,000Wh of power for your visit. This may be plenty of energy to meet your needs – and all from just a 100W system.


Batteries are a major component in solar systems. A number of different types and capacities are available. The best battery for your system depends on many factors and often requires analysis and advice from a solar energy professional. Many small to medium sized systems can use photovoltaic or marine grade batteries. These are designed to be deep-cycled (discharge-recharge) many times and are generally maintenance free. They are available in capacities of about 120 Amp-hours.


Batteries must be able to store enough energy for daily operations. A reserve should be considered to have additional capacity to operate the loads during anticipated periods of cloudy, sunless weather. This reserve capacity is referred to as system ‘autonomy’ and is rated in days. The amount of autonomy needed varies. For critical loads such as telecommunications you may want 10 or more days of autonomy, for a residence perhaps five days, and only a day or two for a vacation cabin.


Of course, solar design can be easy, just call one of the qualified Siemens Solar System Integrators.


Solar modules should be installed at the correct ’tilt-angle’ to achieve the best year-round performance. Generally, this is an angle equal to the site’s latitude plus 20?, with modules facing south in the northern latitudes and north in the southern latitudes.

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