How to size your photovoltaic system?

When choosing equipment to meet your power needs, you will need to determine the following, at a minimum:

• The number and type of solar panels required to capture enough solar energy to support your load.

• The minimum capacity of the battery. The battery will need to store enough

energy to provide power at night and through days with little sun, and will

determine your number of days of autonomy.

• The characteristics of all other components (the regulator, wiring, etc.)

needed to support the amount of power generated and stored.

System sizing calculations are important, because unless the system components are balanced, energy (and ultimately, money) is wasted.

For example, if we install more solar panels to produce more energy, the batteries should have enough capacity to store the additional energy produced. If the bank of batteries is too small and the load is not using the energy as it is generated, then energy must be thrown away.

A regulator of a smaller amperage than needed, or one single cable that is too small, can be a cause of failure (or even fire) and render the installation unusable.

Never forget that the ability of the photovoltaic energy to produce and store

electrical energy is limited. Accidentally leaving on a light bulb during the

day can easily drain your reserves before nighttime, at which point no additional power will be available. The availability of "fuel" for photovoltaic systems (i.e. solar radiation) can be difficult to predict. In fact, it is never possible to be absolutely sure that a standalone system is going to be able to

provide the necessary energy at any particular moment.

Solar systems are designed for a certain consumption, and if the user exceeds the planned limits the provision of energy will fail.

The design method that we propose consists of considering the energy requirements, and based on them to calculate a system that works for the

maximum amount of time so it is as reliable as possible.

Of course, if more panels and batteries are installed, more energy will be able to be collected and stored. This increase of reliability will also have an increase in cost.

In some photovoltaic installations (such as the provision of energy for telecommunications equipment on a network backbone) the reliability factor is

more important that the cost. In a client installation, low cost is likely going to

be a the most important factor. Finding a balance between cost and reliability

is not a easy task, but whatever your situation, you should be able to determine what it is expected from your design choices, and at what price.

The method we will use for sizing the system is known as the method of

the worst month.

We simply calculate the dimensions of the standalone system so it will work in the month in which the demand for energy is greatest with respect to the available solar energy. It is the worst month of the year, as this month with have the largest ratio of demanded energy to available energy.

Using this method, reliability is taken into consideration by fixing the maximum number of days that the system can work without receiving solar radiation (that is, when all consumption is made solely at the expense of the energy stored in the battery.) This is known as the maximum number of days of autonomy (N), and can be thought of as the number of consecutive

cloudy days when the panels do not collect any significant amount of energy.

When choosing N, it is necessary to know the climatology of the place, as well

as the economic and social relevance of the installation. Will it be used to illuminate houses, a hospital, a factory, for a radio link, or for some other application?

Remember that as N increases, so does the investment in equipment

and maintenance. It is also important to evaluate all possible logistical costs of equipment replacement. It is not the same to change a discharged battery from an installation in the middle of a city versus one at the top a telecommunication tower that is several hours or days of walking distance.

Fixing the value of N it is not an easy task as there are many factors involved,

and many of them cannot be evaluated easily. Your experience will

play an important role in this part of the system sizing. One commonly used

value for critical telecommunications equipment is N = 5, whereas for low

cost client equipment it is possible to reduce the autonomy to N = 3.