How to size a Solar panel system

How to size a solar panel system for your house is a difficult decision to make with a lot off variations. Your search for the most efficient PV system what is covering as much of the electrical bill costs.

Information is not always clear, and residential solar installers like to sell you often more then you need. It is possible because you do not have the information you need. And maybe you do not have the time and like to trust the photovoltaic equipment suppliers.

But let's face it, it's a long term investment, and preferably less than ten years. Efficient budget and a lower energy invoice from the national electricity supplier going hand in hand.

How to size a solar panel system

To convert the sunlight into electricity and then use it, various components are necessary; light-sensitive modules, A DC to AC converter, a mounting system and the cabling. Together they form the solar PV system.

Are Solar Panels Affected by Weather?

Before determining the size of a home solar panel system for your roof or carport, investigate the energy consumption bill first. Some info and electrical energy production calculation goes with that.

In Portugal, Algarve we have, on average, 8.38 sun hours per day. In the wintertime December just 5.44 hours while in summertime Juli, 12.15 hours. ( according climates.com )

This is essential information because photovoltiac cells are sensitive for the Sun radiation. The productive daylight hours are, therefore, a significant factor in the solar panel system calculation. And will be a determining factor for 50% of the system size.

Do PV Panels Work in Cold Weather?

Another factor is the environment temperature. Quality solar panels have a lower temperature coefficient. This factor determine the total efficiency of the PV module.

Electricity tariff scheme and feed in tariff:

For the electricity kilowatt-hour price, you have to ask the local energy supplier. If it is the EDP, you can follow this link for actual rates.

Calculate your average usage. In this example, we are going to assuming an average electric grid consumption of 1.5 kilowatt hours kwh. For a solar system without batteries, the goal is saving 50% with batteries 85% to 100% savings yearly. Therefore, the tariff for every kilowatt will be 25 cents, including tax.

Furthermore, if you have or going to install an SMA connect to grid installation, it is wise to install also the SMA sunny home manager. For controlling the feed-in grid and the surplus for a hybrid PV system with lithium batteries.

How to size a solar panel system

Now we have the basic elements to plan a home renewable energy system.

  1. It would help if you had 1.5 kW/h, that's making a total of 36 kW a day.
  2. The first goal is fifty percent savings on the electrical bill.
  3. Grid electricity for one kilowatt is 0.25 cents.
  4. Average sunshine hours is 8.35 Hour/day.
  5. Temperature coefficient of -0.34

You also have to know what the average weather conditions are to make it accurate! A precise calculation is going further than this article allows. The best and free tool for that is to find at The European Commission's science and knowledge service. It is the  PVGIS 5 interface.

Before you accept any solar quotes from a supplier, ask for the specifications of the used equipment and panels. And above all, how they have calculated your power quote. A small study of your energy usage compare to the recommended products, can save euro´s that are better spend elsewhere. Free quotes are not existing!

For a small fee, ask another commercial solar company to recalculate the offered quote with your electricity usage. For 50 euro´s we can professionally calculate the provided renewable energy quote for you.

The free solar power system calculator:

Roughly, and for clarity of this article, the number of solar panels we take, is five. I have good experiances with panels from Ulica 370 Wp. Five times 370Wp a hour clean energy is 1850Wp sunlight into electricity. That will cover temperature corrections, and cable length loses. The practice has shown that solar panels produce approximately 90% of the specified value, measured at the ends of the string.

If we use that in the above mentioned free PHOTOVOLTAIC GEOGRAPHICAL INFORMATION SYSTEM tool. The output result is this:

Click on image for full calculation.

Do solar panels generate DC or AC?

As light falls on the PV module, the cells are starting generating direct current (DC). The direct current must first be converted to alternating current (AC power) before the solar energy flow is usable in your household. The inverter converts the direct current to alternating current.

The generated power goes via DC cables from the solar panels on your roof to your inverter. Here the current is converted into usable alternating current.

When converting direct current from the including solar panels to alternating current for the electricity grid, the inverter takes into account the synchronous running of the alternating current and the current of the network. The inverter also has a safety function. If the voltage exceeds the upper limit, the inverter switches itself off.

The ultimate goal of the inverter is to generate as much power as possible from the solar panels by MPP tracking techniques. This current flows to your meter cupboard via AC cabling. In the meter cupboard, the inverter is usually connected to a separate group to prevent overloading.

Depending on the phases off the household electrical system, you need a single phase PV inverter or a three phase. They come in different types and techniques.

  1. Micro inverters:

    With micro inverters, each panel has its inverter. These are all directly connected to the 230 V alternating current. The advantage here is that each board has its MPP tracking.
    Such systems are often used for installations where the panels all have different angles and orientations. Also for small plants of 3 or 4 modules, because string inverters only work from a minimum number of panels.
    The disadvantage is that the system gets more parts and therefore becomes more susceptible to malfunction. The inverters themselves have harsh conditions (high temperatures, just below the panels) and if they were to break, it would be harder to reach and replace them.
    On the other hand, the yield of each board can be viewed separately, instead of just the return of the system as a whole. There are different brands of micro-inverters, of which Enphase is the best known.

  2. String inverters:

    With string inverters, the panels a serial connection with each other. They are in series in long strings, and the strands have then a link to the inverter. It is the most common type of installation. Most usable when the panels are all facing the same direction.
    The advantage of these systems is that they are relatively simple with few different parts and therefore, a small chance of malfunctions.
    If there is a malfunction in the inverter, it is usually easily accessible and therefore easy to replace.

  3. Hybrid inverters:

    A hybrid inverter, or smart inverter, is meant to be able to use your solar power. And when the battery is al up, electricity will be taken from the grid. But even when the lithium battery is full, the surplus will be supplied back to the electrical network.
    A hybrid inverter can do the same as a regular inverter. Only this inverter has several smart additions. The hybrid Inverter guides the power to where equipment is asking for it in the household.
    But when more power is available, it will go to the batteries. And then when there is a demand for electricity, and the PV is not generating enough, the battery becomes handy.
    It ensures that power is always flowing correctly.

How to choose the PV inverter:

If the PV modules line up nicely, facing all the same way and without shadows, a string inverter is always a safe choice. If you are thinking of having batteries for solar storage, then hybrids are the right choice. But with hybrid inverters, you can mostly not go off-grid. So energy power drops still have an impact on the household system.

One of the hybrid inverter suppliers with a solar PV inverter is RCT-Power. A PV inverter, battery converter and home management system in one. Suitable for the grid and off-grid application. The same equipment for one and three-phase electrical systems with battery connection.

However, if you choose an SMA inverter, like we are going for our example, there is no need to think about having a lithium battery immediately. SMA has separate converters for lithium batteries, like the Sunny boy Storage and Sunny Island. You can implement it in the renewable energy system any moment, any time.

Do it yourself solar installation.

There you have it, a simple explaination for a general complete solar energy system. We just like to point out that there is more to consider. But as we are also renewable energy installer, lets not give everything away. Ofcours there are the system costs and the waranty of the equipment and the photovoltaic panels.

Most SMA equipment have a waranty of five years and can be extend to 20 years. For the PV modules, they mostly have a functional waranty up to 25 years. Its also depending on the type of solar, and where on this earth you buy a solar panel.

The cost of solar systems is very dependable on the situation of the client. As the available space, is it facing south, etc. etc. Most calculations have a payback period less the ten years with solar electricity.