Facts and fables about solar panels
Just imagine: Instead of paying bills, you actually receive money from the electricity company.
And: You never again pay for fuels at the petrol station, because your car runs now for free on the
power you generate yourself. In the process, you positively contribute to the protection of the
environment. All this thanks to a few photovoltaic panels placed on your roof or in your garden.
But as the saying goes “if something seems too good to be true, it usually is”. Where is the line
between myth and reality in this matter?
Before we start: Photovoltaic solar panels generate electricity. They should not be confused with the
similar looking water-boiler panels that directly heat up water. These are also interesting, but worth
Off-grid or grid-connected
The average of 300 days a year with sunshine in the Algarve is a reality. And so is the tendency for
energy prices to grow. This combination makes many people wonder if they can become “off-grid”,
charmed with the idea to become fully self-supporting.
Although the latter ambition certainly follows logic and is also technically possible, theory and
practice differ when it comes down to the pondering of pros and cons among a wider range of
A first question is motivation; Is the wish to become independent based on the fear of grid
blackouts, on the desire to help protecting the environment, or on the idea to save money?
Even if all three are true, saving money is often the decisive factor and in being so, a logical starting
Identifying such starting point is crucial because it influences directly one basic ingredient: The
number of panels required.
From an economic standpoint, the best calculation starts with the energy consumption in its average
demand, not in its peaks. Illustrating this: Modern panels are able to produce around 410 watts
each. A hairdryer may require 2000 watts. So, feeding just a hairdryer in operation requires five
panels. But then again, one uses a hairdryer just a few minutes per day. In short: From a pure
financial point of view, the most interesting is to have enough panels to cover the average
consumption, and to use the grid as a “back-up” for the short peak demands above that.
Disconnected from the grid, however, that back-up option obviously does not exist. So off-grid
systems should be large enough to always cope with full demand, and must therefore have a
considerable number of panels.
Off-grid also means that no energy is available at night or during dark days, so a reserve of energy in
batteries is needed. And that, in turn, requires a double job from the panels: Providing enough
energy for all requirements during the day, and at the same time supply enough energy for the
charging of batteries for all night-time requirements. This is another reason why an extra-large
number of panels is needed for off-grid situations. By the way, not having the grid as a back-up
makes it wise to also invest in a power-generator, in case the batteries go flat. And they easily may
when a few dark days follow in a row.
All in all, going fully off-grid is a necessary option for who cannot get a grid connection in the first
place, or perhaps a comforting idea for people who like to feel independent. But it requires
investments not yet fully justifiable from a pure economic point of view.
Of course, there is also the hybrid option to have both batteries and a grid connection, acting as a
kind of double back-up.
With or without batteries
When the choice is made to stay “grid-connected”, the choice between a system with or without
batteries is therefore the next to be made. Here again, both ideological and rational aspects are of
The idea to have the sun charging batteries for energy use at night is appealing. But batteries are
expensive, and as explained earlier, they require enough panels to perform a “double job”.
If the extra investment for batteries is economically justifiable or not furthermore depends on the
use of the system: If people are out all day and just come back home at night, then batteries
obviously make sense. But if people are at home during the day and able to do a lot of things in
daytime, such as washing laundry, using water pumps and so on, that sense already becomes
smaller. Especially so when a differentiated day and night tariff is chosen from the grid provider. This
turns night power considerably cheaper, whereas the higher price during the day doesn´t hurt for
having solar energy.
What can be done in cases of doubt is to start with a so-called “day-system” without batteries, but
with an inverter that would accept them in a later stage. The advantage of that is double: First of all,
a new calculation can be made after finding out how much money is spent on electricity bought for
night hours. See what batteries cost, and calculate the time required for such extra investment to
pay off. Secondly, by then the right battery capacity to cover night consumption is also known. That
is interesting not only from the investment point of view, but also to help the lifespan of such
batteries. Because just like a cell phone or a laptop, batteries suffer from going flat, as well as from
being overcharged. And since batteries are mostly provided in stackable modules, the adjustment of
capacity is perfectly possible.
Realised energy consumption can be read on the bills. If not available, the expected energy
consumption in the future can be estimated by adding up all the electricity requiring items in and
around the house.
Reading electricity bills in Portugal, however, can be a bit tricky. First of all, they all specify what was
consumed during what part of the day, divided in “Vazio”, “Ponta” and “Cheias”, even if you just pay
one flat rate. The flat rate is called “simples”. Differentiated day and night tariffs are specified as “bihorário” or even “tri-horário”.
Furthermore, many people mistakenly believe they have differentiated rates when looking at the
progressive specification whereon the VAT (I.V.A.) tax rate grows from 6% for the first Kilowatts
consumed, to 13% and 23% thereafter.
Another important detail on the bill is the power contracted, see the amount of kVA under
“Potência”. Usually this is the maximum power a certified electrician has approved for the house,
although in some cases higher power was approved but not installed.
The approved maximum amount of energy is important to know because it is also the maximum
what may be generated with solar panels! This limitation follows double safety rules:
First, the grid may be unable to handle eventual injections from powerful domestic systems, which
may even provoke nearby transformers to burn out. And secondly, internal wiring may also be
weak, especially in older houses, posing a fire threat upon receiving higher loads than what they
were conceived for.
Another common misunderstanding is that a system with batteries will guarantee continuing
electricity in case the grid goes down. You will be disappointed to know that it will automatically
shut down in case of a blackout, as required by law. That is, unless one is fully off-grid.
Components and installation
If the amount of kVA allowed for the house sets a limit, a much bigger limit often is the available
space for solar panels. Mentioned 410W panels measure 115 by 173 centimetres each.
These can be placed on the ground or on the roof, but have to face as much southwards as possible
or be in a combined east-west configuration. Even more important is being free of shades.
Solar panels produce direct current, or “DC”. This has to be changed to alternating current “AC” for
the house, which is done by a so-called inverter. Which brings us, once more, to different options.
Some systems work with small “micro-inverters” on the back of each panel. Panels are then placed
parallel to each other, and directly feed AC from the roof down to the house. Other systems place
panels in series, so-called “strings”, feeding one large inverter, that usually sits inside the house. The
first option has the advantage of panels being parallel whereby shading on one panel does not
hamper the others. But a big disadvantage is the placement of sensitive electronics on a roof, where
it can easily suffer damage. And worse: If one panel fails, one may not notice it immediately. So
regular inspection becomes advisable for this kind of installation. Furthermore, these systems will
always inject all surplus energy into the grid. And that requires a bi-directional meter to be supplied
The second system has several advantages above the first: It is more efficient for leading a higher
voltage through the cables from the roof, it has no exposed sensitive electronics on the outside, and
it does therefor not require special or regular maintenance. Apart from that, the centralized
inverters usually offer more management and monitor options.
Mentioned central inverter should ideally sit in the house or garage or even in a special box, but
always in a moist free, albeit ventilated, space. The same applies for eventual batteries. Apart from
that there will be two safety boxes, one for AC and one for DC, as well as a smart-meter. All that
stuff ideally not too far away from the main switchboard / “Quadro Elétrico”.
Cabling is another issue. Between the panels and the inverter, heavy DC cables must run in an
exclusive, not shared, conduit. When going through the ground, they should be placed in a special
trench dug 80 cm deep with netting and a warning lint on top. When coming down from the roof,
they must be protected in a suitable tube or housing.
From the inverter to the main switchboard goes another heavy cable, being an AC one. And finally, a
thin internet cable should be connected between the smart-meter and the router in the house. All
this can be lead in discrete housings running on the outside of the walls, but will still be visible.
The only alternative to that is to grind channelling for the hiding of conduits, fill those up with
cement and paint them over.
Obviously, this calls for the recommendation of timely planning for anyone who starts refurbishing
Selling surplus energy back to the grid is an option, but bluntly put, for individual households the
prices obtained lately do not justify the bureaucracy involved. Economically speaking it is better to
focus on the return-on-investment by the supply of energy for one´s own consumption. Maybe this
will change in a future when the grid infrastructure can better handle return loads, and also when
energy companies find better ways to store energy.
Via the smart-meter one can measure how many surpluses eventually occurred, and see when a
contract with the grid supplier and the installation of a bi-directional meter becomes interesting.
Apart from this meter to be supplied by the energy company, no further adaptations are required
from most PV-systems.
Honest advice and sober ears
Who buys a system too small to cover all energy requirements knows two things for sure: One is that
electricity will still have to be bought from the grid, but the other is that the system bought will at
least be used for a hundred percent. Each and every panel will, after all, help to reduce the bill.
And yes, by reducing the energy bought from the grid one also helps to protect the environment. So,
starting with the wish to save money is not so selfish after all.
Often forgotten in the equation is the raise in real estate value. Price comparisons between identical
houses have shown that such raise is often higher than the investment in the PV-system installed.
Very important is also to know that all PV-systems in Portugal need to be inspected and certified by
a classified engineer. Not only is this obliged and people who do not comply risk fines, it is also
important for the purpose of insurance, and of course for one’s own safety in the first place.
Serious PV-system suppliers take care of registration and certification for their customers.
The general conclusion is that both facts and fables exist when it comes to solar panels. Fortunately,
in most cases enough positive facts help to overcome eventual disappointments of too high
What it comes down to is the need for honest advice, as well as open and sober listening ears.