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 influence.

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

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 “bi-horário” 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.

Once having an idea about the kind of system wished for, we can dive into dimensioning and see what is required and possible for its installation. We will do that in the following article.

by Hans Mulder - hansmuldernow@gmail.com

Hans is a Dutch citizen who helps a local supplier of PV-systems to answer the high and growing number of daily quote-requests from Algarve residents. The above text reflects the most common doubts and curiosities he encounters while providing advice and suggesting solutions.

For the Dutch speaking community, he will be guest speaker at a webinar organized by the NCA Association, to be held May the 9th starting at 7pm. Information can be obtained via info@ncaportugal.pt

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