How many solar panels and batteries are needed to generate energy in a house?
One third of the energy produced worldwide last year was generated by renewable sources. Hydroelectric power plants remain the main source of clean electricity, but photovoltaics and wind power are growing increasingly strong, according to data from the International Renewable Energy Agency (IRENA). In the midst of this trend, solar self-consumption has been gaining weight thanks to the falling cost of photovoltaic panels and, little by little, of batteries.
But there are still many doubts when it comes to taking the plunge and opting for solar energy in a home. Is it possible to cover the entire energy demand of a home with solar panels? And if so, how do you know how many to install? What happens when the sun is not shining or during the night? And is it possible to somehow store surplus energy production for use at another time?
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Can solar energy cover the needs of a house?
All electricity grids deployed in most countries on the planet operate in a similar way. Energy is produced from many sources (with renewables, nuclear power plants or fossil fuels), but all the electricity is fed into the same grid. This is then managed more or less centrally so that there is never a shortage in households. On a sunny or windy day, renewables will have more weight in this energy mix, but during the night or on a day without wind, thermal or nuclear power plants will need to produce more.
Self-consumption of energy through renewables radically changes this paradigm. If energy is consumed where it is produced, high-voltage transmission grids become less relevant. However, there is one previously insignificant factor that is now becoming increasingly important: storage. Since renewable production is intermittent, it is necessary to be able to store energy from peak production for those times when generation is minimal.
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“Today, it is technically and economically possible to cover the energy needs of a household with solar panels and batteries,” explains Elías Gómez, head of Energy and Sustainability at BBVA in Spain. “Evidence of this are the isolated homes that have no possibility of electrical connection and address their needs with solar panels and batteries to store energy when there is no solar production. Even so, a prior study is always necessary to size both elements and to confirm that the solution is viable”.
Necessary elements for solar self-consumption
The size and capacity of the installation of solar panels and batteries will vary depending on the needs of the house and the objectives sought. It is not the same to bet on photovoltaic energy as a way to save and reduce the environmental impact as it is to be completely self-sufficient or to bring electricity to an isolated building. Even so, an installation of this type must always have the following elements:
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Photovoltaic panels. They are responsible for producing electricity from sunlight.
Inverter. The panels generate direct current and the inverter transforms it into the alternating current normally used by household appliances.
Accumulators or batteries. These are not always necessary, since surplus production (i.e., electricity generated that is not currently used in the home) can be fed into the grid. However, they are necessary if what is sought is real self-consumption to cover the energy needs of the household when photovoltaic production drops.
Monitoring equipment. It controls the production in real time and the state of the batteries.
Types of storage: lead, lithium and virtual?
If there are no storage sources involved, electricity is always consumed at the moment it is produced. Thus, in a solar self-consumption installation it is common for there to be imbalances between demand and energy generation. Let’s say, for example, that we have six solar panels on a house and that at midday on a sunny summer day they produce 4.5 kilowatt hours (KWh). If at that time we have the oven, washing machine and air conditioning running, the demand will be quite in line with the production. But if no one is at home at that time, what happens to the energy?
In this second case, the electricity can follow two paths: it can either be fed into the grid, where it will become part of the overall energy mix (the combination of the different energy sources that cover a country’s electricity supply), or it can be stored in batteries for later use. “The two main storage technologies today are lead-acid batteries and lithium batteries. The former are used in isolated installations because of their high power and low cost. But lithium batteries are taking the lead in the domestic sector because they have no memory effect, the charging cycles can be done as the user wishes, they are lighter and require no maintenance,” explains Elías Gómez.
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In addition, some electricity retailers have developed the concept of virtual batteries, also known as piggy banks or solar wallets. This system monetizes the surplus electricity that is fed into the grid from the home based on the wholesale energy market price. Thus, these surpluses are accumulated in the form of money and can be used to offset the cost of electricity consumption through the grid (when solar generation does not cover the household’s demands). This is a middle-of-the-road solution in which the user is charged for the energy he produces and does not use.
Factors influencing a solar panel and battery system
When deciding how many solar panels and batteries are needed in a home PV system, it is important to consider the following factors:
What is the goal? A single lithium battery may be sufficient to reduce the cost of the bill or to provide emergency back-up in the event of a power outage. If the objective is to be completely independent from the grid, it will be necessary to have an installation with a high storage capacity.
What are the consumption habits? As is done when choosing between the different electricity tariffs available on the market, it will be necessary to make a study of the household’s energy consumption habits and basic energy needs. For example, if at night the demand is minimal, you will not need too much storage.
What is the photovoltaic generation capacity? A single-family house with a large south-facing roof in the province of Almeria does not have the same production potential as a multi-storey building in the center of Oviedo. Some online tools can give a rough idea of the PV generation capacity of a house, but it is important to always have a detailed study done by an expert before making a final decision.
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“The key is to analyze the collection area to be installed in relation to household demand. This way we will be able to know if there is a significant volume of excess energy generated that can be stored and used outside production schedules. This analysis should also be completed with a study of the offers made by the commercialization companies to find out if what is of interest is the installation of batteries or to manage the electricity contract with a modality that is better suited to the model with solar panels,” adds the head of Energy and Sustainability at BBVA in Spain.
How to calculate how many solar batteries are needed at home?
Taking into account all the elements seen so far (objectives, production capacity and consumption habits), when it comes to know how many batteries are needed at home it will be necessary to calculate the average energy produced and the real energy that will be needed at different times of the day. The difference between the two will give us clues as to how much minimum storage capacity to install. To know the minimum energy generation that we can expect and during what times of the day, it is best to rely on the detailed analysis of the installers or distributors of photovoltaic technology.
To know the energy demand and consumption profile of the household, the best way is to analyze how the house has behaved in the last year. “We must take into account at least the energy consumption of the last 12 months and include whether in the coming months or years there will be changes, such as electric cars or the installation of aerothermal systems,” says Elías Gómez. “In the current context, it is very important to take into account in the analysis if the use of gas in the home is going to be reduced and to include more electric systems.
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In the spirit of being more exhaustive, you can go one by one reviewing the electricity demand of each appliance and electrical system and multiplying it by the number of hours we expect them to operate while the photovoltaic panels are not producing at their maximum. So, for example, if we need the refrigerator, which consumes 50 Wh, to run for 12 hours regardless of solar production, we will need a battery capable of supplying 600 Wh per day.
The calculation must be repeated for each of the household appliances. The final sum will provide the data of the storage capacity we should count on according to our objectives. “Currently the batteries for an average home with an average installation of solar panels can mean between 7,000 and 9,000 euros of investment,” adds Elías Gómez. “The relevant thing is that European funds have created an item for this equipment that can be subsidized with up to 70 % of its cost”.
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A decade ago, having a home powered by photovoltaic panels with its own storage system was practically impossible due to the costs. Today, producing solar power is cheaper than burning fossil fuels in much of the world, according to IRENA data. And lithium battery prices have plummeted: they have fallen by 97% since 1991. Today, solar self-consumption is more affordable than ever.
