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How does grid energy storage work?

In this article I will explain how to store electricity from photovoltaic energy, this storage serves as a reserve for the moments of the day that there is little or no production of electrical energy.

How to store electricity from photovoltaic energy

Solar photovoltaic installations generally produce more energy than our homes consume, at least not when the demand for electricity is higher. Here, two solutions come to the rescue:  the prosumer program and home energy storage  .

Of course, the maximum production of electricity by solar panels falls in the middle of the day, when the sun is at its highest point and brightens the solar installation. The main problem with photovoltaics is the fact that at this time the demand for electricity in our homes is the lowest. Usually then no one is home – household members are at school, work, etc. Even if someone spends all day at home, they do not need lighting in the middle of the day, which consumes a lot of electricity. So how was this problem solved?

Equilibrium as a virtual energy storage method

The simplest solution, which requires absolutely no initiative on the part of the owner of the photovoltaic installation, is to become a prosumer  . A prosumer is both a producer and consumer of electricity. This solution is chosen almost by default in our country  .

It is enough to inform the will to become a prosumer to the electricity grid operator, so that he can replace our  meter with a bidirectional one  , that is, the one that counts the net energy, that is, the difference between the amount of electricity we have. we take from the network and the amount of electricity that we have introduced into the network. We basically pay for this difference … on the condition that not all the electricity that enters the grid is subtracted from what we get.

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In accordance with the  Prosument  program  regulations , depending on the size of our installation, the owner of a photovoltaic installation can download 70 to 80 percent of the energy previously introduced from the grid. And the 20..30%? It is lost: this energy is absorbed by the operator of the transmission network in exchange for the possibility of using its infrastructure as, in fact, an electricity storage.

Energy storage

What alternatives are there to become a prosumer? Well, we can store energy ourselves in our home. There are several approaches to this topic. First of all, you can invest in a classic home energy storage. Depending on its design, the operating costs of such a system, its capacity and performance vary. Let’s take a closer look at the most promising technologies used to store electricity from photovoltaics.

Batteries as energy storage

The classic solutions are systems that use batteries to store electrical energy. They work like a power bank for our home  . They can use  cheaper lead-acid batteries (like the ones in our cars), which offer 80% to 90% energy efficiency and an impressive lifespan of  up to several dozen years. Unfortunately, these parameters are obtained at the expense of the size of the solution. A warehouse based on lead-acid cells stores approximately 65 kWh / m3.

Systems using more modern  lithium-ion cells achieve even higher efficiency: up to 95% and an energy density of up to 400 kWh / m3  . Unfortunately, these types of batteries are more expensive than lead batteries, and also more delicate than them: they have a limited number of charge and discharge cycles, usually 1000 to 10,000 cycles (after this time, the cell is not damaged , it only loses its capacity; typically, the number of charge cycles is given for the decrease of the cell capacity to 90% of its initial value).

Alternative energy storage

In addition to battery storage , in large installations, there are, among others, flow accumulators, flyers, hydrogen or compressed air systems and, above all, pumped storage plants. However, these are not installations that, at least for now, can be used on a domestic scale.

Electric cars as energy storage

An alternative to dedicated electricity storage systems is the use of electric vehicles as emergency electricity storage. This is problematic because when connecting our vehicle to the network, we want it to charge to its maximum capacity. However, the facility owner may want to use the electric vehicle as an energy storage facility to smooth out larger electricity demand peaks, thus relieving power plants and the transmission grid. Electricity providers may, in return, offer people who lend their cars as such warehouses certain promotions in terms of the services they provide.We write more about this in a separate article Energy storage in an electric car

How does hot and cold storage work?

However, it is not only electricity that can be stored at home. It is also possible to store energy in the form of heat or cold. For example, if we use a photovoltaic  heat pump  to produce hot water (domestic or central heating),  we can equip our installation with an excessively large hot water tank, the so-called heat accumulator  . Once properly insulated, the water maintains its high temperature for a long time, thanks to which the losses are small and the efficiency of the system is high, from 60% to 95%. Furthermore, analyzes show that even a small thermal energy store enables significant optimization of the energy system.

Why store energy from photovoltaic energy?

The main motivation for using energy storage is savings. By becoming prosumers, we lose 20% to 30% of electricity, which we cannot use immediately. The maximum electricity consumption (usually in the afternoon and evening) falls at a completely different time of day than the maximum efficiency of the solar system (noon and early afternoon). Therefore, it is safe to assume that all energy used at night must come from some type of storage facility. This assumption will help us simplify a more detailed analysis.

The following table summarizes the results of a simple profit analysis for a 10 kWp PV plant  , where all energy “goes” through some type of storage. In Poland, we have an average of 4.38 hours of sunshine per day, which translates into an average production of 43.8 kWh per day in the system in question. The average price of electricity in Poland at the time of writing is PLN 0.63 / kWh and this value was used to calculate the benefit of using a given type of energy storage.

Warehouse type Medium efficiency Lost energy (daily) Profit relative to prosumer (daily)
Prosumer – “storage” in the transmission network 80% 8,76 kWh
1: 1 balance 100% 0 5.52
Lead acid batteries 85% 6,57 kWh 1.38 PLN
Lithium ion batteries 90% 4,38 kWh PLN 2.76
Electric car cells 90% 4,38 kWh PLN 2.76
Heat storage facilities 60% 17,52 kWh


As you can see from the table below,  1: 1 balancing is the most efficient way to virtually store energy  . Using physical energy stores allows you to save between PLN 1.40 and PLN 2.80 per day. This translates to roughly PLN 500-1,000 in savings per year. Assuming optimal system operating conditions, even a demanding and relatively delicate lithium-ion cell-based system can function properly for 10 years without reducing its capacity by more than 10% of the initial value. This translates to a total amount saved of up to PLN 10,000. How does it compare to purchase costs?

How much does an energy storage cost?

A warehouse with a capacity that can accommodate all the energy analyzed in the previous example (40 kWh) has an impressive cost of almost PLN 50,000, but in a real home system no more than 6-10 kWh of capacity is needed, which This translates into investment costs of PLN 12-20,000 with the use of cheaper batteries.

The above list is not optimistic, but it does not take into account one important aspect: co-financing for energy storage. According to the most recent information, the National Fund for Environmental Protection and Water Management decided to expand the work covered by the co-financing in the next edition of the My Electricity contest with, among others, electricity storage facilities at home. In addition, as we reported some time ago, the Ministry of Climate and Environment is pushing for renewable energy producers to support self-consumption. This means supporting electromobility, systems using heat pumps, and electricity storage. The possibility of obtaining financing means that the storage of electricity can pay off in a few years and generate money for us in the next few years .

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