How much energy does my (photovoltaic) PV system produce? How much of it ends up in my sonnenBatterie? And, how much of this can I actually use? As a sonnenBatterie owner, you''ve certainly asked yourself these questions. You can find answers at any time in your sonnen App. But how can the differences between the energy produced and the energy available — conversion losses — be explained? And what are the standard efficiency values for battery storage systems on the market? We''ll explain all of this in the following article!
When sunlight hits the solar cells of your PV system, electricity flows, and the electrons make their way from your roof to your electricity storage unit. They carry the energy from the sun with them.
However, they have to overcome numerous obstacles on the way. They pass through cables, electrical components (such as inverters), and finally through the batteries of your storage system. At each obstacle or resistance, they release a small amount of their energy – this is when conversion losses occur, similar to the way people lose energy when overcoming obstacles. In an electrical circuit electrical energy is converted into thermal energy. This is why electrical appliances heat up after a while.
Efficiency shows how much electrical energy is converted into heat on the journey from the source to the target. If the efficiency is 80 per cent, 80 per cent of the original electrical energy reaches its destination. In this case, 20 per cent of the electrical energy is referred to as power loss.
The classic light bulb exemplifies how high this power loss can be. An incandescent light bulb can have an efficiency of as low as five per cent. Here, the bulb only converts five per cent of the original electrical energy into light, the rest is converted into heat. LED bulbs, on the other hand, achieve efficiencies of 30 to 40 per cent and are therefore six to eight times more efficient than incandescent bulbs.
From a physical perspective, there can be no such thing as 100 per cent efficiency, as minimal conversion losses always occur. Therefore, the aim of increasing the efficiency of technical devices is to achieve the highest possible efficiency and minimise avoidable losses as much as possible. This is something that we at sonnen achieve with our batteries, which have a high efficiency rate.
Solar panel inverters, for example, which convert the direct current (DC) of solar modules into alternating current (AC) now achieve efficiencies of between 96 and 98 per cent.
High efficiency is a key factor in the development of electrical appliances, though it''s not the only one. Quality, safety, availability in large quantities, and cost are also considered.
For example, the fuel consumption of a car per 100 kilometres is an important parameter, but this alone does not allow any fundamental statement to be made about the quality of other properties of the car.
A very high level of efficiency can lead to significantly higher costs because the necessary components are extremely expensive or in short supply. Here, a car manufacturer will weigh up which route they want to take, higher efficiency or lower cost.
The same consideration applies to battery storage systems, which also differ in efficiency. A comparison between different manufacturers should be exercised with caution, as the basis for calculating the information in data sheets can vary considerably. In turn, this makes a neutral comparison between different manufacturers difficult.
This is where independent electricity storage inspection comes in, such as those done by ITP''s Battery Test Centre in Australia.
Our sonnen customers can view a transparent visualisation of their energy consumption data at any time in the sonnen App. This also includes the efficiency of their sonnenBatterie. To do this, the value of the total discharge can be divided by the value of the total charge.
It is important to use the annual value here, as a complete picture can only be obtained by looking at a whole year. Only then are annual effects or different utilisation patterns balanced out. In practice, these values vary from household to household.
As an exemplary use case, we assume an efficiency of between 75 and 80 per cent, which frequently occurs. This is the actual value for the entire storage system and not the best values or the values of individual components.
Read more about the technology behind our sonnenBatterie here.
In our example, the efficiency of the sonnenBatterie is approximately 75 to 80 per cent. What is behind the power loss of around 20 to 25 per cent? Remember, the electrons have to cross many obstacles before they reach the batteries.
1. The path through the sonnenBatterie
The efficiency mentioned here starts with the inverter of the sonnenBatterie - in other words, where the alternating current from the PV inverter is converted into direct current.
It then passes through the inverter to the batteries themselves, where the electrical energy is converted into chemical energy. When discharging, it goes back the same way. Chemical energy in the batteries is converted into electrical energy and this flows through the inverter back into the domestic grid. Without taking into account the resistances in the cables, the electrons have to overcome two components during storage and discharge, both there and back, where they naturally release energy.
Let''s take a look at this using an example calculation.
Assuming the inverter has an efficiency of 96 per cent for charging and discharging and the batteries have the same, the calculation is as follows:
0.96 (inverter charging)
* 0.96 (inverter discharging)
This is more than the 75 to 80 per cent we see in our example. We''ll look at why this is, in the following points.
In addition to the conversion losses of the inverter, other factors influence the calculation. The values given for efficiency in the point above are best values and not average values. Such optimum conditions are not always achieved in practice.
The temperature or the level of output plays a role here. Many inverters work most efficiently when they have to deliver high power, roughly in the power range between 50 and 100 per cent.
In the case of the sonnenBatterie 10, this range would be between 2.3 kW and 4.6 kW. However, there are times when the sonnenBatterie is charged with less power, for example, because the PV yield is lower in winter.
If the sonnenBatterie is only charged with 800 watts, for example, the efficiency is lower than 96 per cent. High outputs are not always necessary for consumption either; outputs of a few hundred watts are common, especially at night. Efficiency is also reduced at such lower outputs.
If we take into account the not-always-optimal range with lower outputs, our calculation could look something like this:
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