The use of solar batteries has really taken off in recent years. Not only home users, but also the small business market, are increasingly using energy storage, whereby the excess energy generated by solar panels is stored for a time when there is a shortage of energy, such as on dark days or at night.
The challenge is to find a perfect balance, and match the capacity of a solar battery to the energy need. By calculating the capacity of the solar battery, the end user enjoys maximum benefits. With the right storage capacity, the electricity grid is used as little as possible and users enjoy optimum benefits from the energy generated by the solar panels.
In this article, we will discuss how to calculate the capacity of a solar battery, what you should pay attention to when purchasing and what costs you should take into account:
- Solar battery terminology
- How does a solar battery work?
- Calculating solar battery capacity
- How do you calculate the capacity of a solar battery?
- What does a solar battery for solar panels cost?
Solar battery terminology
When choosing a solar battery, confusion often arises due to the terminology used. In particular, the words ‘power’ and ‘capacity’ are often used interchangeably as if they mean the same thing, while these battery properties differ. For this reason, below we are providing a brief overview of the most important terms that you are sure to come across.
Capacity: capacity is measured in kilowatt-hours (kWh) and is the total amount of electricity a solar battery can store. You can compare it to a jerry can, which has a storage capacity of five or ten litres.
Depth of Discharge (DoD): Most solar batteries cannot be fully discharged as this would damage them. The DoD is the percentage of energy discharged relative to the total capacity of the solar battery. If a solar battery has a DoD of 90%, this means you can use 90% of its total capacity.
Power: Measured in kilowatts (kW), power is the amount of electricity a solar battery can deliver at one time. Think of it as “how far can you turn on a tap”. A high-capacity, low-power solar battery provides a low level of electricity (enough to power a few appliances) for a long time. A low-capacity, high-power solar battery can provide electricity for the whole house, but only for a short time.
Round-Trip Efficiency: This is the efficiency with which electricity can be stored in a solar battery and then extracted again. This is expressed as a percentage. If you charge 5 kWh of electricity in the solar battery and you get 4 kWh back, the battery has a Round-Trip Efficiency of 80%.
Battery life & guarantee: Battery life usually refers to the number of charge/discharge cycles it can undergo before its capacity drops to a certain percentage (usually 80%) of its original capacity. The supplier often gives a certain capacity guarantee after a certain number of charge/discharge cycles.
The most commonly used for solar energy storage are lithium-ion batteries, known for their high DoD, long life and decreasing cost.
How does a solar battery work?
A solar battery stores electricity for later use. The operation of a solar battery can be explained as follows:
Charging: A solar battery is charged with solar energy produced by solar panels. When the solar panels produce more electricity than is being consumed, the excess energy is stored in the solar battery.
Discharging/consumption: As soon as the solar battery is charged, this energy is available for times when consumption exceeds the amount of energy generated, for example, at night or on dark days.
Automatic transfer: In the event of a power failure, the solar battery’s software detects this and automatically switches to the stored energy, so the power supply is not interrupted.
Storage of excess solar energy can help owners maximise solar self-consumption and reduce reliance on the electricity grid.
Calculating solar battery capacity
When selecting a suitable solar battery, several factors are important with regard to capacity and power. As an installer, it is important to make an inventory of how your customer will use this solar battery. You can use the following questions for this and select a suitable solar battery together with the customer, based on the answers.
- What is the current and future energy consumption?
The capacity of the battery must be adapted to the current energy consumption, but must also be suitable for any future plans. Is a family expansion on the cards? Children leaving the parental home? Purchasing an outdoor pool? Switching to electric cooking?
It is wise to take any electric car into consideration, as this can also be used as a solar battery. In addition, it has a significantly larger capacity than a solar battery. On the other hand, it may also be wise to exclude this from the capacity calculation because otherwise, the user will have to purchase considerably more solar battery capacity, resulting in negligible yield. - How much power does the solar panel system have?
In a solar battery has insufficient storage capacity means it will fill up quickly and the excess solar energy is returned to the grid; thus a reduction in yield. Multiply the wattage of each solar panel by the total number of solar panels.
If there are no solar panels yet, the peak load or maximum wattage must be determined. This is done by adding up the wattage of the appliances that are being used at the same time, such as microwaves, tumble dryers, lamps and computers. The total is the peak load and it must be covered by the power and capacity of the solar battery.
Multiply the wattage of each device by the runtime to get the energy in watt hours per day. Add all watt-hour values together for the total. This estimate is probably too low because there will be efficiency losses. Multiply by 1.5 for the actual value in the case of system losses. - Calculating daily energy consumption
Example: Light bulbs burn 5 hours a day. A computer runs 2 hours a day. 120 x 5 + 300 x 2 = 1200 watt-hours. 1200 x 1.5 = 1800 watt-hours
Especially when your customer is not yet sure what their future energy needs will be, it is advisable to recommend a modular solar battery. This can easily be expanded with an additional battery module as the energy need increases. - How will the solar battery be used?
An important question for your customer is about the context in which the solar battery will be used. Does it serve as a backup in the event of a power failure? Does everyone in the house need to ‘be kept up and running’? Or will the solar battery be used in an off-grid situation? All these factors can influence the choice of capacity, power and other properties of a solar battery. - Is the solar panel system retrofit?
With a retrofit system, a solar battery is added to an existing solar panel system afterwards. In most cases, you will then need a separate battery inverter, because the existing inverter is not bidirectional.
How do you calculate the capacity of a solar battery?
With a solar panel system with a capacity of 5 kWp, the capacity of the solar battery would be 7.5 to 8 kWh (5kWp x 1.5 due to system losses).
Is the daily energy consumption higher? If so, the user may consider installing solar panels and purchasing a solar battery with a greater capacity.
The table below shows the recommended storage capacity for each power consumption. With a consumption of, for example, 6000 kWh per year, you advise the customer to pick a battery storage capacity of 9 kWh. If your customer has plans to start cooking electrically or to expand their family, roughly calculate how much power consumption will increase and multiply that by 1.5.
Consequences of capacity being too high or too low
If the solar battery has excessively high storage capacity, you will not be able to fully charge it during the winter months, because there aren’t enough daylight hours. During sunny summer months, you won’t be able to drain the battery, because you produce enough energy during the day and you can also use that energy directly. When a solar capacity has overcapacity, you earn back the investment less quickly.
When the solar battery has too little capacity, you can maximise self-consumption to about half. When the solar panels generate a lot of energy in the summer, the battery capacity is not sufficient to store all the energy.
What does a solar battery for solar panels cost?
The cost of a solar battery for solar panels can vary greatly. This is because the price depends on a number of factors, including the capacity of the battery, the brand and model, installation costs and any additional components. Here are some things that affect the cost of a solar battery:
Battery capacity: solar batteries come in different capacities, measured in kilowatt-hours (kWh). The larger the capacity, the more expensive the battery usually is. The capacity depends on how much excess solar energy you want to store.
Brand and model: There are several suppliers of A-brand solar batteries on the market, such as SolarEdge, LG Chem, Huawei and Enphase. Each brand and model has its own pricing and features.
Installation costs: The cost of installation can vary considerably, depending on the living situation and the complexity of the installation. If you do not have technical knowledge yourself, you may have to get a professional installer to come.
Additional components: If you do not already have an existing solar panel installation, you will face additional costs for solar panels, an inverter, cabling and security equipment. With this, the cost can add up considerably.
The cost of a solar battery for solar panels ranges roughly from around a thousand euros to tens of thousands of euros, depending on capacity and configuration. The advice is to map out your needs well - also for the future - and research the options well online. Compare offers, brands and models and possible compatibility with the existing system. The development of solar batteries is very fast and prices can also change significantly over time due to ever-evolving technology. So always make sure you have up-to-date information.