Everything you need to know about solar batteries

As a solar rooftop owner, you may not use the solar energy at the exact time it is produced. That is the main reason solar batteries exist.

The reasons may vary from climate and geography to culture and lifestyle. 

As an example, according to the U.S. Energy Information Administration, peak power usage in the U.S. often occurs on summer evenings, when solar energy generation is at its lowest. 

On the other hand, for households where people work during the day time, the highest energy consumption is likely to take place in the evenings, rather than afternoons. 

All these differences between energy production and consumption creates a need for storage technology. 

In short, solar batteries store surplus energy generated by solar panels. This means you can use the extra energy to power your house on cloudy or rainy days, or after the sun goes down – i.e. when energy production is low.

• First of all, you get the most out of the solar energy generated by your solar rooftop system, thanks to solar batteries. Wasting the surplus energy would definitely contradict  initial motivation which was to protect the planet and use your resources wisely, wouldn’t it?  

In case you are interested,  read more about the environmental benefits of solar energy.

• If your rooftop solar system and battery are large enough, you can run your home mostly on solar power.
• Using electricity from your battery can be cheaper per kilowatt-hour than using electricity from the utility grid. This depends on the time of day and electricity tariffs in your area. 
• Solar storage also eliminates the risk of electricity prices going up and feed-in-tariffs going down.
• Last but not least, solar batteries can help ease variations in the solar energy flows (the changes in the amount of sunlight that shines onto photovoltaic (PV) panels or concentrating solar-thermal power (CSP) systems.)

There are four main types of batteries used to store solar energy — lead-acid, lithium-ion, flow batteries, and nickel cadmium.

Let’s deep dive into each of them. 

1.  Lead-acid: This type is the oldest solar battery type. Thanks to its long history, it has been developed alongside clean energy resources. 

Lead-acid solar batteries come in two different types. Sealed lead acid batteries are designed in a way that they reduce the release of toxic gas into the atmosphere, during their charging process.

The second lead-acid battery type is flooded lead acid battery. This is like the bigger version of a traditional car battery.

When it comes to the features, lead-acid solar batteries have a shorter lifespan in general, and their depth-of-discharge is lower compared to the other storage options. They also require regular maintenance. That is why lead-acid batteries are considered to be the cheapest option among the four. 

Keep in mind that experts recommend not to run lead-acid batteries to a depth-of-discharge any higher than 50%. Discharging the battery beyond that percentage may negatively affect its lifespan.

2. Lithium-ion: We can call this type “the new kid on the block”, compared to the much longer history of lead-acid batteries. It has been improved in parallel to the developments in battery technology required for electric cars in recent years. 

Lithium-ion solar batteries are popular among residential home owners. There are three reasons for this:

• Their lifespan is longer.
• They require less maintenance. 
• They are more lightweight and smaller compared to lead-acid batteries. 

In addition to these features, these batteries are able to easily handle deep discharges of 80% or more, meaning they have a higher usable capacity.

With that being said, an important drawback is that lithium-ion batteries pose the risk of thermal runaway, meaning they can catch fire if they are not installed properly.

3. Flow batteries: Another new kid on the block, truly a new player in the solar battery technology scene is the flow battery. 

Despite the fact that it is a fairly new storage solution, and it still requires improvements, there are a couple of points that make flow batteries a popular option. 

• Its depth of discharge is 100%. So, you can use the entire stored energy. 
• Unlike nickel cadmium batteries which we will be discussing in the next section, flow batteries are non-toxic. The reason is that they are water-based. 

Here comes the downsides of using a flow battery. 

• As we said before, its technology is still in the development stage, yet it is more expensive than the other options.
• It occupies more space due to low storage capacity. 

4. Nickel cadmium: As another tried and tested solution, nickel cadmium batteries – also called “nickel batteries” and “Ni-Cd” – have been in the battery technology scene for a while. 

They are known for the ability to operate at extreme temperatures without any complex battery management systems. That is why they are more popular among commercial-scale projects.

One important note to keep in mind is that we can consider nickel batteries as a very old technology. Because of their high toxicity, they are banned in a large number of countries. 

Segmentation based on timeframe

Another way to segment solar batteries is the timeframe — short-term and long-term storage.

Short-term storage refers to an operation which is completed in as little as a few minutes. The goal is to make sure that the solar system still works despite a passing cloud, for example. 

As the name suggests, long-term storage refers to a longer time frame providing supply over days or weeks, rather than minutes or hours. 

It makes sense to choose it when the solar energy production significantly decreases due to a dramatic change in climate. 

Starting with the personal needs 

A solar rooftop system is great, but coupling it with the right solar battery is even greater. 

When you choose a solar battery, in addition to the battery features we will soon mention, you also need to consider:

• Geography and climate: Where you live.
• Energy production: What type of a solar rooftop system you have. 
• Energy consumption: How much energy you consume. 
• Lifestyle: What your budget and priorities are. 

To solidify all these with some examples: 

• If you are living in an area where the energy generation is highest in the afternoon but you consume the most energy at night when you get back from work, you will definitely benefit from a storage system. 
• If you have a 5kW solar system, you will most likely need a battery with a capacity of at least 10kWh, and even up to 13kWh.

Technical Features of Solar Batteries

Every battery type comes with some advantages and disadvantages. This is why it is important to take into account six important factors when choosing a solar battery for your commercial premises or residential house. 

Battery Size/kWh

The first thing to know about the battery size is that it is measured in kilowatt-hours (kWh). As mentioned earlier, the kWh in a battery bank is used at night or in lowlight conditions when the solar production is not enough to power your appliances. 

All batteries come in different sizes based on preferences. As an example, smaller batteries can be 2kWh sizing up to 20kWh or even more. 

To make it more clear with a comparison, if you are consuming around 0,75-1 kW energy an hour, 10kWh battery would last around 10-12 hours and a 13 kWh battery will last 13-16 hours.

Energy Capacity/kW

Unlike battery size which is measured in kWh, the energy capacity of solar batteries is measured in kilowatt-hours (kW). 

To avoid any confusion beforehand:

• kWh refers to the total amount of energy used. 
• kW refers to the rate of electricity usage. 

A battery’s power rating tells you both how many appliances your battery can power at once and which appliances those are. 

Keep in mind that most of the batteries available on the market today have a continuous power output of around 5 kW.

Round-trip Efficiency

Round-trip efficiency is the percentage of electricity put into storage that is used later. 

In short, there are two scenarios when you are likely to lose some kWh of electricity:

• When you invert some kWh of electricity from direct current (DC) electricity to alternating current (AC) electricity;
• When you store electricity in your battery and use it again. 

The higher the roundtrip efficiency, the less energy is lost in the storage process. Typically it is about 80%.

Depth of Discharge

You are using a solar battery to store surplus solar energy, but what about the amount you can actually use out of that?

Depth of discharge (DoD) is one of the most important factors when making a decision about which solar battery to go with. It refers to the amount of the energy you can use out of your solar storage system. 

As an example, let’s say you have a battery of 8kWh with a DoD of 80%. That means you can use a maximum of 6.4 kWh. 

Battery Lifetime

Battery lifetime is measured with three different metrics: 

• Expected years of operation
• Expected throughput
• Expected cycles.

Battery life is defined in years, while a battery’s expected throughput and cycles are like a car’s mileage warranty. 

Solar batteries have a super high lifecycle. This means that they don’t require frequent maintenance or replacement. Good news is, after charging and discharging solar batteries thousands of times, they still remain highly functional.

Safety

Last but not least, safety comes first. 

Solar batteries are considered quite safe in general. However, all solar batteries have to meet certain safety requirements in order to be certified for installation in homes and businesses. 

For this reason, make sure the solar battery you are planning to buy meets all these requirements. 

If you are:

• planning to reroof and go solar at the same time
• looking for design as well as technology
• looking for some guidance along the way

You are definitely at the right place.

As an award winning integrated solar roof solution, Roofit.Solar offers full packages of solar systems, including solar batteries,  solar inverters, and other necessary devices. 

Contact us to get a fully customized premium solar roof that will protect your home for the decades to come!