How to Store Energy from Solar Panels

For those who like to live as efficiently as possible, wastage will be one of your biggest enemies! 

Unfortunately, wasting resources has become accepted in modern day society. Whether it’s food waste created in homes, companies using unrecyclable, single use packaging or generating renewable energy that’s never used. Whatever it may be, waste isn’t good. It’s inefficient and, a lot of the time, avoidable. 

When it comes to solar panels, it’s possible for energy to be wasted too. This is why storing excess generated energy is essential – it makes sure all of that renewable sunlight is used as effectively as possible by you and your appliances. This stored energy can then be used by homes and businesses at a later time without depending on the national grid.

And the key to storing solar energy? A good battery storage system.

This article will take you through the basics of solar energy storage, how it works, plus what your best options are when it comes to solar battery systems. 

Understanding the basics of solar energy storage

Solar energy storage provides a back-up against the unpredictability of solar power. By capturing excess energy when generation is high, stored energy can then be used during periods of low or no sunlight.

But let’s take it back a little bit first to understand the basics. 

Solar power relies on photovoltaic (PV) panels. When sunlight hits a PV panel, the little packets of light energy, called photons, ‘excite’ electrons in the panel, which start to move around. The internal semiconductor technology is able to harness the energy from this movement to create electricity. 

This process is almost instantaneous and involves no moving parts, making solar panels a clean and highly renewable source of energy. 

When there’s plenty of sunlight around, panels can generate more electricity than is immediately needed. In these instances, electricity supply outweighs demand. Without a battery storage system, this excess energy would simply go to waste. But with solar battery storage, it’s possible to store the energy for later use. 

Battery energy storage systems, also called BESS, allow households and businesses to:

• Use solar energy to power appliances in the evening, at night or when the sun isn’t shining
• Increase energy independence and reduce reliance on the grid
• Provide back-up power in case of power cuts
• Move away from fossil fuels and use more sustainable forms of energy 

Solar energy storage also enables what is known as peak load shifting. This process involves storing energy during off-peak hours, such as at night when demand is low, and releasing it during peak hours. 

This can result in cost savings for consumers and help to balance demand on the grid. Depending on the tariff, the consumer can then be paid for this service thanks to the Smart Energy Guarantee (SEG) scheme.

How does solar energy go from panel to battery?

When solar panels absorb sunlight and create electricity, they do so in a form called direct current (DC) electricity. Batteries also make use of DC to store the electrical energy. 

There are two types of systems for sending solar energy from panel to battery:

1. DC-Coupled
2. AC-Coupled

In a DC-coupled storage system, the DC electricity from the solar panels is sent to a charge controller and then to the battery for storage without being converted to AC. When the stored energy is needed, it passes through a hybrid inverter to convert it to AC so it can be used by your household devices.

In an AC-coupled storage system, the solar energy goes through three conversions before it can be used around your home. First, the DC is converted to AC by the solar inverter, then a separate battery inverter converts the AC back to DC for storage in the battery. When it’s time to use the energy, it goes back through the battery inverter to be converted into AC. 

So, why all of the switching from DC to AC? 

It’s because our home appliances and national grid use AC as the main form of electricity. There are a few technical reasons:

• It’s easier to change the voltage of AC electricity. For the grid and system-wide distribution, AC voltage can be stepped up for efficient long distance power transmission, and then stepped back down for safe use in homes and businesses. This isn’t possible with DC.
• AC motors tend to be simpler, cheaper and more durable than an equivalent DC motor. This made AC the preferred distribution method when electric motors started becoming more common during the early 20th century.

Summary on what happens to the electricity from solar panels

1. Solar panels: Capture sunlight and convert it to DC electricity.
2. Inverter:
   • In DC-coupled systems, DC electricity goes to the battery without being converted or is converted to AC to be used straight away. 
   • In AC-coupled systems, DC is converted to AC for immediate use or to AC then back to DC for battery storage.
3. Battery storage: Stores the electricity in DC form until needed.

As you can probably tell, a DC-coupled system is more energy efficient, due to the limited number of conversions. For this reason, they’re the system of choice for new installations. On the other side of the coin, they also tend to be more expensive. 

As AC-coupled systems have two separate inverters – solar and battery – they are easier to retrofit into existing solar panel set-ups. However, they are a little less efficient than DC systems.

Types of solar batteries for energy storage 

Ok, so now you know the various ways in which electricity is generated from solar panels and sent to the battery system. Now, onto the batteries themselves. 

There are two main choices when it comes to residential battery energy storage systems:

• Lead-acid batteries
• Lithium-ion batteries

There’s a third type of storage battery, known as flow batteries, but these are more applicable to large, grid-scale type storage.  

Lead-acid batteries

Before lithium-ion, lead-acid batteries were the go-to for energy storage. As a reliable and affordable battery type, they have a long track record in storage systems. However, they have limitations in relation to energy density, depth of discharge and lifespan, lasting for just 3-5 years have seen lithium-ion batteries rapidly displace them.

While cheaper upfront, these limitations have made lithium-ion a more sensible long-term investment, especially as costs have declined. Lead-acid batteries also come with an environmental issue in disposing of the lead. 

For these reasons, lead-acid technology has largely faded from modern solar storage applications and been replaced by lithium ion. 

Lead acid pros

• Cheaper upfront costs
• Reliable performance history

Lead acid cons

• Lower energy density – Bulkier and heavier
• Short lifespan of just 3-5 years
• Use an environmentally toxic substance in lead

Lithium-ion batteries

In recent years, lithium-ion batteries have taken over as the dominant storage medium for solar power systems. Their high energy density allows decent capacity storage in relatively small spaces, while their 5-15 year lifespans and low maintenance needs make them easy for homeowners to live with long-term.

There are a few types of lithium-ion batteries, including nickel manganese cobalt (NMC) and lithium iron phosphate (LiFePo4), which are the most common in solar usage. NMC, such as that used by Tesla’s Powerwall system, offers more energy density while LFP provides longer cycle lifespans. 

Lithium-ion pros

• High energy density – Compact storage for their capacity
• Longer lifespan of 5-15 years
• Low maintenance – Mostly set and forget operation

Lithium-ion cons

• More expensive upfront than lead-acid
• Potential issues with overheating if not installed properly

Best battery type to store solar energy?

When it comes to the best battery technology for storing residential solar power, lithium-ion batteries are the clear winner. 

With their high energy density providing compact storage capacities, long operational lifespans, low maintenance requirements and rapidly declining costs, there are plenty of practical and performance-related benefits for going with lithium-ion. 

As lithium-ion production continues to accelerate, we’re likely to see further reductions in cost, just as we’ve seen with solar panels themselves over the last decade.

Is solar battery storage for you?

Solar battery storage provides a way to capture excess renewable energy generated during the day to use at night or when sunlight is limited. If you have solar panels or are thinking about having them installed, a solar battery is a no-brainer. 

Installing a battery at the same time as solar panels is your best bet for energy efficiency and reduced electricity bills, but this may depend on your budget constraints. Installing a solar storage system takes advance planning and isn’t the cheapest endeavour. Prices for battery storage range from around £4,500-£10,000.  

But they can provide more sustainable, efficient use of renewable energy and are a smart choice to save money over time. If it’s not possible immediately, then retrofitting solar battery storage is possible at a later date.