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What Size Solar Battery Do I Need - UK

Like millions of others in the UK who are considering renewable energy, you might have your eye on a solar PV system. Solar panels are an excellent way of reducing your energy bills as well as your carbon footprint, which is why so many of us are investing in them.

For example, solar panel sales in 2021 were up by 75% from the previous year. This figure is likely to be much higher now, driven by the twin incentives of astronomical electricity prices and the threat of climate change.

But choosing the right solar panel system can be a challenge. You need to ask questions like, how many solar panels will I need? What kind of solar inverter is best? And if you opt for battery storage, you'll need to ask, what size solar battery do I need?

We're focussing on that last question right here to give you all the information you need in order to pick the right battery storage for your solar system. We've also added some basic information about solar PV systems just in case it's needed.

The world of solar panels used to be pretty straightforward, but as solar battery technology has advanced and new products are released on a regular basis, the subject has become complex and can be a little confusing.

The experts at Skylamp Solar have an in-depth knowledge of this subject and have put together this mini-guide to give you a helping hand.

What Is Solar Battery Storage?

A solar battery storage system is a brilliant way of maximising the solar energy from your panels. They are used with solar photovoltaic systems (often shortened to solar PV), as solar thermal panels only produce hot water. You can also connect them to a wind turbine or any other renewable energy method that produces electricity.

Although solar battery storage systems are optional in most cases (as your solar PV system can run perfectly well without one), it makes sense to invest in them under certain circumstances. In fact, more people than ever are choosing to include solar batteries when having solar panels installed, as they serve to offset the massive hikes in energy prices that the UK has experienced recently.

Not only do they allow you to produce cheaper, greener energy, but they can also be configured to draw energy from the grid at night (using a special type of solar inverter) when the rates are lower. This stored energy can then be used at your leisure, and you'll avoid paying the higher tariff.

Solar Battery

Do I Need A Solar Battery?

It all depends on your lifestyle and your main hours of energy usage.

Homes or businesses without a battery system might generate more energy than they can possibly use during the day. This is usually the case for homes where the occupants are out for most of the daylight hours. Their solar panels produce electricity that powers any appliances that might be running, but any excess energy is wasted. And at night, when the solar panels aren't generating electricity, the occupants will need to rely on their energy company.

Wintertime can be especially problematic here, as the panels won't have much daylight to work with, so they'll stop producing electricity much earlier.

For households where people are at home all day, this isn't a problem; they will probably use all the energy generated by the panels during daylight hours. Once again, they will need to switch to using electricity from their supplier in the evenings.

However, if any excess energy is stored in a solar battery, this can be used in the evenings, meaning that it's not wasted and you get cheaper electricity.

In some cases, a solar PV system can be configured to send the surplus electricity to the national grid, but the homeowner won't benefit from this unless they have applied and qualified for the Smart Export Guarantee scheme (SEG).

The Solar Battery

Electric Vehicles

If you have an electric car (or any other electric vehicle), a solar battery storage system would be a wise investment. You can charge it with 'free' electricity from your solar panels, or use any excess electricity stored in the battery.

While the initial investment might be high, the running costs are kept to a more manageable amount.

Solar Battery Types

Although there are dozens of different solar batteries, they are generally split into two categories:

Lead-acid Batteries

These can also be divided into two basic types: sealed or flooded.

A flooded lead-acid battery has a lifespan of between five and seven years, and requires more maintenance*, while the sealed type only lasts between three and five years. Sealed lead-acid batteries aren't fully enclosed, as they have vents in the sides to allow gasses to escape.

Lead-acid batteries are the cheapest option in the short term but will need to be replaced much earlier than lithium batteries.

*also known as 'wet' lead acid batteries, they have removable caps and need to be topped up with distilled water.

Lead acid Batteries

Lithium-ion Batteries

These are a much more attractive option - literally. They are compact, with most examples being half the size of a typical lead acid battery, and they are designed to look good.

They also have a far superior lifespan, with an average of around ten years! While they are undoubtedly more expensive, they are highly efficient and last much longer.

Considering that modern solar panels can last between twenty-five and forty years, you might be expected to replace a lithium-ion battery only two or three times during this time. With a lead acid battery, you might get through six or seven in the same amount of time!

Lithium ion Batteries

What Does 'Battery Size' Mean?

This refers to battery capacity rather than the item's physical size.

A battery's capacity is the maximum amount of energy that it can hold at any one time, and this is measured in kilowatt-hours (kWh).

However, the most important figure to note is the usable capacity, also known as the depth of discharge limit (or DoD). In plain English, this means the actual amount of power you can use without damaging the battery. You will rarely be able to use the full capacity of any solar battery!

For example, take a 6.5 kW battery with a depth of discharge limit of 94.5%. You can only use 6.14 kW of the energy stored in the battery before it needs charging again. If you exceed this, you will affect the battery cycle (the number of times it can be charged and discharged) which will shorten its lifespan.

It is essential to know this, as all solar battery systems have a different maximum depth of discharge limit. If you regularly discharge the battery (use up all the stored energy) beyond this depth of discharge limit, you will drastically reduce its lifespan and potentially cause irreversible damage.

A lead acid battery will probably have a much lower DoD, usually around 40-50%, while lithium-ion batteries are generally between 80-95%. You might even find some very new lithium-ion batteries that boast 100% usable capacity, but these are fairly rare.

So, when you choose a solar battery be sure to check the usable capacity (or DoD) against the nominal capacity. Generally speaking, the higher the DoD, the better, as you get to use more of the power without the battery needing to be charged.

How Much Does Solar Battery Storage Cost?

A lead-acid battery generally costs between £2,500 and £4,500 depending on its usable capacity in kilowatt-hours.

Lithium batteries are typically between £3,500 and £6,000 due to their superior lifespan and discharge rate.

While it might be tempting to go for the cheaper option, remember that you'll potentially be replacing the lead battery several times. It's definitely worth considering the lithium-ion variety if you can afford the initial investment, not only for its lifespan but also because of the improved performance and high efficiency.

Solar Battery Sizes: Which One Do I Need?

There are several different sizes, ranging from 1kWh up to around 25kWh, so you'll need to choose carefully.

This involves a bit of work as you'll need to gather some information and calculate the ideal size. Here's a list of things you'll need to know:

The Size of Your Solar Panel System

Again, we're not talking about the physical dimensions here (although that plays a role, as the amount of available roof space may limit the number of panels you can install). Instead, this refers to the total wattage output of the solar panels.

Your Energy Consumption Each Day

This is pretty easy to do if you have a smart meter, otherwise, you can find the information on your energy bills.

How Much Energy You Use During The Night

Again, a smart meter or electricity bill will tell you this. Ideally, we would need to be able to get figures relating to daily and hourly energy usage to calculate accurately.

The Amount Of Energy Your Solar Panels Generate

For those with an existing solar PV system, you can check the display on the inverter. Some systems come with an app that monitors the output. If your solar panels haven't been fitted yet, then your installer or the manufacturer can tell you how much electricity you can expect them to produce.

Solar Panel 1

Are You Planning To Go Off-Grid?

If you want to get cheaper electricity, then a single battery will probably do (depending on electricity usage), but for off-grid use, you'll need several to ensure that you don't lose power during a heavy winter.

The size of your property also plays a role, so here's a rough guide as to what energy storage capacity you might need according to the building size:

Four Bedroom - 10kWh

Three Bedroom - 8kWh

Two Bedroom - 4kWh

One Bedroom - 2kWh

Using this information, you can find a lower and upper limit, a minimum and maximum amount of solar power storage you might need. You would then go for a battery system somewhere in the middle - in most cases, it will match the above list relating to property size, but it all depends on the output of your solar panels and the size of your solar array.

Although it can seem a complex process, it all makes sense in the end. And if you are still uncertain, Skylamp Solar will do all the calculations for you to ensure that you get the right solar battery storage for your home or business.

Why Is Battery Size Important?

You need a battery storage system that suits the size of your solar array. If you buy one that's too small, you'll have to rely on an energy provider when you run out of stored electricity.

And if you choose one that's too big, your solar panels won't be able to charge them fully. Why is this a problem? Because if the battery never reaches full charge it often leads to chronic undercharging that will affect performance and lifespan.

Given that you've probably paid somewhere between £1,600 and £6,000 for your solar battery, that's not very welcome news! The lesson here is that bigger is not always better.

What you need is a battery system that's just the right size: not so small that it can't store all the electricity produced by your panels, and not so large that it never gets full.

Do I Need An AC or DC-Coupled System?

What's the difference? you might ask.

Well, it's all to do with where the battery is located within the system.

As you may know, AC stands for alternating current, while DC stands for direct current. With AC, the current flows backwards and forwards from positive to negative, and DC voltage flows only in one direction. AC is better for transmitting over long distances (as there's less energy loss) and is more suitable for running household appliances, which is why UK grid power uses this form.

AC-Coupled Systems

With an AC-coupled system, solar panels send a DC charge to the inverter which converts it to AC for use in the home. But if the energy is not needed, it is sent back to the inverter (sometimes to a separate inverter incorporated into the battery itself) that turns it back into DC form for storage.

So, with an AC-coupled system, the electricity typically gets converted three times before it is used.

Overall, these are preferred to the DC system, as they are more reliable and flexible, and it's easier to retrofit them to an existing system. AC-coupled batteries can also be charged by the panels and the national grid, so you are never without power.

However, they are generally more expensive and not as efficient as DC-coupled batteries because of the need for constant conversion between AC and DC.

DC Coupled Systems

With these systems, the battery is connected directly to the solar panels before the generation meter or inverter via a charge controller. The charge will only pass through the inverter once when it is turned into AC electricity for your appliances or sent to the grid.

DC-coupled systems tend to be more affordable (as you probably only need one inverter) and they are more efficient as the charge only has to be inverted once. And, in theory, you can also oversize your system, meaning that you can add more solar panels to produce more power than you need using your existing inverter. This extra energy can be used to charge an electric vehicle, a water heater, or a home battery.

The main downside is that the battery has to be situated close to the panels, so there's less flexibility in this matter.

Can You Fit Battery Storage To An Existing Solar PV System?

Indeed you can. In most cases, we can retrofit your new solar battery to your old PV system without too much hassle.

It will need to be either an AC or DC-coupled battery system, depending on the type of inverter your panels are using. As mentioned above, retrofitting an AC system is a much easier process. And while it's not impossible to fit a DC system, it is a more complex job and will incur more expense.

Multiple Solar Batteries

If your solar array is very large, you'll probably need more than one battery.

This is also the norm for off-grid systems, where properties are entirely energy independent and disconnected from the national grid. Without adequate home battery storage, there's a chance that you'll run out of power, especially during a bad winter.

Thankfully, there's plenty of choice when it comes to scalable solar battery systems. These allow you to start with one battery and add more storage as and when the need arises. It's a more manageable option that avoids a huge initial investment.

Will A Solar Battery Work As An Emergency Power Supply?

Not all solar batteries are able to do this, but there are plenty that will automatically step in if there's a power cut.

This also depends on the type of inverter you have, as most will cut off the flow of electricity in the event of a power outage. The reason for this is that engineers might need to examine and work on the electrical grid system. And if the current is still flowing in grid-tied solar panel systems, it could present a risk to the workers.

If you specifically want a system that keeps the lights on during a power cut, you'll need a battery backup that acts as an emergency power supply. When there's a drop in voltage, the system automatically cuts off the grid connection and switches to the backup supply. It also allows your solar panels to keep working, sending solar-generated electricity to the battery storage.

A Few Last Words

Finding the right solar battery system can be a frustrating task, as there's so much information to digest and so many factors to consider - not least, the cost. These components require a sizable investment, with the average solar battery costing around £4,500 plus installation costs.

Because of this, you need to get it right.

When calculating the size, it's always better to opt for a slightly bigger one than you need. A larger battery storage capacity will be handy if you invest in a heat pump or more panels at a later date.

Finally, if you need help or advice about finding the best solar battery for your solar PV system, Skylamp Solar is here for you.

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