Solar Battery FAQ

What are the Best Types of Solar Battery?

There are 2 main factors to consider when reviewing the best types of battery; the first being its internal chemical composition, and the second is the connecting system. Although specifications of batteries can vary, it is always necessary to review the correct sizes and voltage requires for each individual task.

Lithium-Ion vs Lead-Acid Batteries

The majority of solar batteries used in recent years have lithium-ion compositions, with this battery type being found in many everyday electrical appliances such as mobile phones, laptops, calculators and laser point printers. Whilst lead-acid batteries are still commonly used, the associated technology is far older than that of lithium-ion batteries. Accordingly, lithium-ion batteries operate more efficiently, with longer life spans, and provide greater depth of discharge.

For anyone looking towards a better-quality battery, Lithium-ion models are the suitable choice, however, lead-acid batteries can be appropriate in circumstances where bulk purchases and space allocation is not an issue.

Direct Current vs Alternating Current Batteries

In addition to considering the correct internal battery composition, it is also important to understand the manner in which a battery does or doesn’t connect to your Solar PV system and there are two main ways of linking a battery storage system.

  • DC Coupled: These batteries are installed on the same side of the solar inverter as the solar PV panels. They charge from the panels and their DC is only converted to AC when it’s used. If installed at the same time as a Solar PV system, the battery and panels would usual share the same inverter, commonly known as a ‘Hybrid’ inverter. DC-coupled systems can be retrofitted without changing your current inverter; however, you will need a charge controller for your battery.
    • Pro – More efficient power transfer than AC by up to 3%.
  • AC Coupled: These batteries are installed on the grid-side, where the solar PV’s DC has already been converted to AC. A separate inverter converts the AC back to DC for storing in the battery. When the battery discharges, the same separate inverter converts the DC back to AC.
    • Pro – Higher energy capacity as you get the combined power from battery and solar PV system.

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Is it Worth getting a Solar Battery?

Solar batteries can be an excellent long-term investment for any homeowner or small to medium business owner. It is, however, vital to consider a number of factors when deciding whether a battery will save you money or not.

The main cost for a solar battery comes with its initial purchase and installation. A number of batteries will have a 10-year warranty included, providing piece of mind, and supporting the fact that solar storage systems will provide a return on investment and not require frequent replacements.

As prices have decreased significantly overtime, with the push for renewable energies taking centre stage, it is highly likely that these prices will drop even further in the coming years, making these storage units even more attractive to prospective purchasers.

Solar PV systems have a long lifespan, being able to last for upwards of 30 years. As such, solar batteries which also can last up to 20 years will provide a long-term solution for energy subsidisation and storage. Whilst many people may be off put by the initial price, these prices have become far more affordable than over ten years ago, with the prices decreasing by over 65%.

Prospective owners ought to account for the utility of solar batteries allow the use of up to 30% more electricity produced by the solar panels than if this storage was not present. This is according to E.ON which based these figures on a 12 x 315W panel setup configured to a 9.6 kWh battery. The total savings would amount to £560 per year.

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How much is a Solar Battery for a House?

Comprehensive solar batteries systems usually cost between £4,500 up to over £10,000, depending on a number of specification variables such as a battery’s energy capacity, brand, construction material, lifespan and the installation cost.

A Which? Survey conducted within the UK found that 25% of the participants paid under £3,000, alongside 41% paying between £4,000 and £7,000 for a battery storage system.

This survey did exclude the cost of solar PV in a number of cases, and when looking to get the best deal it would be financially wise to consider purchasing a solar panel and battery system at the same time.

Whilst this will be beneficial, it does not prevent the possibility of finding a great deal for a solar battery as a purchase.

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How Good are Solar Batteries?

Solar Batteries can greatly improve the money-saving potential created when a solar PV system has been installed on a house. Having a solar battery system in place will increase the self-consumption of your existing solar PV system. As well as reducing your daily electricity costs, having this unit in place will also reduce your environmental impact, lowering your carbon footprint.

In addition to this, it is also possible to operate independently from the UK power grid for a period. With energy companies frequently increasing their energy prices, having a solar battery in place will go a long way to lowering the impact of these price hikes.

In the event of a power outage, having a solar battery may also allow for sustained power use, allowing homeowners to continue on with their activities in place will also allow activities which were underway at the time to be restarted. Any solar battery with backup capability will provide the opportunity for a small business to continue with work through electronic devices during downtimes.

For particularly heavy electrical usage it would be recommended to have a charge controller which allows for significant depth of discharge, and a number of units which have substantial useable capacities.

Business owners should keep in mind that not all solar batteries are ‘Uninterrupted Power Supplies’ which would allow for a seamless transition from the National Grid to the stored energy. If the aim of an Employer is to have this function it is essential to ensure that the battery has this function, and not only an ‘Emergency Power Supply’ capacity.

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How long can a Solar Battery hold a Charge?

This question can address a number of aspects which need to be considered. A general answer when determining how long a fully charged solar battery can deliver power to a house in the UK would dictate that it can last overnight when the solar panels are not producing energy. To give an accurate duration it is important to understand a number of variables; the average daily power consumption of your household, what the capacity and power rating of the solar battery is, and whether or not you are connected to the National Grid.

To understand this in practical terms, the LG Chem RESU10 has a nominal capacity of 9.8kWh, Usable Capacity 8.8kWH, its Peak Power is 7kW however this is only for 3 seconds, so the Max Power is 5kW. The RESU10 can provide 5kW Max Power, but as this is only a battery, it’s the charge controller that provides the charge/discharge rate. In some situations, this could be between 2.5kW – 5kW.

To determine the duration for which this solar battery will last, the energy storage capacity will be divided by the accumulative total power consumption of all connected household appliances used.

An LCD TV, on average will use 150 Watts, a stereo receiver uses 450 Watts, and a 40-Watt LED Bulb will use 10 Watts per hour. If all of these items were the only appliances used throughout a day, and where left on for a 24-hour period the power demand would be:

  • LCD TV: 150 x 24 = 3.6 kWh (3600 Watts)
  • Stereo Receiver: 450 x 24 = 10.8kWh (10,800 Watts)
  • 40-Watt LED Bulb: 10 x 24 = 0.24kWh (240 Watts)

These 3 devices would total 14.64 kWh of energy usage within a day. Keeping in mind that the usable capacity for an LG Chem RESU10 is 8.8kWh, it becomes clear that the battery would not last a full 24-hour period, without being supplied energy through the connected solar panels.

This basic example gives a general idea on how to calculate battery duration during a power outage, however, remember that many appliances such as garage door opener, blow dryers and kettles are only used for a very small fraction of a day. With a 500-Watt item which is used for 10 minutes only consuming 50 Watts of energy. With this intermittent usage, and high demand devices not being active for lengthy periods of time it is highly like that a battery with 10 kWh capacity could last multiple days.

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Do I need a Battery for my Solar Panel System?

It is not necessary to have a Solar Battery present in order to have a functioning Solar PV System as Solar Panels and Solar Batteries are independent components.

A Solar Battery will require an installation of panels in order to store energy, however, Solar Panels can supply energy in real-time and feed any excess energy produced back into the grid. Doing so can allow for a secondary power source to be available, independent of the National Grid, allowing for continued power usage during any potential power outages.

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What is the Depth of Discharge (DoD) of a Solar Battery?

A solar battery’s DoD represents the percentage of a battery which has been discharged, relative to the Nominal Capacity of a battery. A Tesla Powerwall 2 has a Nominal capacity of 13.5 kWh of electricity. If 12 kWh were to be discharged, the DoD would amount to 88%.

It is important to note that different battery models will have different maximum discharge rates before requiring a recharge. The Powerwall 2 has a maximum discharge of 100%, meaning 1 cycle would allow a user to draw all 13.5 kWh of electricity from the unit without affected the battery.

In comparison, the LG Chem RESU10H has a Useable Capacity of 95%, based on a Nominal Capacity of 9.3 kWh. This means that the maximum DoD possible from this unit would be 8.84 kWh’s before a recharge becomes necessary.

From this, it becomes clear why DoD is important, as higher DoD translates to being able to use more energy stored within a battery. Many modern lithium-ion batteries advertise a DoD of over 90%.

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What is the Cyclic Life of a Solar Battery?

The lifespan of a solar battery is determined by the number of cycles it can use. A battery cycle is defined as the number of times a battery can be fully charged and discharged before they reach the end of their functional life.

Cycle life specifications can vary substantially depending on their internal chemistry. Fortunately, lithium-ion batteries, which solar storage units primarily use, have the greatest number, typically having 4000-8000 cycles within their lifetime.

In practice, a solar battery may be used four times at 25% to reach one full cycle, provided the DoD of the battery in 100%.

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How many Solar Batteries does it take to power a House?

There is no universal answer to this as there are diverse energy requirements for different houses. Whilst a large 4-bedroom detached house will nearly always use much more energy than a small bungalow with only 1 bedroom, the energy consumption may disproportionately differ for reasons such as the bungalow resident using numerous electrically demanding appliances frequently whilst a family in a 4-bedroom detached house may be far more conservative in their energy usage. Most energy guidelines revolve around the principle of ‘the more electricity you use the more solar panels you will require to offset this’.

It is advisable to review your homes previous yearly energy usage, with specific reference being made of your electric bills. The average 4-person house uses approximately 3,600 kWh of energy per year in the UK, however, depending on the appliances and devices used, the frequency of use, and the number of users will significantly affect the kW’s used.

Energy outliers can consume a great deal more than 3,600 kWh, going about 10,000 kWh of use per year, whereas a conservative energy user may only use the bare essentials and consume 1,000 kWh. To discover how much electricity your home uses, review previous energy bills, which will display your electricity usage in kWh over a specified period of time.

To calculate the system size with a system performance that will contribute to the 3,600-kWh used in an average 4-person house depends on a large number of factors. The Microgeneration Certification Scheme (MCS) created the methodology described below which is necessarily simplified in order to create a standard method that can be used to achieve a reasonable estimation of performance without it being an unduly complex procedure. The purpose of a standardised procedure is intended to prevent miss-selling and overestimation of PV systems – such that all customers will receive a system performance estimation completed to a standardised procedure.

The estimated annual electricity generated (AC) in kWh/year of installed system shall then be determined using the following formula:

Annual AC output (kWh) = kWp x Kk x SF

Kk – kWh/kWp Value. Tables of kWh/kWp (Kk) values are provided for each postcode zone in the UK. The tables provide kWh/kWp values for the zone in question for 1° variation of inclination (pitch) and 5° variations of orientation. During the site survey, you will make note of the roof pitch (inclination) and the orientation from the south. So, for example, a solar array facing due south has an azimuth value of 0⁰ and in this example has a 40⁰ roof pitch. Using these values for our Postcode Zone (LN = 11) you look up the table for the Kk value to match i.e. 893

SF – Shading Factor. Where there is an obvious clear horizon and no near or far shading, the assessment of SF can be omitted and an SF value of 1 used in all related calculations. Where there is a potential for shading from objects further than 10m away from the centre midpoint of the array then MCS has provided a procedure to calculate the SF value which will be less than 1.

kWp – Size of the Solar PV Array. The kWp value used shall be the sum of the data plate value (Wp at STC) of all modules installed (the value printed on the module label). So for this example, 14x 285w panels would equal 3.99 kWp

Calculation based on the above information:

Annual AC output (kWh) = 3.99 kWp x 893 Kk x 1 SF = 3,563 kWh

So, in this example, this 14 panel 3.99 kWp Solar PV system will provide 3,563 kWh of annual electric. This would contribute highly towards the 3,600-kWh used in an average 4-person house; however please note that the annual generation is only during the sunlight hours, whereas the average use will include night time use.

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What should I know about the Solar Battery system before buying?

After over 10 years of being in the Solar industry, it still surprises us of how many people don’t get more than one quote, as well as making decisions on quotes without the data to compare.

You may now be interested in a Solar Battery system, and to fully understand what you are buying, UK Alternative Energy recommends getting the answers to the following questions before making a decision:

What is the Useable Capacity of the battery?

Capacity is the amount of energy in kWh (units) that a battery can store. Batteries should never be drained completely. However, some are misleadingly sold quoting ‘total/nominal’ capacity. Check what’s being stated. ‘Useable capacity’ is the figure you need to know. The LG Chem RESU10 has a nominal capacity of 9.8kWh, however, it’s Usable Capacity is 8.8kWh. It will never fully discharge to prevent damage to the battery cells.

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What are the Charge/Discharge rates?

So, in the previous question, we’ve discussed on ‘Useable Capacity’, so you can potentially store a large amount of energy ready to use whenever needed. The next important thing is how much your Solar PV can charge the battery as well as deliver back to your house at any one time.

Discharge rates depend on the charge controller, even for an LG Chem RESU10 which can store 8.8kWh of energy, the charge controller installed may only be able to charge/discharge 2kW at a time. So, if the battery can only deliver 2kW of power to the house, this is just enough for a kettle with a little spare for other requirements.

Likewise, if you’re generating 4kW then 2kW can go to the battery, potentially wasting the other 2kW and directing it to the National Grid.

It’s vital that you check the power output before you make a purchase, otherwise you may find yourself drawing a lot of energy from the grid even though you have energy in your battery.

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Will it work in a power cut?

Despite expectations, some storage systems will not provide power during a power-cut. Most systems are just for energy storage to increase the self-consumption of the electricity generated by your solar panel system.

However, some offer the backup capability to provide power when there’s a power cut. These can be limited for an ‘Emergency Power Supply’ (EPS) to selected circuits in the house. Again, you will be limited by the amount of power the battery can provide during a power cut based on its discharge rate. A battery may run out of power before the power cut ends – or have already run out of power if it’s been discharging all evening and the power outage starts late at night or early in the morning.

If you require your battery storage system to provide power during power-cuts it is important to talk this through with your storage installer from the outset as the system will need to be carefully designed and set-up to your specific requirements.

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What is battery lifetime?

A battery’s efficient lifetime depends on the technology and the way in which the battery is used – this is significantly affected by the number of ‘cycles’ (complete full battery charge and discharge) that they undergo. Manufacturers generally give an expected lifetime in years and/or in ‘charge-discharge cycles’.

For example: ‘Life expectancy = 10 years or 10,000 cycles, whichever is the sooner’. Lithium-ion batteries last longer than lead-acid: with a 10-year lifetime expectancy as standard, with this longevity improving all the time.

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What is the product warranty?

A product warranty is a guarantee that an item will perform to a certain standard for a designated period of time. Most battery storage systems come with a 5- or 10-year product warranty, very much like your Solar panel system inverter.

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What is the price (£) per kWh of Useable Capacity?

If you have the information from above, you will be able to calculate this yourself and we recommend this as it sheds a light on the comparison of products and quotes. For example, you may have two quotes:

(1) £4,000 and;

(2) £8,000 but broken down,

(1) may have a useable capacity of 3kWh and;

(2) 10kWh.

So (1) is costing £1,333.33 per kWh of useable capacity and (2) is £800 per kWh of useable capacity. This makes it clear to see that whilst (2) is the larger outlay, you are getting more for the money as well.

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