5 Top Tips For Choosing A Home Solar Battery in 2024

All in one battery storage systems

Over the last five years, we have seen a significant increase in the number of UK households opting for an electrified lifestyle. The adoption of electric vehicles, heat pumps and electric cookers, etc. allowed us to reduce our dependence on fossil fuel, but the rise in electricity costs over the last two years is making us ponder if we can maintain low carbon footprints and low energy costs simultaneously.

There are two ways to keep the cost low: 1. produce our own renewable electricity with e.g. solar panels; 2. switch to Octopus Energy, a supplier of 100% renewable energy and take on one of their time-of-use tariffs. In 2023, an additional 189,826 UK households had solar panels installed (source: MCS database). Over 3.4 million UK families are now using Octopus Energy as their electricity supplier and using their low-cost overnight electricity.

Our peak electricity hours tend to be in the evening (7-11 PM) when we are back from work, turning on/up the heater and making meals. Renewable energy can only be generated intermittently (e.g. solar energy can only be harvested when the sun is up), and off-peak rates are generally applied after midnight. So, we need some temporary storage to hold the electricity for the peak-time usage, and this is where a home battery system comes into play.

If you are on Octopus, you can charge up a home battery (and your EV) using their off-peak electricity (7-12 pence per kWh) and power your peak-time consumption using the battery instead of the standard rate (30p) electricity. This saves you over 20p per kWh you use. If you use 10 kWh a day, that is approximately £730 saving per year on your electricity bill.

If you already have solar panels, a home battery system can help you better utilise the energy you generated during the day. You can either store the excess solar energy in the battery for your evening usage or sell it at the most valuable time to make a profit (See Octopus Outgoing Agile). An AC-coupled battery system would also allow you to keep your current solar setup as is, i.e. no need to replace your current inverter or disturb your FIT tariff. The electricity is stored only after your inverter has converted it to AC and your meter has measured it.

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What to consider when buying a home solar battery system?

Although home battery system is a relatively new product category, there are already a number of brands on the UK market. These include Tesla Powerwall, GivEnergy, PylonTech, Sonnen, AlphaESS and Huawei. There are many differences among the products, but we will focus on the key factors that are relevant for the storage of excess solar and off-peak electricity for reducing electricity costs.

1. Cost per kWh of storage

To calculate this, you have to factor in the cost of the battery as well as its inverter charger. Most of the brands sell them separately. For example, GivEnergy’s 3kW AC-coupled inverter charger costs an additional £1352 (source: Midsummer Energy).

Another important factor is the depth of discharge. It is the maximum usable percentage of a battery’s capacity. For example, Pylontech’s 2.4 kWh battery has a depth of discharge of 90%. This means that although its total capacity is 2.4 kWh, only 2.2 kWh can be used.

To calculate cost per kWh: (Battery price + inverter charger price) / (stated capacity x depth of discharge %)

2. Ease of Installation

Electricians often quote by an hourly rate (£45-£90 per hour depending on where you are). If the system is relatively straightforward to set up, it would certainly save you money. The complexity often rises from matching an inverter and a battery from different brands. Fault-finding and troubleshooting may also take longer when an inverter and a battery come from different manufacturers.

All-in-one modular battery systems are now gaining popularity. Their inverter and battery modules are made by the same manufacturer and are made to be 100% compatible. The inverter knows what the battery is doing and can instruct the battery management system to carry out tasks without communication errors. They are also easy to put together thanks to their Lego-like plug & play design.

We suggest that you choose a qualified local electrician who has past experience with your system of choice. Ideally, you want someone who knows the ins and outs of the system to commission it and expand its capacity (i.e. adding more batteries or connecting it with solar panels later) when your energy usage increases.

3. Charging cycles and warranties

Like the batteries in our smartphones and laptops, home solar batteries can only be charged and discharged a certain number of times before their capacities drop significantly. A complete charge and discharge is a cycle. Now, most manufacturers offer warranties to cover 6000 cycles, but a few can warrant 10,000 cycles (in general, these have their battery cells made by CATL which also makes EV battery cells for Tesla, BMW and Ford, etc.).

A 10-year battery performance warranty is now the industrial standard. It guarantees the battery will perform and retain a significant percentage of its capacity before the 10-year mark, otherwise, it will be replaced free of charge. However, different manufacturers have different standards for the “significantly retained capacity”. Premium manufacturers can guarantee 80% of the original capacity by the end of the 10 years. This figure is important for calculating your return on investment but is often omitted on the product datasheet. Make sure you ask your supplier for it if it is not listed on the datasheet.

Another important factor to consider besides warranties is how long the manufacturer has been around. There are many newcomers joining the battery storage sector. Can these companies still deliver on their promises after 5 to 10 years? If they go bust who’d provide the ongoing warranty?

4. Charging and discharging power

Max. charging current from the inverter charger determines how quickly your batteries can be charged. For example, the KSTAR BluE-S 3680D inverter has a max. charging current of 50 A, it takes 3.6 hours to charge two KSTAR 5 kWh batteries, whereas the BluE-S 5000D has a max. charging current of 100 A, it only takes 1.8 hours to charge the same batteries. If you plan to use off-peak electricity to charge your batteries, make sure you pick a system that can be fully charged within the off-peak duration.

The maximal power output you can get from your battery system is often determined by its maximal discharging current  (most UK households use low voltage ~50 V batteries).

Max. Output Power = Voltage x Max. Current

For example, AlphaESS SMILE 5 battery has a voltage of 51.2 V and a maximal discharging current of 56 A. It would give you a maximal of 51.2 V x 56 A = 2867 W output at any given time. The KSTAR battery has the same voltage of 51.2 V but a higher maximal discharging current of 80 A. It can give you a maximum of 51.2 V x 80 A = 4096 W output.

Just to give you some perspective, a typical kettle consumes around 2 kW. If your power consumption at any given time exceeds your battery’s maximal capable power output, you may find yourself drawing electricity from the grid even though you have energy in your battery. So, it is critical that you check the power output before purchasing. A higher power output means that your battery is capable of handling higher base loads and surges.

5. Availability and rising battery prices

The demand for home solar batteries has increased drastically in 2022. You may have to wait for some time before you can get the system you desire and get ahold of a qualified electrician for its installation. Both battery price and installation costs are going up and will continue to rise in 2023. As the overall cost increases, you may want to recalculate how long it would take to reach breakeven on your proposed solar battery investment.

Say that you plan to pay £6000 for installing a 10 kWh battery storage with a usable capacity of 9 kWh. You use off-peak (e.g. 10p) instead of peak rate (e.g. 45p) to charge the battery, so you save 45p-10p = 35p per kWh stored. If the 9 kWh stored matches your daily usage, you would save 35p x 9 = £3.15 a day and £1150 per year. So, you would break even in 5.2 years (£6000/£1150). However, if the total cost increases, it would take longer to recover it.

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We will use the provided information to suggest a home battery system that aligns with your energy needs. Our team will contact you to discuss the system’s design and installation costs. Should you decide to move forward with our proposal, we’ll arrange for an on-site survey to assess your current electrical setup.

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We will use the provided information to suggest a home battery system that aligns with your energy needs. Our team will contact you to discuss the system’s design and installation costs. Should you decide to move forward with our proposal, we’ll arrange for an on-site survey to assess your current electrical setup.

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