M&E Contrax Resources

How Home Solar Battery Storage Works

Resource Highlights:

A home solar battery stores surplus electricity generated by your panels during the day so you can use it in the evening instead of buying it from the grid.

The two main system types are DC-coupled (integrated with a hybrid inverter) and AC-coupled (a battery added to an existing solar installation). Each suits different situations.

Battery capacity is measured in kilowatt-hours (kWh). Most homes with a standard 3.5 to 5kWp solar system get the best balance from a 5 to 10kWh battery.

Adding a battery typically increases self-consumption from around 30 to 40% without storage up to 70 to 80% with it.

Backup power during a grid outage is not automatic. Most standard battery systems are not configured for power cut protection unless specifically designed and wired for it. If resilience during outages is a priority, it needs to be discussed at the design stage.

Store your solar energy and use it when it matters most

A solar battery lets you hold onto the electricity your panels generate during the day and use it in the evening, overnight, or when the sun isn’t shining. Without storage, that surplus leaves your property at a fraction of what grid electricity costs you to buy back. With the right battery, most of it stays in your home.

At M&E Contrax Renewables, we design and install solar battery systems for homes and businesses across Devon and the South West. Whether you’re adding storage to an existing solar installation or planning panels and a battery together from scratch, we’ll assess your system, your usage, and your goals before recommending anything.

We install a range of leading battery products including Tesla Powerwall, Fox Energy Storage, and more, and all electrical installation work is carried out by our in-house NICEIC-accredited engineers.

What a home battery actually does

When your solar panels generate more electricity than your home is using at that moment, the surplus normally flows out to the grid. The Smart Export Guarantee pays you for that export, but at a rate considerably lower than what you pay for grid electricity when your panels are not producing.

A battery changes that equation. Instead of exporting the surplus, the system stores it. Later, when your panels are no longer generating, in the evening, overnight, or on a dull day, the stored electricity discharges into your home circuits, reducing or eliminating what you need to draw from the grid.

The financial benefit is the difference between what you would pay to import grid electricity and what you would earn by exporting. At current unit rates, that difference is significant, which is why adding a battery to a solar installation shifts the financial returns considerably.

Batteries can also charge from the grid during off-peak periods if you are on a time-of-use tariff. Some households use this to buy cheap overnight electricity and avoid peak-rate grid electricity in the evenings, with or without solar panels.

How the electricity flows – the basics

A solar panel produces direct current (DC). Your home runs on alternating current (AC). An inverter handles the conversion.

A battery also stores electricity as DC. Where the battery connects in relation to the inverter, before or after the conversion, determines whether the system is described as DC-coupled or AC-coupled.

DC-coupled systems

DC-coupled systems connect the battery on the DC side, before the main inverter. A hybrid inverter handles both the solar output and the battery in a single unit. Electricity from the panels can flow directly into the battery without being converted first, which is slightly more efficient. This arrangement is typically used when solar and battery are installed together at the same time, and the system is designed as an integrated whole.

AC-coupled systems

AC-coupled systems connect the battery on the AC side, after the solar inverter. The solar inverter continues to operate as normal, converting panel output to AC for household use. A separate battery inverter then converts some of that AC back to DC for storage. When the battery discharges, it converts back to AC again. There are more conversion steps involved, which reduces efficiency slightly, but this approach works with any existing solar inverter and is generally the most practical route when a battery is being added to a system that is already installed.

In simple terms: if you are installing solar and battery together for the first time, a hybrid inverter with DC coupling is usually the cleanest solution. If you have existing panels and want to add storage, AC coupling is typically more practical.

What battery capacity means in practice

Battery capacity is measured in kilowatt-hours (kWh). One kilowatt-hour is the amount of electricity a 1,000-watt appliance uses in an hour, roughly what a kettle and a washing machine cycle use between them.

A 5kWh battery contains enough stored energy to run typical evening household loads for several hours. A 10kWh battery stores more and provides a greater buffer, which matters particularly in winter when solar generation is lower and more of your energy use falls outside daylight hours.

Most battery systems specify a usable capacity, which accounts for the depth of discharge limit built into the battery management system to protect cell longevity. A battery rated at 10kWh may deliver around 9 to 9.5kWh of usable energy.

A three-bedroom household with a 3.5kWp solar panel system will usually require around a 5kWh battery. This is smaller than the home’s average daily usage of 9 to 10kWh, which makes sense, since a large chunk of household usage comes during the day when panels are producing electricity that can be used immediately.

For a household with a larger solar array, an EV charger, or a heat pump that adds significantly to electricity consumption, a larger battery in the 10 to 15kWh range is likely to deliver better returns. The goal is matching storage capacity to actual evening demand rather than maximising the number on the specification sheet.

Oversized batteries sit partially empty most of the time, worsening payback. The rule of thumb is to size the battery to cover your evening electricity demand from sunset to bedtime, based on the solar surplus generated during that day.

Battery chemistry – what LFP means and why it matters

Most home batteries installed in the UK in 2026 use lithium iron phosphate chemistry, commonly written as LFP. This has become the standard for residential storage for practical reasons.

LFP batteries are thermally stable, meaning they are very unlikely to overheat in normal use. They handle frequent daily charging and discharging well, a quality battery should be rated for 4,000 to 6,000 cycles, which at one cycle per day corresponds to 11 to 16 years of regular use before the capacity drops to the warranted level. They also tolerate being kept at high or low states of charge better than older lithium chemistries.

The alternative is NMC (nickel manganese cobalt), which offers higher energy density in a smaller physical footprint but has a shorter cycle life in daily-cycling applications and is less tolerant of heat. NMC still appears in some products, but LFP now dominates the residential market.

When comparing battery products, checking the cycle life warranty and the chemistry type is more useful than comparing headline kWh figures. A battery warranted to retain 80% of its capacity after 6,000 cycles is a more durable long-term asset than one warranted for 2,000 cycles, regardless of their initial capacity.

What self-consumption means and why it matters

Self-consumption is the percentage of your solar electricity that your household uses itself, rather than exporting to the grid.

Without a battery, self-consumption for most households sits somewhere between 30 and 40%. This is because solar panels generate most strongly in the middle of the day, when consumption is often lower, and household demand peaks in the morning and evening when panels generate little or nothing.

With a battery, self-consumption typically increases to around 70 to 80%, depending on system size, battery capacity, and household usage patterns.

The financial significance of this is meaningful. Electricity you use from your own battery is worth the full unit rate you would otherwise pay to your supplier. Electricity you export to the grid earns the SEG rate, which is considerably lower. Shifting generation into self-consumption rather than export is one of the most direct ways to improve the return from a solar installation.

Smart tariffs and charging from the grid

Some battery systems can charge from the grid during off-peak periods, not just from solar. This is useful if you are on a time-of-use tariff that offers significantly cheaper electricity at certain hours, typically overnight.

Octopus Flux, Intelligent Octopus, and similar tariffs allow households to buy cheap overnight electricity, store it in the battery, and use it during the day. Some tariffs also offer higher export rates during peak demand periods in the early evening, which can make exporting from the battery financially worthwhile at certain times.

This grid charging and discharging strategy can meaningfully improve the financial case for battery storage, but it requires a compatible inverter, a smart meter providing half-hourly data, and active management through an app or automated controls. Whether it makes sense for your household depends on your usage pattern and which tariffs are available to you at the time of installation.

Backup power during a grid outage

This is one of the most commonly misunderstood aspects of home battery storage, and it is worth being clear about it.

A standard solar and battery system connected to the grid is required by regulation to disconnect from the grid automatically when a power cut occurs. This safety requirement protects electricity network workers who may be working on the affected lines. It means that, in a standard installation without additional configuration, a power outage will cause your solar and battery system to shut down along with the grid.

For the battery to provide power during an outage, the system needs to be specifically designed and wired for it. Some solar installers call this Emergency Power Supply, or EPS — it involves a relay or transfer switch that disconnects the system from the grid during an outage, allowing the battery to power selected circuits from an isolated internal supply.

There are two main approaches:

Partial backup (EPS): A separate sub-board is wired from the inverter’s EPS output. Selected circuits are moved onto it, typically lighting, essential sockets, the boiler, a freezer, and a broadband router. During an outage, only these circuits stay live. The rest of the house remains off until grid power returns.

Full-house backup: A backup box or automatic changeover switch allows the entire consumer unit to switch from grid supply to the inverter output during an outage. This is more expensive and requires more electrical work, but means the whole house continues to operate normally.

If backup power during outages is a priority, which is particularly relevant for rural properties where power cuts are more common, this needs to be specified at the design stage rather than added as an afterthought. Some inverters have EPS capability built in; others require additional hardware. We discuss this with every customer before installation.

Adding a battery to existing solar panels

Many homeowners with solar panels installed in recent years are now considering adding battery storage. Whether this is straightforward depends on what inverter is currently in place and how the existing system was designed.

If the existing inverter is a standard string inverter, the most practical route is usually AC coupling, a battery with its own inverter is added to the AC side of the installation. This works with any solar inverter and does not require replacing the existing setup.

If the existing inverter is already a hybrid model that supports battery connection, it may be possible to add a compatible battery directly without any additional inverter hardware.

For most existing solar-only systems, AC coupling is the practical answer. It works with any inverter, and it does not require replacing the current setup.

Before committing to a specific battery product for a retrofit, it is worth having an installer assess the current system. The existing inverter’s age, condition, and compatibility with available batteries all affect what makes most sense. A battery that integrates cleanly with a well-functioning existing inverter is a better outcome than replacing the inverter unnecessarily.

What to think about before a survey

A few things worth considering before you speak to an installer:

Your evening electricity use. If you know roughly how much electricity you use between late afternoon and bedtime, that figure is the most useful guide to battery sizing. It is available from a smart meter app or can be estimated from your annual usage and your daily patterns.

Your solar system’s current export profile. If you already have solar panels, your generation and export data from the system monitoring app shows how much electricity is typically exported on an average day. That is the energy a battery would be capturing.

Future plans. If you are planning to add an EV charger or a heat pump within the next few years, those add significantly to electricity consumption and may mean a larger battery makes more sense now than it would based on current usage alone.

Grid outage resilience. If backup power during outages matters to you, say so at the start of the survey conversation. It affects inverter selection, system design, and wiring, and is much harder to add after the fact.

FAQs

You can add a battery at any point after the panels are installed. Adding storage at the same time as the panels can be more cost-efficient because installation costs are shared, and a hybrid inverter can be specified from the outset. But retrofitting a battery to an existing system is entirely possible and is something we carry out regularly.

Not automatically. A standard system disconnects from the grid during an outage for safety reasons. If you want backup power during power cuts, the system needs to be specifically configured for it — either with EPS on selected circuits or a full backup arrangement. This needs to be discussed and designed before installation.

A quality LFP battery is typically warranted to retain 80% of its original capacity after 4,000 to 6,000 full charge and discharge cycles. At one cycle per day, that equates to roughly 11 to 16 years. The battery does not stop working after the warranty period — it continues to operate at reduced capacity.

Not strictly, but a smart meter significantly improves how well the system can be managed, particularly if you want to take advantage of time-of-use tariffs, grid charging during cheap rate periods, or peak export tariffs. Most installers recommend confirming smart meter availability before installation.