Battery Storage

What a battery changes

Solar without storage has an awkward shape. Generation peaks in the middle of the day; household consumption peaks in the evening. If nobody is home between eight and six, most of what the array makes is exported at a low rate and bought back a few hours later at a high one.

A battery closes that gap. It stores what the house is not using and gives it back when the house needs it. That is the whole proposition, and it is why storage often improves the economics of a solar system more than adding another two kilowatts of panels would.

It also does something panels cannot: it lets you buy electricity at one price and use it at another. On a time-of-use tariff, that price difference is worth money by itself — which is why a battery makes sense in houses with no solar at all.

AC-coupled or DC-coupled

There are two ways to connect storage, and which is right depends mostly on whether the solar already exists.

DC-coupled puts the battery on the same side as the panels, managed by a hybrid inverter that handles both. Energy is converted once rather than twice, so it is more efficient. It is the natural choice when solar and storage are installed together — see solar and battery systems.

AC-coupled puts the battery on the house side with its own inverter. It is slightly less efficient, but it does not care what solar system is already there, which makes it the usual answer for a retrofit — see battery retrofit and adding a battery to existing solar.

The inverter architecture is covered properly in hybrid inverters and inverters explained.

Chemistry

Most current home batteries use lithium iron phosphate — LFP. It has a lower energy density than some alternatives, which means a slightly larger box for the same capacity, and in exchange it offers good cycle life and favourable thermal characteristics. For a stationary home battery, where physical size matters much less than it does in a vehicle, that is a sensible trade.

More detail on LFP batteries and battery technology generally.

Sizing it properly

The most common error in battery specification is buying capacity that never gets used. A battery only earns its keep when it cycles — filling and emptying most days. Oversize it and you have paid for kilowatt-hours that sit idle for most of the year.

Backup and EPS — what it does and does not do

Emergency Power Supply is the function that keeps selected circuits running when the grid goes down. It is not automatic: a battery without EPS shuts down in a power cut along with everything else, for the safety of anyone working on the network.

Where it is specified, it usually covers a subset of circuits rather than the whole house, there is normally a brief interruption while it switches over, and how long it lasts depends on the load and the state of charge. It will not make a house off-grid, and anyone suggesting otherwise is overselling it.

Siting, ventilation and safety

Batteries dislike heat and dislike being difficult to reach. A loft is the worst of both. Where there is any alternative — a garage, a utility space, an external wall in a weatherproof enclosure — we use it.

Siting also covers manufacturer clearances, ventilation, distance from escape routes, the surface the unit stands on or is fixed to, and the practicalities of getting a heavy unit into position. All of it is assessed at survey, not assumed.

Installation and notification

A storage installation involves the battery, its inverter or hybrid inverter, AC and DC isolation, protection, metering and the connection into the consumer unit. As with solar, the network operator is notified: G98 where total generation capacity is 16A or less per phase — about 3.68kW single-phase — and a G99 application requiring approval before installation above that. See G98 and G99 applications.

Warranties and end of life

Battery warranties are more complicated than panel warranties because there are three ways to measure wear: elapsed years, number of cycles, and total energy throughput. A warranty typically expresses all three, whichever comes first — so a battery worked hard on a tariff may reach a throughput limit long before a calendar limit.

Capacity retention is the other term to read: the percentage of original usable capacity the manufacturer undertakes the battery will still have at the end of the term. Warranty terms vary by manufacturer and product, and the terms that apply to your installation are confirmed in your written quotation before work begins. More in warranties explained and on warranties and guarantees.

Cost and value

What drives the price is usable capacity, power output, chemistry and warranty terms, whether a hybrid inverter is needed, whether backup is included, and the siting and electrical work involved. What drives the value is different: it is how often the battery cycles, and the gap between the price you avoid paying and the price you would have been paid for exporting.

Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.

Set out fully on what affects the cost of battery storage.