Home battery storage: sizing, siting and tariffs in Malta

With solar panels, a battery keeps surplus daytime generation for the evening instead of exporting it at a lower rate. Without solar, it can charge on a cheap overnight or time-of-use tariff and run the house through the expensive hours. Some households want both, and some mainly want the lights and the fridge to keep going in a power cut.

These aims lead to different systems. Backup during an outage is not automatic: many batteries shut down with the grid unless the system has an emergency power function or a changeover switch. If backup matters to you, say so at the start, and ask exactly which circuits will stay on.

Capacity, in kWh, is how much energy the battery holds. Power, in kW, is how fast it can charge and discharge. A battery with plenty of capacity but a low power rating cannot run an electric shower or charge a car at full speed, and a large battery that never fills up in winter is money sitting idle.

A sensible size comes from your half-hourly usage, which a smart meter provides, your solar generation across the year and the tariff you plan to use. Ask the installer to show how full the battery would be on a typical summer day and a typical winter day.

A battery can be added to most existing solar systems. The question is how: a DC-coupled battery shares a hybrid inverter with the panels, while an AC-coupled battery has its own inverter and connects to the house wiring. AC coupling is often simpler for a retrofit; replacing an old inverter with a hybrid one can make sense if the old one is near the end of its life.

If your panels are on an older export arrangement such as the Feed-in Tariff, ask how a battery affects the way exports are measured before you sign. The installer should also confirm that the new equipment is compatible with what is already on the roof.

Home batteries are usually lithium-based and must be installed where a fault cannot trap anyone or spread quickly. PAS 63100, published by BSI, sets out fire safety requirements for installing battery storage in homes, including the choice of location. That is why an installer may turn down the spot you had in mind and propose another, such as a garage or an external wall.

Temperature, ventilation, access for maintenance and the length of cable to the consumer unit all affect the position.

A battery counts as generation equipment when it discharges, so your installer must tell the local network operator, the DNO. Depending on the combined output of the battery and any solar inverter, this can be a notification after installation or an application before it; the second takes longer, and the operator may limit how much you can export.

A battery that can export may be used with a Smart Export Guarantee tariff. You apply to a SEG supplier yourself, and the supplier decides whether and how battery exports are paid, so ask before you rely on that income.

• usable capacity in kWh and continuous power in kW
• DC- or AC-coupled, and whether a new inverter is included
• where the battery will go and how that position meets current fire safety guidance
• whether backup power is included, which circuits it covers and how it switches over
• DNO notification or application, and any export limit
• which tariffs and control apps the system works with
• warranty length and the guaranteed remaining capacity at the end of it

For solar PV with a battery, check the current schemes of the Regulator for Energy and Water Services (REWS). REWS runs incentive schemes for residential users, such as the Renewable Energy Sources Scheme and the Water Heater Scheme, and publishes the conditions for each year's scheme. Conditions and deadlines change from year to year, so check them before you sign.

Ask the installer whether the equipment and the installation meet the scheme's conditions and who submits the application.

Source: rews.org.mt

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