Home Battery Backup for Power Cuts: How to Size It

Where the grid fails every day or every week, a home battery is the quietest way to keep the lights on. Size it in two steps. The battery's capacity (kWh) must cover the energy your essential loads use during the longest typical outage. Its backup output (W) must cover everything that runs at the same moment, including the start-up surge of pumps, fridges and air conditioners. Solar panels then recharge the battery between outages, which is what makes the system work day after day.

Why ordinary solar goes dark in a power cut

A standard grid-tied solar inverter must switch off when the grid fails. This protection, called anti-islanding, stops the system from feeding power into lines that engineers are working on. So a rooftop system without a battery produces nothing during a power cut, even at midday.

A backup system needs three things:

A battery that holds energy for the outage.
A hybrid inverter with a backup (EPS) port. When the grid drops out, it disconnects from the grid and keeps supplying the circuits on that port from the battery and the solar panels.
A separate backup circuit. An electrician moves the essential loads (fridge, lights, router, fans, water pump) to a small backup loads panel fed from the EPS port. The rest of the home stays on the grid side and goes dark in a power cut.

Normal day and power cut

Grid on

GridSolar panels
Hybrid inverter and batterykeeps the battery charged as a reserve
Home circuitsBackup loads panel

Power cut

Solar panels
Hybrid inverter and batterydisconnects from the grid, switches over in ≤ 10 ms
Backup loads panelfridge, lights, router, fans, pump

Only the circuits on the backup loads panel keep running in a power cut. The rest of the home waits for the grid.

Switchover time decides what you notice. With a switchover of about 10 ms, lights do not flicker visibly and most computers, routers and CCTV recorders keep running. Slower systems or generators leave a gap of seconds to minutes.

Whole home or essential loads? Backing up the whole home needs a much larger inverter and battery, mainly because of air conditioners, electric water heaters and cookers. For most homes it is better value to back up the essential circuits and leave the heavy loads on the grid side.

How to size the battery

Sizing in five steps

List essential loadswatts and hours of use
Add up the energykWh per outage
Check the peak powerincluding motor starts
Add reserve and lossesabout 10–20%
Check the rechargesolar between outages

Capacity comes from energy (kWh); the inverter's backup output comes from the loads running at the same moment (W).

1. List the essential loads. Write down each appliance, its power and how long it runs during an outage. The rating plate or the manual gives the power. For a four-hour evening outage, a typical list looks like this:

Load	Power	Hours in the outage	Energy
Fridge-freezer (compressor runs about a third of the time)	150 W	4 h	0.2 kWh
LED lighting, 10 × 8 W	80 W	4 h	0.3 kWh
Router, phones, laptop	60 W	4 h	0.2 kWh
TV	100 W	3 h	0.3 kWh
Two ceiling fans	120 W	4 h	0.5 kWh
Water pump, short cycles	750 W	0.5 h	0.4 kWh
Total			about 1.9 kWh

2. Add up the energy. Here the evening outage needs about 1.9 kWh. If there is also a two-hour outage in the morning, add that too: about 0.8 kWh in this example. One inverter air conditioner at 1,000 W for four hours adds another 4 kWh on its own, which is why air conditioning decides the size of most backup systems in hot climates.

3. Check the peak power. Add up everything that can run at the same time: in the example, about 1,260 W. Then allow for motor starts. Pumps, fridge compressors and non-inverter air conditioners can draw several times their running power for a moment. The inverter's backup output must cover the running total continuously and the start-up surge for a few seconds.

4. Add a reserve and losses. Batteries keep a small reserve, and some energy is lost in conversion. Adding about 10–20% is a practical rule. For the example, 2.7 kWh for both outages becomes roughly 3–3.3 kWh of usable capacity.

5. Check the recharge. A battery is only useful if it is full again before the next outage. With daily outages, the solar array should be able to cover the daytime loads and recharge the battery on an ordinary day. In the rainy season, or where outages last for days, plan for less solar and more capacity.

An illustrative day with two power cuts

Solar used directly 3.5 kWh
Solar stored in the battery 4.4 kWh
Solar not needed 5.9 kWh
From the battery 4.4 kWh
From the grid 2.5 kWh
Power cut 06:00–08:00, 18:00–22:00
Solar output 13.8 kWh
Essential loads 10.4 kWh

Example: about 3 kWp of solar, a 5 kWh battery that is held in reserve while the grid is up, and essential loads of 300–800 W. Solar refills the battery by late afternoon, and the battery covers the morning and evening cuts with some charge to spare.

Battery or generator? A generator handles long outages and large loads, but it needs fuel, is noisy, gives off fumes and takes time to start. A battery switches over instantly, runs silently and is recharged by solar, but its energy is limited. For short, frequent outages, a battery with solar usually wins. Where outages last for days, homes often combine the two, with an electrician making sure only one source can supply the circuits at a time.

What outages look like in key markets

The right size depends on how long and how often the grid fails.

South Africa. The national grid has been much more stable. In September 2026 the government news agency reported some 476 days without load shedding and a winter without it. Eskom's separate load reduction in some areas continues, although seven provinces are now free of it. Many homes keep backup for local faults and load reduction, so a battery sized for a few hours remains useful.
Nigeria. Supply depends on the tariff band. Under the service-based tariff set by the regulator, NERC, Band A feeders should deliver at least 20 hours a day and Band E at least 4 hours. The national grid collapsed in January 2026; NERC later reported no system disturbance between April and June 2026. Many households still rely on generators, so batteries are sized for several hours a day and solar does most of the recharging.
Lebanon. Électricité du Liban (EDL) reported a national average of about four hours of supply a day in July 2026, down from seven to nine hours in February. Private generators cover much of the rest. Here the grid is the backup rather than the main supply, and systems are sized almost like off-grid systems.
Iraq. In summer 2026, many cities ran on rotating schedules. In May, central Basra had five hours on and one off, and parts of Najaf two hours on and six off. Because cooling is the main load, the backup output and the number of air conditioners on the backup side decide the size.
Thailand. The grid in cities is generally reliable. Outages are mostly short. They come from planned maintenance, which MEA and PEA announce in advance, and from storms in the rainy season. A modest battery covering a few hours, with a fast switchover for computers and CCTV, suits most homes and shops.
France and Europe. Outages are rare, but storms can cut rural lines for days. After storm Ciarán in November 2023, Enedis reported 1.2 million homes without power, most of them in Brittany; the last were reconnected about three weeks later. Here backup is mainly about comfort and security.

Safety and installation

Fixed wiring by a qualified electrician. The EPS port, the backup loads panel and the battery are connected by an electrician, in line with national and local rules.
Never backfeed the grid. A generator or inverter must never be plugged into a socket to power the house. Without a proper changeover, it can push power into the grid and endanger line workers. The hybrid inverter's own disconnection, or a changeover switch for a generator, handles this.
Ask the utility. Hybrid systems connected to the grid usually need approval or notification, as for any grid-connected solar system.
Place the battery properly. Follow the manual for location, ventilation, clearances and the anti-tip kit for stacked batteries.

Checklist before you quote

How often does the grid fail, and how long is the longest typical outage?
Which circuits must keep running? Make the load list with power and hours.
What is the peak power with everything on, and which loads have motors?
How many air conditioners, if any, should be on the backup side?
Is there enough roof for solar to recharge the battery between outages?
What switchover time do the loads need (computers, CCTV, medical devices)?
Who wires the backup loads panel, and what do the utility and local rules require?
Is there room to add capacity later?

How VaySunic can help

VaySunic designs and manufactures hybrid storage at its R&D and manufacturing centre in Wuxi, with headquarters in Germany.

PowerCube is an all-in-one hybrid storage system: inverter, four MPPT solar inputs and a 2.5 kWh battery in one unit, expandable with up to five VB5000D packs to 27.5 kWh. Its dedicated EPS port switches over in 10 ms or less and supplies up to 3,000 W continuously, with 4,500 W for 15 seconds for motor starts. With the built-in 2.5 kWh battery alone, battery output is limited to 1,200 W; the full 3,000 W needs 7.5 kWh or more, or solar supplying at the same time.
SolarHub is a wall-mounted hybrid microinverter for on-grid, backup and off-grid use, with 5–25 kWh of VB5000D storage. Backup and off-grid use need at least one battery pack.

Our backup and off-grid page shows typical set-ups for weak-grid homes, islands and farms. Send us your load list and outage pattern through the contact form, and we will suggest a configuration. Installers and distributors can see our distributor programme.