How a backup battery works in a power cut
In normal use, the battery stores the solar power you don't use during the day and releases it in the evening, which lowers your power bill. When the grid goes down, the battery switches to its own AC outlet. Anything plugged into that outlet, directly or through a power strip, keeps running from the stored energy. Your solar kit itself switches off during a blackout, for safety, as every grid-connected inverter must.
The Iberian blackout of April 2025 showed how quickly a home without power loses its fridge, internet and lights. A backup battery keeps those running for hours, or for days if you recharge it from solar panels.
Outlet backup or whole-home backup
- Outlet backup. Every battery here has a backup outlet: you plug in the devices you want to keep running. It needs no installation work and covers the essentials: fridge, router, lights, phone chargers, a laptop.
- Whole-home backup. With the Marstek Smart Box, up to three Marstek Venus E 3.0 batteries supply your whole home, switching over within milliseconds when the grid fails. The Smart Box is connected at your distribution board and needs the Marstek Smart Meter; we recommend an electrician for that step.
How much power and capacity you need
Two numbers matter. Output power (kW) is how much the backup outlet can supply at once, so add up the devices you want to run at the same time. Capacity (kWh) is how long it lasts. A fridge, a router and a few LED lights use roughly 2 kWh a day, so a 5 kWh battery covers them for about two days.
- Marstek Venus E Mini: 2 kWh, 1.5 kW outlet. Fits small homes and kits up to 800 W.
- Marstek Venus E 3.0: 5.12 kWh, 2.5 kW outlet. Up to three units with the Smart Box for whole-home backup.
- Marstek Venus E 4.0: 5 kWh, 3 kW outlet, expandable to 15 kWh.
- Marstek Venus E Max: 10 kWh, 3.6 kW outlet, expandable to 30 kWh.
- Anker Solarbank 3 Pro: 2.7 kWh, 1.2 kW outlet, expandable to 16 kWh, with up to 8 solar panels connected directly.
- Anker Solarbank 4 Pro: 5 kWh, 2.5 kW outlet, expandable to 30 kWh, with up to 12 solar panels connected directly.
Charging during a long outage
Batteries with solar panels connected directly, such as the Anker Solarbank, keep recharging from the sun while the grid is down, so they suit off-grid homes and areas with frequent outages. The Marstek Venus E batteries charge through the household socket, from your solar kit or from the grid, and can also charge at cheaper hours on a time-of-use tariff.
Installation
Outlet backup needs no installer: place the battery indoors or on a sheltered spot outside, plug it into a socket and connect it to the app. Compare all our home batteries, or pair a battery with panels in our balcony solar kits with battery.
