Powering Change
Installing since 2010 · 0118 951 4490 · info@spiritenergy.co.uk
Image source: Tesla.
You've installed solar PV, and now you want to make sure you can keep the lights on in a power cut. This page explains how solar battery backup works, which systems we recommend, and what to think about when designing your backup setup.
Our current recommendations for solar panel backup are the Tesla Powerwall 3 and the Sigenergy SigenStor. Both are DC-coupled hybrid systems, meaning a single unit handles the solar inverter and battery management together, and both include integrated backup gateway functionality as standard.
The right choice between the two will depend on your system size, budget, and specific requirements. We can advise on which suits your situation best.
Both the Powerwall 3 and SigenStor support standard domestic single-phase backup. Here is how each system works.
The Powerwall 3 is a fully integrated DC-coupled hybrid system. Unlike the Powerwall 2 which was AC-coupled and required a separate solar inverter, the Powerwall 3 includes the solar inverter built in, meaning it handles the solar array, battery charging and discharging, and backup switching in a single unit.
Powerwall 3 technical summary:
Multiple Powerwall 3 units can be stacked for larger capacity, each additional unit adds 13.5kWh of storage and 11.5kW of additional power capacity.
The SigenStor is Sigenergy's hybrid battery system, offering flexible configuration for both residential and light commercial applications. Like the Powerwall 3, it integrates the inverter and battery management in a single system with backup capability built in.
SigenStor technical summary:
The SigenStor is particularly well suited to larger homes or properties with higher consumption, where its scalable battery architecture and higher inverter ratings offer flexibility that a single Powerwall 3 unit may not cover.
Both systems are DC-coupled. Solar panels produce DC electricity, which can be fed directly into the battery without an intermediate conversion step. DC is only converted to AC when electricity is needed for use in the home, or when the battery is full and surplus generation is exported to the grid.
This reduces conversion losses compared to AC-coupled systems, where electricity is converted from DC to AC and then back to DC for battery storage. DC-coupled systems also simplify the installation when solar and battery are being installed together, since a single hybrid inverter unit handles both.
DNO permission is required for any battery system that will operate in island mode (i.e. backup mode) during a power cut, regardless of system size. Battery inverters must be certified to G98/G99 standard, as with solar inverters.
We handle DNO applications on your behalf as part of the installation process, and recommend applying for a higher export permission level than you currently need to allow for future expansion.
Both the Powerwall 3 and SigenStor allow you to reserve a percentage of the battery's capacity specifically for backup use. For example, you might choose to reserve 30% of a 13.5kWh Powerwall 3, giving you 4kWh held in reserve at all times, while the remaining 70% continues to be used for day-to-day solar self-consumption and tariff optimisation.
This reserve setting can be adjusted at any time via the respective app.
With any backup system, there is a practical limit to the size of the solar array that can operate on the islanded side of the system, the part that functions when the grid goes down.
When the grid is offline, solar electricity can only go to immediate household loads or into the battery. If the battery is full and there is insufficient load to absorb all the solar generation, the system must throttle or shut down the solar input. Both the Powerwall 3 and SigenStor manage this automatically.
The Powerwall 3's six MPPT inputs support up to 20kWp of solar directly connected to the unit, more than sufficient for the vast majority of domestic installations. For larger arrays, panels can also be connected upstream of the backup gateway, where they will operate normally when the grid is live but will not function during a power cut.
Solar battery backup can be configured to back up the whole house or just selected circuits.
Both the Powerwall 3 and SigenStor support whole-home backup as standard. Partial backup, protecting only certain circuits while leaving others unprotected, can also be configured, which is worth considering for high-draw loads like EV chargers. Running an EV charger from backup power overnight could drain your entire battery reserve, leaving nothing available for the essentials if the outage continues.
To estimate how much backup capacity you need, consider the key loads you want to maintain during a power cut and how long you need them to run. As a rough guide, reserving 4kWh, around 30% of a Powerwall 3, is sufficient to run lights, a TV, a laptop, a fridge freezer, and make several cups of coffee for around four hours.
If longer backup duration or higher continuous power is a priority, the modular nature of both systems means additional units can be added to increase both capacity and power output.
Both the Powerwall 3 and SigenStor switch automatically to backup mode in a power cut, with switching times fast enough that most household loads are unaffected. In most cases you will notice only via a notification on the app rather than any interruption to your appliances.
Neither system should be relied upon as UPS-standard backup for critical medical or life-support equipment, where a dedicated UPS with guaranteed switching times would be the appropriate solution.
For help choosing and sizing the right backup system for your solar installation, get in touch with us today.
