Tesla have just launched a Virtual Power Plant in the UK

Tesla PR Dept. wrote:

When temperatures are low, all batteries have a reduced ability to charge. To help Powerwall counteract this, Powerwall uses Preconditioning. When temperatures are below freezing, Preconditioning turns on and heats your Powerwall to improve operation and charging performance.

To heat itself, Powerwall draws a small amount of energy, which then allows high-power charging. During a cold night, your Powerwall automatically preheats before sunrise so that maximum solar energy can be captured during the day.

You cannot customize this mode because Powerwall can best detect its internal temperature and knows when to enable Preconditioning.

During Preconditioning, the Power Flow screen may show energy flowing to your Powerwall from solar or the grid. This is normal behavior, and the energy is only being used to heat, not to charge.

During winter months, a small amount of your Powerwall capacity is reserved to improve performance in cold weather. This may change your visible total capacity in the Tesla app. Rest assured, your total capacity has not changed – this small reserve is just set aside to improve performance.

Preconditioning, in combination with Tesla’s unique liquid thermal management system, allows your Powerwall to operate at lower temperatures than any other home battery.