The most common form of cheaper night-time electricity is known as Economy 7, but at present (April 2022) many supplies have removed these tariffs.  

Octopus and Ovo also have interesting tariffs intended for owners of electric vehicles. For example, the Octopus Go tariff is currently 7.5p per kWh for the four hours between 00:30 and 04:30. If you have a battery with capacity at least as large as your use on a winter’s day, you could be able to use very little peak electricity any season. 

Using our example, we assume that we use 1,020kWh direct from our solar panels (30%) and 1,380kWh more by storing solar energy in the battery (an additional 40%). This leaves 1,000kWh at an economy 7 or Octopus Go rate. Assuming this is 10p per kWh, we pay £100 instead of £300 for grid electricity for the year, a further saving of £200 on top of the £408. With £608 pa of savings the payback time is a bit over eight years. 

In practice you won’t achieve the ideal of not using any peak rate electricity and the battery can be expected to degrade with time and store less electricity. Thus the payback time is probably around 10 years, similar to the reasonable expectation for battery life.