Easiest, cheapest option was to fit 2 x 18 amp/hr batteries in series to get 24 volt, fitted into a standard pvc battery box.

I fitted a 5 amp smart charger under the lid of the battery box, which can be removed and the 230 plug inserted into the charger.

A plug socket flush mounted to the side of the battery box, which is the same as the one mounted on the side of the electrical control panel.

Can run for around 8 hours.