The original ceiling lights were fluorescent tubes with two power settings, approx. 20W per tube which adds up when running on battery. I made some aluminium panels to replace them with LED strips, which are a bit brighter and much lower power - 2.8W on single row (“low” power), 8.6W with all three strips lit:

They re-use the original covers, and the much flatter profile also means we can stick some 25mm celotex in the original hole, making the ceiling that bit more insulated.