Last night as I lay awake in the wee small hours (not an infrequent occurrence) I found myself recalling 'O' level physics with remarkable accuracy and setting myself the exercise of calculating how long it would take to heat the water in the portable baptistery from "flippin' freezin' " to "pleasantly warm"... answer - almost certainly longer than we have available, even if I neglect heat losses to ambient.  Good job, then, that the instructions come with advice to use lots of hot water (once you have got a few inches of cold in and can switch on the heater safely).

My guess is we have potentially 12-14 hours heating time, which with a baptistery of approximately 3500l will take some heating... At least we will get the chill off, even if we use totally cold fill, and I am planning on getting some hot in there by fair means or foul!!

Anyway, I leave it as an exercise to the student to calculate how warm you think the water will be on Sunday morning.  Assume specific heat capacity of water is 4200J/kgK, immersion heater is 4.8kW and water starts at approx 6C...

(I won't make you do the O level maths to calculate the volume of a dodecagon, you can assume it's a cylinder 2m in diameter and 1m deep and round down to allow for it not being totally full of water! Archimedes principle comes in too when you have to allow space for the people!...)

Things they did not teach you at vicar schoool number.... very large.

And proof nothing is wasted - never thought O level physics would come in for Baptist ministry!