Some kind of pool heater?

Beetlemama

We got a second hand inflatable hot tub type deal - but having spent several hours and a lot of clever trickery involving a hose pipe running out of my bedroom window, attached to a pond pump in the bath and three tanks of hot water, we think we need a heater.

Now, I was all about a best ways 3.5kw until someone mentioned that it's best to heat it overnight as it takes a long time to warm up - I am not thrilled at the idea of running a 3.5kw heater ALL night, that sounds awfully expensive.

So, how to cheaply heat my new tiny tub please, it's only about 4ft across, takes three full baths to fill.