"Solar" Greenhouse: frost free in winter?

I thought I would share some details of my new 'solar' greenhouse, which I am just completing (see photo). It is based on a design I found on the Internet.
It faces south and uses twin wall polycarb panels steeply inclined to catch the winter sun. Ideally I should have used triple wall sheets but these were beyond my budget!
The back wall is built into a steep bank which gives it protection from the cold northerly winds, while the rear roof is metal panels but insulated with fibreglass and lined inside with wooden panelling. The ends are floorboard panels on the outside and more wood panels on the inside, filled with fibreglass for insulation.
There will be a 'heat sink' consisting of a line of old 20 litre plastic containers which will be filled with water, painted black, and stacked along the inside of the rear wall. I will put a shelf above these for the really tender small things.
There will be small electric fans above the two doorways (where there will be solid wood doors) to ventilate and let out any excess heat.
The main aim is to have somewhere where my tender shrubs and trees, like the guava, avocado and other sub tropical things can survive the winter. They make it through in another building which has walls on three sides and a polycarb roof but that is too dark and I needed something with more light. If the temperatures drop too far, and here we do often get minus 19 and 20 C, I will see if I can afford to run a small electric fan heater set to switch on if temperatures get towards 3 or 4C. I'm hoping as the place will be well insulated it shouldn't be needed much, but winter always brings those dull grey days where the sun won't get a chance to heat the place up.
I hope others are inspired to do something similar. I am going to keep the max/min thermometer readings to see how much heat it loses on the first cold nights.
Even if doesn't work, I will have a very nice melon house for next summer!