Although none of us want to waste energy we’re also desperate not to lose our plants so then we may inadvertently set our thermostats rather high in order ‘to be safe’. Obviously we need to maintain a sufficiently high temperature so our greenhouse, i.e. not-hardy, plants do not sustain any damage.
Now the old recommendations of which plants would survive which low temperatures were decided when fuel was very cheap and often the houses were both very hot and steamy. However the famous Thomas Knight proved he could grow ‘modest’ pineapples without extravagant heat just through keeping them VERY dry.
In the end it’s often Moulds not the Cold that takes out many plants so be careful to not only water sparingly but never to splash it about. There’s much to be gained from increasing air circulation and/or ventilation as both these reduce stagnant conditions and the high humidity which promotes Moulds. Thus it’s often better adding a De-humidifier rather than yet another heater (provided there’s sufficient capacity in existing heaters to cope with those coldest nights).
Given drier air and dryish roots enables many plants to survive lower temperatures than otherwise and thus we can save fuel. After all many Cacti come from deserts that often drop below zero over-night. But give Cacti a milder temperature, with dim damp days over weeks and months, and many are likely to succumb unless kept arid.
Likewise even fleshy plants such as Aloes and Agaves if given drought conditions can come through very cold periods unscathed. Sadly even when kept on the dry side many tropical and sub-tropical plants just cannot take temperatures down near zero so always need some background warmth. But these vary with some more needy than others. E.g. Mangoes, Guavas and Custard Apples must be kept much warmer than Coffee and Passion fruits.
Mountain Guavas seem much tougher, as are Bougainvillea, Physalis and Daturas (Brugmansias). And Avocados and Monstera have proved almost hardy, indeed alongside Citrus.
However some, especially Papaya and Banana, desperately crave even more warmth, indeed need ‘room temperature’. My idiosyncratic solution, keep them warm enough and simply move in there with them.
P.S. I strongly recommend you also follow the excellent suggestions Hartley Botanic are making in their recent release ‘Keep your greenhouse costs down this winter’.