One way to do this would be to treat your garage like any inside enclosure and what I would do is to use a halogen lamp on a dimming thermostat during the day. For night time heat you could use a ceramic heat emitter on a pulse proportional thermostat if you think it's necessary in the garage.
If the tortoise is going to be spending a lot of time in the garden then probably you don't need to worry about uv lighting. But if you do decide to provide it I suggest using the 24watt Arcadia Flood Uv lamp alongside your halogen lamp for uv.
I couldn't give you an actual amount but none of this costs a lot to run. The bit that could end up costing most is the ceramic heat emitter at night depending how many months of the year you plan to use it. Will the tortoise hibernate?
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Testudo Hermanni Hermanni (Corse) tortoises
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