Oven is not the only way to dehydrate the beetroot slice. You can use microwave or if it is extremely hot and sunny in your place, you can opt for sun drying.
And the time taken to dehydrate the slices depends on how thin the slices are. Mine are slightly thick and I think that is why it took a day for the drying. But if you can slice it extremely thin, the time taken will be drastically lowered.
Someone asked whether grating beetroot will work. I think grating will result in loss of so much juice and in turn the colour. But it doesn't hurt to try.
How much electricity would the oven take for a day of dehydration? - I have no idea. I felt this was way too easier than roaming around the city in search of the powder. So if you are worried about your electricity bills, then go for sun drying. It will be economical. But it can only be seasonal.
Note 0n Using the powder: Mix a spoon full of the powder with a tablespoon of water and set aside for it to dissolve. Varying the quantity of water and the powder will give you various shades of red. Add it to the recipe instead of food colours.