most economical setup

[Crow River]

Cheapest setup uses natural sunshine: outdoors or window sill / skylight.

 

Alternatively, as you're growing autos they really don't care about light cycles. I've had good results on 20/4, this might help with high temperatures if you put the dark period say noon to 4pm, warmest part of day, unlikely your heating will come on.

 

As others have said, 26ºC seems high for the heat mat, 23ºC is plenty, put bubble wrap round the pots so they stay cosy longer. Also you can place ceramic tiles on the floor, they store heat from the mat so evening out temperatures during dark period.

[Should Know Betterer]

Currently I'm using a .6x.7x.8 iirc 2heatmats set around 22°c an AC Infinity cloud line 4" never really set above 1 occasionally upto 4 and a Migro Aray2 on various settings, I can't be arsed with working out shit but mate tells me it's not much more than £1 a day to run it all. Totally for my own use, as meds to replace doctor opiates, £1 ish a day, first run of 2 I  pulled 5jars ish, this run I've added 1plant , I know someone, including my mate, will know the exact amount etc but I don't care so don't worry about it, my point was you can still grow enough for personal use indoors at a reasonable cost, much cheaper than popping to the dealer. Sorry to ramble, I made some millionaires shortbread with about 2g of Mighty iso clean oil in the chocolate, the shortbread was utter dogshit and is going out for the birds, no oil in it don't worry, but a surprisingly small amount of the caramel and the chocolate has spangled me blah blah etc etc sorry good night