It boils down to your budget. A kwh costs about $0.27 a heat mat uses about half a kwh a day. How much can you afford to pay to have the soil at 75 degrees and then keep the frost off the leaves? You could double greenhouse build a poly tunnel inside your regular tunnel and put your heat mat on insulation and flats on top of that this reduces the amount of heat energy to keep them warm. Watering them then becomes an issue. You can only scale from zero to hero based on your budget otherwise you end up borrowing money to keep the homestead afloat. (typical farm practice across America) If you use electricity to generate income its part of your expenses and should come off your profits before you start paying the tax man on the income you generate from selling produce. plan ahead for next year with a war chest for how much electricity your going to need to pay for going forward.
Do you have livestock? Many farms in Europe lived in the same building as their livestock in winter due to the heat generated. I've often opened the old style barns with three cows and been met with 70 degree air. However that was in Ireland and doesn't have the same arctic chill you guys experience.
