What is the Best Garage Heater Sizes? Single-car garages (0-450 sq ft) require a 2000-3000 W unit (or 6,800-9,000 BTUs/hr) Two-car garages (450-700 sq ft) require a 3600-7000 W unit (electric heaters) or 12,000-24,000 BTUs/hr (for the propane ones) A basic rule of thumb for forced-air ...
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