That depends on how you're making that gas. IIRC algae (the primary ingredient in most fossile fuels) are less than 10% efficient at converting sunlight into ch

That depends on how you're making that gas. IIRC algae (the primary ingredient in most fossile fuels) are less than 10% efficient at converting sunlight into chemical energy after metabolic expenditures. (There's also the part where the sun is <<0.00001% efficent at turning fusion energy into sunlight-that-hits-earth, but that problem isn't specific to gas heating.)