Greater Belfast’s Best Gas Offer for the Average Customer: Comparison is based on effective unit rates which include all supplier discounts and welcome credit in Greater Belfast and calculated using the Ofgem average domestic usage of 12,000kWh. Customers obtain firmus energy’s effective unit with an average consumption of up to 13,500kWh. If your usage is above 13,500kWh, this is not the best unit rate. Actual average consumption varies due to weather conditions, changes in energy efficiency and householder behaviour. Contact us and we will be happy to talk you through our rates, the Welcome Bonus or any queries that you may have. Correct as of 29/05/2025. Verify at Best Gas Offer. New customer offer only, terms and conditions apply Welcome Bonus T&Cs.