Can my solar export credits pay my gas bill?

Generally no. Clean Export Guarantee (CEG) credit for the electricity you export is applied to your ELECTRICITY account, not your gas. On a dual-fuel plan most suppliers keep electricity and gas as separate accounts on different billing cycles — for example, Electric Ireland does not let an electricity export credit pay down a gas balance. A few suppliers may handle a single combined account differently, so it is worth asking yours directly, but the safe assumption is that your solar credit offsets electricity only. To actually cut a gas bill you reduce gas use or move that load onto electricity you generate: a hot-water diverter sends surplus solar to your immersion instead of heating water with the boiler, and longer term a heat pump shifts heating onto electricity you can partly self-supply.