Now the solar hot water. 13% less carbon emissions than a comparable
home and 25% lower gas bills do not have to be incompatible figures.
I don't know how gas is billed but 25% lower bills may not mean 25%
less gas used.

Alternatively, the 13% refers to carbon emissions and the 25% refers to gas.
There are also carbon emissions from electricity, so it makes complete
sense. You could completely eliminate the gas consumption of the house and
still only halve the carbon emissions, as the house still uses electricity,
which is unaffected.

Christian.