These can be installed indoors but require a flue for combustion gases. An outdoor high-efficiency gas hot-water cylinder might be a better option, saving space and eliminating the need for a flue. While more efficient, outdoor units may lose heat in colder conditions and cost more upfront. 

Gas and electricity prices are similar, but gas comes with extra costs like daily service charges, year-round, plus a connection fee. Gas water heaters are pricier to buy and install, less efficient at converting energy into hot water, and have higher standing losses. Using gas for heating water isn't cost-effective for many households. However, if you already have a gas connection, especially with a larger family, it can be a good option. 

Main benefits: plenty of hot water for large families, as gas cylinders reheat faster than electric ones. 

Main drawbacks: gas hot-water cylinders are less efficient, have higher heat losses than electric ones, and can't be fitted with insulation wraps. 

If you prefer gas but lack a street connection, a cylinder-swap LPG system could be a good option.