Heating bill. It will go someway to covering the extra gas used because MrsW has had to work from home through the winter, which of course her employer doesn’t care about. Unlike MPs who have been getting get an allowance…
Presumably she’s saving money by not having to travel to work and not spending money at lunchtime whilst she’s there though? I don’t know the detail, but isn’t there an adjustment you can make to your PAYE tax code if you work at home to offset a proportion of your costs of working from home?