Its is pretty near useless.
I monitor oil usage closely: we burn next to Nowt June/July/August around 70% is burned December January February and the remaining 30% is spread over the spring and autumn months, as the weather dictates.
That winter usage is ALL DAY usage. It is simply not possible to store enough overnight to not be freezing in the evening: With our underfloor (admittedly high lag) the heating needs to be on all day and off all night. In prolonged cold weather it runs 24x7.