alt Hacker NewsA big problem towards full electrification are those resistive extra heaters, due to their correlation across entire cities and correlation with extremely poor solar yields.
Unless there's a particular good reason to it's actually not too bad of an idea (with a hot water radiator loop in the home at least) to keep a fuel burning radiator loop heater around and just valve it off to not interfere on normal days. Especially if the heat pump is air source (which the mentioned swedish one isn't).
It's not that I'm against full electrification, but peak cold spell heating in excess of the heat pump capacity is among the last things we ought to "electrify by force."
We'd have massive summer overcapacity at the point where we can generally afford to "just do that". And chances are that it'd not be a resistive heater but a heat pump freezing a pool worth of water to cap the cold side temperature at just under 0C, or something the like, just because of how grid intensive the correlated load of those resistive heaters is.
It might even be cheaper to use synthetic fuel produced in summer than to 100% electrify that cold spell heating capacity. Carbon capture is expensive but there are options from e.g. domestic waste incinerators, paper making, and generally waste wood/plant matter decomposition, where hydrogen can cheaply be added to produce methane (pipes) or propane/butane (tanker trucks with liquified gas).
One alternative to a freezing pool of water that I'm intrigued by is using thermochemical heat batteries to transport heat energy from summer to winter.
For example, evaporating a 40,000 L solution of calcium chloride brine can store 4.6 megawatt-hours(*) of thermal energy and release it in the winter by passing water vapour over the solution to dilute it. This can heat geothermal temperatures from 10°C to 27°C which can be used in in-floor heating.
But the big asterisk is that only 11% of that 4.6 megawatt-hours of that thermal energy being "stored" is held in the enthalpy of dilution, and 89% of that heat is coming from the enthalpy of vaporization, and acquiring water vapour in winter isn't so easy.
One would probably need a source of water and use geothermal heat to raise dry cold air temperature to the needed 10°C, while absorbing water vapour from a (relatively small) tank of water. The net result is more of a thermochemical heat pump rather than a thermochemical battery per se. But the point is that, other than some fans, this is not electrically powered.
Then there is the issue of storing such tanks safely underground, and more problems. Still, there might be some innovative solutions lying in this direction. For example, one can replace this open-loop system with a closed-loop vacuum system to boil water, but at the cost of far more complex and dangerous engineering.