So, heat pump efficiency vs temperature only tells part of the story. With good insulation you will almost always need more cooling in the summer than heating in the winter outside of the arctic circle.
Ground source heat pumps have a typical efficiency of about 400%. You're moving heat from the ground to your house rather than heating up the air, so you get considerably more useful energy out of the system than you put in.
Ground heat-pumps are being championed as a cost effective replacement for keeping houses warm. They provide a 4:1 heat:electricity ratio so have lower running costs than gas. They can also be attached to sun panels and can work with storage heaters (heat the water when the sun shines, use it when it doesn't).
Unfortunately pumps are expensive and very invasive to install in existing houses - you need to dig gardens up to lay the pipes and installation for an average house is about 15-20k EUR ($17k-$23k).
(They’re special for several reasons. A big one is that they have huge thermal mass, so your heat pump can operate at whatever time of day you want. This means you can optimize for electric rates vs outdoor temperature. Heat pumps, unlike gas boilers, are considerably more efficient when it’s warm out.)
1. We need to eventually switch our winter gas heaters to electric which while helping balance summer vs winter electric usage, electric heating isn't as efficient and we have to increase the outside air a lot more in the winter than we have to cool it in the summer (for most people at least).
2. I forgot to add in the extra electric demand needed to power the cars which is currently powered by oil.