It's the insulation. While it depends on the location and geography, I'd wager that American homes are probably less well insulated than Swedish homes because they didn't have to be.
That contrasts quite a bit with Swedish home standards, which have long been built more air-tight and with considerably better insulated even if they're of comparable age. This has been true for decades, became even more stark in the 1980s, and likely remains very different on the balance: https://www.aceee.org/files/proceedings/1984/data/papers/SS8...
That contrasts quite a bit with Swedish home standards, which have long been built more air-tight and with considerably better insulated even if they're of comparable age. This has been true for decades, became even more stark in the 1980s, and likely remains very different on the balance: https://www.aceee.org/files/proceedings/1984/data/papers/SS8...