... there’s just so much to unpack in this thread... you’re not really doing the ducks any favours by having that swing in temps for them. Much more stressful to go from toasty warm to cool, even with a heat lamp, than to be cool to cold. Ducks are WAY more cold hardy than chickens, too, but even so that’s a lot of stress on their bodies each and every day. Most livestock prefer temperatures much lower than a person does, because they have built in defences against the cold.
my suggestion, since you’re bringing them inside at night, would be to gradually reduce and then remove your heat source outside, and acclimate them to an area of the house that is just moderately warmer than outside. That’s actually what I do for my dog, who wants to be outside all day but much prefers to come in at night. She stays in the unheated porch overnight, which hovers right around the freezing mark in the dead of winter, and happily spends all day out as long as it’s not horrifically cold. There’s nothing wrong with a little extra warmth on those days that drop way way below what’s been normal, but those are few and far between.
As day time temps are usually quite a bit higher than night time lows, why not use the sunlight to your advantage too? Clear plastic, old windows, plenty of ways to create little warmer areas during the day that will be a moderate increase and not stress them unduly.
if you’re adamant about keeping heat on for adult birds (which, in my opinion, is superfluous) it would be much safer for them and you to use a radiant form of heat, like the panels or even a plain ol heating pad. That way they can approach it to warm themselves if they’re cold, but aren’t subjected to heat they don’t need or want. Safer, more economical, and you may be surprised by how little they actually use it.
The only situation that I would ensure there was additional heat for adult birds is if they have some sort of impediment that would make them less hardy. Sick birds, old birds, that kind of thing. there’s a lady I know here that had a very old, arthritic Muscovy. He had a little wooden house with a regular light bulb in (like a lot of people use in dog houses) on a thermostat. If it got below a certain temp that he struggled with, the bulb would come on and warm him up.
just my opinion, but if people can keep chickens in Alaska without additional heat...

Haha.