That's not true. A heat pump produces 1.5 to kwH of heat per kwH of electricity consumed, so it's already much more efficient than a diesel generator.

Even in winter, electricity from the grid is greener than burning diesel. I didn't find specific numbers for winter, but wind is about 30% of Germany's (just picking the biggest country out of DACH to support the point, not trying to come up with exact numbers) electricity production year-round, and wind doesn't tank in winter like solar does.

So, in short. Installing a heat pump and just taking electricity from the grid is still better for the environment. Of course, having your own solar is great if you live in a house, but you don't need it.