[ToddF]

Back to solar thermal, my 200 gallon solar storage tank gets up to 170 degrees this time of year and stays above 120 for two days of clouds (even when we're using hot water normally). Solar heat is usually not sized for 100% of your use. You need a backup. That could be considered a downside. But tanks do not cool down that quickly. Even when insulation is so-so.

Heat storage for homes is on a different scale if done right. Houses built on 2 foot sand beds have 100 times more storage than my 200 gallon tank and do well in the MN climate. Its tough to size the solar to the sand bed, though, but "good fits" provide 90% of your heating if you're willing to live with 60 degrees during a cold snap.