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I respectfully disagree. First, direct solar radiation is around 340 BTUh per square foot, the area being measured in the plane perpendicular to the sun incidence. There are losses, of course, in re-radiation if the collecting surface can do so. If it cannot — if, for example, the collecting surface is behind a IR opaque glass, that is not a factor. Previous work and studies of mine have found that a good approximation for a collector behind glass, whether it is room contents, a Trombe wall, a floor… whatever — can be expected to absorb around 250 BTUh per square foot. This figure will be higher if the solid temperature of the collector is lower — so cooling the collector by moving the heat elsewhere, for instance with air circulation, is always advantageous.

Now given that figure, that 650 square feet of effective collector will absorb somewhere in the vicinity of 162,000 BTU per hour, or on a day with 3.5 hours of sunshine (taken as the average daily insolation in New England) around 570,000 BTU. That is sufficient to supply the entire heat load of a well-insulated, 1100 square foot house in most areas south of about 45 degrees north latitude.

The problem isn't collecting enough energy. The problem is storing it and, related, moving it to where it can be stored if need be. It should be stored at as low a temperature as is feasible, to begin with, and it is ideal if the collector and the storage are the same, such as masonry floors or walls inside the heated envelope. We were working in New England, and were pessimists (engineers have to be), so for the houses and other buildings with which I was involved, all of which worked, by the way, we aimed at a storage of around 2000 BTU with a delta T of not more than 10 degrees per square foot of building footprint. That was sufficient for 4 sunless days in a row.

There are at least a dozen structures — houses, academic facilities, commercial facilities — which we designed and built in the late 1970 and 1980s which are still in use and still working as intended. But that was then, 50 years ago, and two of us are dead and the third is tired…

I might point out in passing, however, that all that is for new builds. Retrofitting rarely works.

Not for me, no. I'll take them and charge the 3.5% fee, but 99% of my clients don't want to pay that fee and neither do I.

@The Steam Whisperer I have a feeling that your idea may be against the law or at least an infringement on someone's right to privacy. I am not sure that it is a viable service that a business can just post Customer John Doe  does not pay their bill.  Unless you get the customer’s permission beforehand.  

I would become a member of a private online service, not open to the general public, that may post this information similar to the way a credit bureau offers the credit score for individuals who provide their SS number and DOB.  Anytime you apply for a credit card or a bank loan, the fine print says that they can report your payment history to any credit bureau that the financial institution has an agreement with.   You gave permission when you signed the application.

The trick is to assemble a “Bureau of Trade Customers” by having all tradesmen have that fine print in their invoices where it states that any customers dealings with tradesmen of any type that subscribe to the “Bureau of Trade Customers” that will give permission to the contractor to post their experience with said customer for all other trades to look up and make a decision if they want to deal with that customer.   Each customer would be rated with a “Trade Score” just like a “Credit Score” 

So every address that has any type of trade work on the property, from original construction including foundations and structures, to masons, to framers and roofers, to electricians, plumbers and mechanicals.  For the most part those first owners (the developers) would get a score based on how the original construction was handled. If the developer ends up not paying and files bankruptcy the personal information of the individual(s) would be part of the public record and you can avoid future develop[ers with that(those) individual(s) as named owners.   

Each proceeding owner would also be rated along with an equipment rating (as to weather or not the systems were designed or operate properly) so as a prospective contractor you can know what you are getting into. Customer rates poorly or equipment rates poorly.

Just a few ramblings from an old man in a wheelchair