Help with passive solar project: want to use 5 gallon buckets to store heat collected from sun, how?

The way most passive solar is done is to fill the buckets, (color and construction materials are not important.) cover them with clear plexi, then embed them under a concrete floor on the sunny side of the house. Make sure the buckets are embedded in the concrete so that they can absorb heat from the sun warmed concrete and then when the sun is gone, slowly return the heat into the concrete. (This is usually done with drums of water, as the larger the volume of liquid in each container, the longer it retains heat.) If you put a slatted cover as a roof over the concrete slab that has openings that only allow the sun to reach the concrete in the cold months and not during the summer, you will have truly passive heating.

Paint the insides of the buckets black and fill them with water. Cover the tops with a clear material that will allow light to enter, but not allow heat to escape. That won't be very efficient, but will store some heat for limited amounts of time.

A more efficient storage method would be batteries charged from solar panels.

Plastic has a very poor coefficient of heat. There are many designs and options for solar heat collectors. DAGS for swimming pool solar heating. You tube had a design video using aluminum cans and a wood frame and a glass cover. How you do what you want is up to you. Copper is about the best heat exchange for most purposes.

If I paint 5 gallon buckets from Home Depot with Black paint, and enclose them in an insulated structure, exposed to the sun what type of solar gain would I get. And should the buckets be filled with water, sand or something else?

https://www.howtogosolar.org/water-tanks-passive-s...

https://search.yahoo.com/search;_ylt=AkamvblDuAuns...

the exterior and interior ambient temperatures of the five gallon buckets are only going to be the same == it is not possible to store heat as you wish ===