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I have a couple of unused 1 cu m (1000 kg) water containers sitting in our field.

Water has a high speciifc heat of 4184 Joules / kg / per degree C change

So solar heating the water in one container to 50C above ambient would store 1000 * 4184 * 50 Joules.
This comes to 209200000 Joules. i.e. 209 mega joules.

If we needed say 1kw heating for say 10 hours overnight, we would need :

1000*60*60*10 Joules over 10 hours = 36 mega Joules

So 36/209 of the tank contents i.e. 0.17 would be used in one night
This is 170kg of water at say 70C.

So, assuming these calculations are correct, using some sort of solar to raise the water container by 50C during the day would allow the hot water
to be used for heating overnight.

Do these figures make sense?

The tank will need to be well insulated, otherwise most of the heat will be lost to the room when the water is being heated, and not be available for later use.

It is IMHO simpler to use metric units these days.

It takes about 1.1 kwh to heat 1000 litres of water through one degree.

If you were thinking of IBCs as a thermal store, these have a limited working temperature, and are not very suitable, cheap though. 1 kw used for 10 hours will raise the temperature of 1000 litres through nearly 9 degrees.

Place a couple of 1000 litre IBCs in a well insulated cupboard perhaps, and heat from say 25 degrees up to say 35 degrees when solar energy is available. Open the doors when you want to let the heat out. Beware the weight, probably best restricted to solid floors only.

Urm, yes, but the conventional approach would be to use the solar during the day to heat the water in your thermal store (hot water tank) then redistribute that heat using pipes, radiators and a pump. There are even electronic gizmos, programmable thermostats, able to maintain set temperatures at different times.

Going up to too high a temperature might be a problem as the IBC plastic might get a bit too plastic.
Bear in mind that if you are using solar PV as the 'heat' source you are converting a useful and flexible source of energy, electricity, into inflexible low grade heat. Saying that, however, supposedly two thirds of UK domestic energy usage is converted to low grade heat.

However you just say 'solar energy' so maybe you are using solar energy to heat up a container of water - certainly doable - when I lived in Queensland i painted one side of several 10 litre water containers black and left them in the sun all day and they heated water to 45 degC. Five were enough for fairly decent bath - carrying them upstairs to the bathroom was a bit of a bind.

Be more specific how you will heat them. Passive solar, solar vacuum tube or similar collector, or electricity.

You would be better getting an idea of energy use and what excess you will have and what number of days of heat storage one IBC that will provide. Also you could perform a stress test on how hot an IBC can get inside before it collapses under its own weight. See also how quickly they cool under ambient conditions. I have heard of people in Australia building a little glasshouse around a metal water tank and using it for hot water production.

BTW I have thought of using one to provide night time heat for my greenhouse so I am interested in all the answers.