A conservatory can be a great way to use solar power. And as well as saving energy, it will provide a pleasant extra room. The big thing to remember is that a conservatory should never be heated - or all the benefits will be lost!
A conservatory is a 'buffer' against the outside weather - the temperature will stay a few degrees warmer than it is outside. For much of the year a conservatory is a very nice place to be. However, you'll need blinds or shutters to prevent overheating in high summer. Growing seasonal vines or creepers across the roof is also a great way to get summer shading. A south-east facing conservatory is best, as it gains from the morning sun but will be slightly shaded from the westerly sun at the warmest time of day.
A conservatory can also act as a lobby for coming and going from the house, so reducing draughts and heat loss. Fresh air coming in to the house via the conservatory will be warmed on its way through. This is good, because lots of the heat loss from a well-insulated house is through ventilation.
A danger with conservatories is that they come to be relied upon as an extra living space in the home. As the coldest months come around, a few degrees above outdoor temperatures is still quite chilly. This leads to the temptation of putting a heater or radiator in, a move that would make your home an energy guzzler! It is impossible to insulate such a highly glazed room sufficiently,so if you want to maintain energy and monetary savings, you'll need to resist temptation and keep it unheated.
Shutting the conservatory off from the main part of the house with solid doors, or glass doors with thick curtains, will stop heat escaping at the coldest times of the year. Plenty of 'thermal mass' within the conservatory will store the heat gained for longer. So if it is being added to an existing brick wall, much of this could be kept for this purpose. Alternatively, a solid stone or brick floor will soak up and then slowly release heat as the evening cools down.
For a sunny but heated space in your home, consider instead a sun-room. This would have double glazing throughout with an insulated solid roof and well-insulated curtains or blinds. An insulated, heated sunroom would need to meet Building Regulations, so the balance of windows and insulated roof or walls would need to be designed carefully.
A conservatory may seem cheaper to build than a proper extension, but this is not always the case - they can be more costly per square metre than the rest of the house! And if it ends up as a heated room then the less obvious heating costs will not be at all cheap.