Thermalite are lightweight concrete blocks. The only way they will save 25% on energy bills is if it was being compared with a garden shed.
The best way to save energy is to insulate the building enevelope as best you can to acheive a low U-Value.
Take a look at
http://www.nearzero.co.uk they explain about energy efficiency and have a product that is know as SIPs structural insulated panels that has been used on hundreds of project in the UK and thousands worldwide.
What are you looking to do with the thermalite blocks?