The present invention relates to a cold-crystallizing material having high latent heat, and a method of using the present material for heat storing. The material of the invention comprises of a phase change material and an additive which is preferably a hydrophilic polymer matrix. Preferably, sugar alcohols, such as erythritol or D-mannitol, are used as the phase change material. The polymer of the invention is preferably cross-linked and ionic, i.e. a polyelectrolyte. PCM stores a high amount of heat energy while melting and releases the stored thermal energy by cold-crystallization. Compared to previously studied cold-crystallizing materials, the latent heat of the cold-crystallizing material of the invention is considerably higher, and cold-crystallization is more repeatable in successive melting-crystallization cycles. The material can be used for heat storing. Heat energy can be released from the material by a heat pulse on demand. The method is particularly suitable for long-term heat storing.