A heat store (11) comprising a reservoir (13), the interior of the reservoir accommodating a first heat exchanger (17) in which the first heat exchanger have a first fluid path connected to a first inlet (31) and first outlet (33) located to the exterior of the reservoir, the first inlet and first outlet being intended in use to be connected to a further heat exchanger (65, 67, 69) associated with a heat source (55, 57, 59) the reservoir being in thermal communication with a second heat exchanger (35) having a second inlet (37) and second outlet (39). The second inlet being intended in use to be connected to a source of a supply liquid and the second outlet being intended in use to be connected to a delivery outlet, the interior of the reservoir accommodating a phase change material capable of changing phase between a liquid and a solid phase, the interior of the reservoir accommodating a nucleation activation means (41) adapted to cause nucleation of the phase change material on the phase change material being at a temperature below its melting point. A heat battery comprising a plurality of heat stores is also claimed