Kilowatt peak refers to the value of power generated by a solar tile or panel system under full solar radiation (under set Standard Test Conditions). Solar radiation of 1,000 watts per square meter is used to define standard conditions. In the UK, one kilowatt peak of solar will generate roughly 850 kilowatt hours (kW·h) per year.
A kilowatt hour is the same as a 'unit' of energy on your bill, i.e. A 100w light bulb running for 10 hours. 100w X 10h = 1kW·h.