The energy used per year for heating a villa is 15,000 kWh. A homeowner thinks getting solar collectors and a hot water tank to store heat from summer to winter. How big does the store need to be if you calculate that half of the annual heating demand is needed is stored and that during the summer the water can be heated up to 80C by the solar collector? Water with included lower temperatures than 40 C is not useful for heating. (Large heat losses are unavoidable but not included in this calculation.)