If you just need enough hot water for occasional handwashing at a single sink, an instant water heater is a good place to start. These are small electrically powered units that heat water 'instantly' as it flows through the product. Instant water heaters have the advantages of being compact, relatively cheap to buy and easy to install above a sink but they can only serve one sink and they can normally only heat water relatively slowly as hot water flow rates are limited by the electrical power rating. This means that you might have to accept quite a slow flow rate when the incoming water is very cold, for example in winter.