In the winter time when house is heated, about 25% of the heat comes from the sun, people and appliances. Rest of the heat has to be done with heaters like gas, electricity, heat pump etc. Summer time house is usually cooled with A/C machine. Electricity for these heaters or coolers can be bought from the grid or produced by solar panels or other generators.

What is zero energy?

Zero or passive energy house is producing energy eg with wind mill or solar panel more than energy is lost. Some zero energy log houses are actually making money by selling electricity back to the network. Solar panel, windmill, heat pump, fuel cell can for example use to produce energy. Eg. batteries and condesators can be used for store energy. House is always leaking energy and thus we try to minimize this leak of energy.

Air ventilation is the biggest heat loss

Air ventilation is one of the biggest energy leaks and that is why modern houses have machine ventilation which has heat recovery. Outgoing warm or cool air heats or cools incoming air. New apartment buildings these are mandatory and many wooden houses also have the system. Older houses one needs to open the the window to let new air in and there is hole (and usually fan) in bathroom and kitchen to let air out.

Modern windows are insulating