The Food Forest

The Energy Efficient Home

This page discusses the energy consumption of a 'normal' home and environmentally/economically sound ways of providing it.
We use the homestead at The Food Forest as a case study.

Hot water

Heating water consumes more than 35% of the energy used in most homes.
With our solar hot water panels and connection to a ‘wetback’ in a slow combustion space heater, which we light in cold, cloudy weather, electric heating backup in the new house is minimal (as little as 90c per quarter).

solar hot water

Retrofitted solar hot water system on old bathroom roof.
Note storage tank at right


Mini pump which circulates water through the lagged copper pipe system

Space heating and cooling

strawbale west wallSpace heating and cooling consumes 25% of the energy used by most homes.
With good passive-solar design and heating provided by a small amount of wood grown on our block we can almost eliminate this energy cost

A ‘normal home’ using electricity for its energy, consumes something over 20 kilowatt Hours per day but by getting hot water and space heating and cooling sorted out we save 60%, leaving only 40% to produce or import. So if we ‘live normally’ we should need about 8kWhrs/day for electrical appliances, lighting and cooking. In the case of The Food Forest we produce about 7kWhrs of power per day.

Using materials to insulate the home from high or low temperatures outside (strawbale west wall in this picture) and providing mass inside (concrete slab and rock walls), evens out the temperature within the home, reducing heating and cooling costs. Using a slow combustion heater to warm the home and produce hot water in winter can virtually eliminate the need for fossil fuel consumption for those purposes. The black vertical lines in this picture are insulated pipes which transport hot water to the storage tank in the roof from the slow combustion heater in the living/dining room.


A hot water urn may use 1000 watts at every instant while it is heating (like a kilometre is 1000 metres, a kilowatt is 1000 watts) so the urn uses 1kW… but because the element switches on and off, it isn’t heating all the time. We need to know how long it is actually heating as well as the rate it uses power to calculate how much in total (kilowatt hours) it uses in a day.

Some typical appliances:

  • A load of washing in a front loading washing machine uses electricity for heating, washing and spinning, totalling about one third of a kilowatt hour
  • A small domestic fridge or chest freezer may use 1 kW hr per day
  • A wall mounted reverse-cycle air conditioner may use 1 kW while running
  • A light bulb may use 60Watts whilst on

The amount of time each appliance is used and its efficiency have a big impact on how energy efficient the home is. A really efficient appliance may use over 30% less power than one of a different brand and 70% less than an old, poorly maintained machine. The new 'compact fluorescent' light bulbs produce 3 times the light of incandescent bulbs from a given amount of power. You can have your appliances tested for efficiency. Getting rid of poorly performing appliances may be best for your pocket and the environment.Use the excellent Energy rating site (see left) to help you choose new appliances.

Producing the power

energy usage chartOur photovoltaic system on the roof will produce 1.74kW while the summer sun is shining directly on it and we’ll produce about 12kWhrs per day, but in winter it will drop to about 5kWhrs/day and end up averaging about 7kWhrs/day over the whole year. With our grid–connect system the highs and lows don’t matter as the system will use mains power while the Mains generators are operating at high efficiency at night, and push power into the mains on hot days when everyone has their air conditioners going and the power stations are struggling to meet demand.

Economics of photovoltaics

Our system will produce about $600 worth of electricity a year at current rates (approx 19cents per kilowatt hour). If electricity prices did not go up at all it would take about 23 years to pay back our outlay but if we imagine the value of power was to go up by 3% per year along with Consumer Price Index it may be more like 17 years. In energy terms the system will only take 2-3 years to pay for its manufacture. The system has 20 year warranty but should last at least 30 years without major maintenance.

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