Self sufficiency revisited

At the back of my mind for the past few months has been the niggling doubt over connecting to mains power. We do have a 19Kv line running through the property, with a power pole approximately 140 metres from where the house is to be sited. So the cost involved to get power to the house is “affordable” while still being a fair whack of cash. For arguments sake lets round the figure to $20k for the transformer and then undergrounding some enormous cables from the pole to the house, all grunt labour supplied by myself of course.

Water pumps
Being dependent on rain water means we also need to pressurise the delivery of it. One option here is to use a header tank and that will do away with the two 400W (400 watts) pressure pumps we would have needed. 800W of power using pumps may not sound like a big power draw, but I’ll explain later on why we are better off not having them if we intend being off grid for power.

I spent some time yesterday looking at the quickly evolving LED “globe” technology which is a major advance in lighting efficiency similar to the step from incandescents to compact flourescents. Doing the sums on our proposed lighting for the house we would need to run about 1000W worth of these and this we can do over a 12 volt network rather than conventional 240 volt.

I’ve done some preliminary costings on a small 200W 12 volt turbine and 4 deep cycle SLA batteries at under $1k to forever eliminate the power bills for lighting. My scrap paper calculations reckon on this setup being able to power lights for 3 days (typical use) should no wind blow. With the average windspeed we have the system should be able to power entirely off the turbine with the batteries there for backup on the very rare occasion it is calm or when the turbine needs to be taken out of service for maintenance.

Since we’ll be using gas to cook, wood to heat and solar for hot water –  the 3 largest users of electricity in electric only houses – I crunched a few figures on exactly what we would need to produce and store in terms of renewable energy to power the house. 

We have the typical “always on” items such as fridge and chest freezer. Then we have the usual appliances that are used in short bursts each day such as the kettle, toaster, microwave and tv. We are now keeping a logbook of the usage of each of these short burst use appliances to see just how much power they are likely to use.

The limitations
It is very important that one thing is kept in mind here, and that is the PEAK POWER LOAD. One thing about being off grid is you are limited in the amount of power you can draw at one time. All of these systems run inverters that take DC power (directly from the turbines or the battery banks) and turn it into 240 volt AC for use inside the house.

Typically a 3kW (3000 watts) 12 volt  inverter will cost $2800 and will let you draw up to 6kW for 1-2 minutes at a time. If we were to consistently exceed 3kW of power draw (done as simply as having the  kettle and toaster running together) we would need to add another expensive inverter to share the load. Constantly abusing the inverter by exceeding the rated power will of course shorten its working life.

By simply using the kettle, then using the toaster we can avoid the above situation but it means being very aware of what we have running and when. Removing the water pumps as discussed above will also help keep down that peak power load. All these systems require a power dump when power excess to needs is generated and we’ll divert that to a pump to fill the header tank. The header tank will overflow back to the storage tanks so it can pump away all day if it needs to.

Wind versus solar
Dollar for dollar wind will deliver more power and by a large margin. Considering the wind blows 24/7 and the sun gives 10-12 useful hours a day for solar energy that fact should be a no brainer. A 1kW turbine will set you back less than $5 000 where a solar array of the same capacity would cost you $14 000. The turbine of course is useable 24 hours a day and the solar array only half of that.

In terms of storage the turbines always being capable of delivering power means we need a much smaller storage solution. A 12 volt battery bank can set you back $3 000 for a low draw scenario such as wind power then go all the way up to $22 000 for high draw capacity as needed for solar.

Calculating what we need
Unfortunately the renewable energy market is focused on city based installations and of course they are only interested in solar installs. Similarly government rebates in the past have been heavily focused on solar since most of our population by default lives in densely populated areas. This means finding any sort of online calculator to work out the specs for a system has proved impossible so far.

Piecing together information has led me to believe we can get a wind powered off grid setup consisting of:

  • a small battery bank,
  • three 1kW turbines (1 spare for redundancy in case of breakdown, maintenance or future expansion),
  • three charge controllers to send power to the batteries,
  • an inverter to deliver the 240 volts required to the house
  • a small diesel generator for battery conditioning and power top up should it be needed
  • a dedicated shed for storage of chargers, inverter and batteries.
  • a seperate 12 volt system for lighting only

for under $25 000. While this initially is more expensive than connecting to the grid, that $5000 gap should be recouped within 5 years at todays power prices.

Comments are closed.