Sustainable House Costs & Projections

For this type of renovation to be appealing to everyone, it has to make sense economically. In this section, we’re documenting all cost and pricing aspects of our sustainable house build, contrasting the costs and savings with a “normal” build where possible, and outlining the methods and techniques we’ve developed to save money.

As part of that, I’ll be displaying a series of calculations to show homeowners where the savings can be made and where the pain points are, starting with the solar solution. The water solution is still being costed and I still have to finish the calculations for the building etc., but once they’re done I’ll put it all together so you can see what the total sustainable house cost/benefits are for going completely off the grid. But first, let’s look at solar.

Sustainable House Design Cost & Pricing – Solar

Solution one: 6.57kW system @ $8,000

Let’s begin with a solution that uses electricity from the grid and doesn’t use a solar battery. This is the solution most people will probably go for given it’s more cost-effective than including a battery (as you’ll see in the calculations where I do include a battery). At the moment my bill is estimated to be around $800 a year for electricity and $160 a year for gas. I have also assumed that I won’t have to pay for gas and electricity connections at $340pa and $396pa respectively, a feed-in tariff of 10c per kWh.  My electricity provider charges me 27.4c per kWh.

Roland and his team of solar experts have estimated that I will use about 12kWh a day (I actually use around 7kWh a day) and that I should be able to generate about 23kWh a day. Of this I will directly consume around 5.7kWh per day with the rest (7 – 5.7)kWh coming from the grid. I will be able to export around (23 – 5.7)kWh to the grid every day.

So my annual savings will be:

= 2,038

This chart shows the cumulative net cash flows discounted back to today’s dollars for each year over 15 years. In the first year I would incur a loss equal to the cost of the system less the annual savings. By year 4, my annual savings would have accumulated to a number greater than the cost of the system. So my payback period is 4 years. A great result!

Solution two: 7.11kW system @ $13,300

With this solution I’ll be generating more electricity so I’ll be able to feed more into the grid. I should be able to generate about 26kWh a day. Of this I will be able to export around (26-5.7 = 20.3)kWh to the grid every day and will directly consume around 5.7kWh per day with the rest (7 – 5.7)kWh coming from the grid. So then my annual savings will end up being:

So my annual savings will be:


The payback period is in year 7.

Solution three: 7.11kW system + battery @ $39,950

As before, this solution should generate about 26kWh a day. Of that 26kWh, I will directly use about 5.7kWh during the day, with 6.3kWh going into the battery, (26-5.7-6.3 = 14)kWh going to the grid and 6.1kWh being consumed from the battery.

So my annual savings will be:

= 2,038

This solution is expensive! And the payback period is LONG! So if the economics are more important than energy security, then this is probably not the solution for you.

Water Engineer needed

We are calling for EOI to design a residential water recycling system.

Sustainable House Design Cost & Pricing – Water

I’m still working on the water solution and we aren’t sure how much the system will cost yet.  But we will have the numbers very soon!

Putting it all together

I’ll be doing a full projection of costs and savings for the “total solution” which will include:

  1. Water
  2. Solar
  3. The incinerating toilet
  4. Savings from being disconnected from the grid
  5. Rebates
  6. Building costs and savings vs a “normal” renovation
  7. Scenario testing (i.e., changing the assumptions, for example feed in tariffs, inflation etc)

I have some preliminary estimates of the total solution below but I need to do more work on the costings, especially for the water solution. At this early stage I estimate that by year 7 I will break even and then I will be making money on my solution without a battery. A really great result! The battery continues to be prohibitively costly. I’m planning on staying in the house for a long time, but maybe not 17 years.

The next step is to do some variations on the assumptions, (i.e. inflation, electricity feed in tariffs etc).  I’ll post them to the site so you can get an idea of how these variables impact the cost effectiveness of going off the grid

Sustainable Building Costs

There are two parts to the renovation. The off-grid part, and the building part. My plan is to detail the costs of the pre-fab and recycling against a “normal” build. I’ll then add those to the projections above.

Does being sustainable increase or decrease the value of your home?

Beyond reduced living costs and reducing my carbon footprint there are other benefits from incorporating sustainability into a home. (Laquatra, 1986) documents the improved re-sale value from improved thermal performance and (Jackson et al., 2009) show how adopting renewable energy adds to a home’s value. Domain has written that energy efficient homes can sell for as much as 10 per cent more.