Case Study – Tom’s renewable energy home and cars

Case Study – Tom’s renewable energy home and cars

Tom Hunt – CCL Hume group convenor – recently wrote about towing with his electric vehicle.

Towing a caravan with an electric car – can it be done?

Now he shows how over time his home and vehicles all became mostly solar powered.

Several years ago my wife and I set out to reduce our emissions through changes to our home. It seems to have worked for us, and I thought it may be worth sharing our story.

We have switched our entire home, and our cars, to run from sunlight (with some hydro, and wind power thrown in) !

We produce enough energy for our household needs from our own roof !

And though we still need the grid to balance things out for us, we are actually helping to flatten out overall demand on the grid (ie helping to reduce the dreaded ‘duck curve’)

I’ll admit what we have chosen may not be the cheapest option, but the solar panels and battery on which our solution depends cost only 2.5% of the value of the house and they effectively give us free power for nearly all our household and driving needs. That is – no cost for electricity, gas or petrol, for the life of the system!

I am convinced that this is not only good for the environment, it is also cost effective.

We started with our dream home, that we built 17 years ago on the shore of Lake Illawarra. It complied with the council’s 3.5 star NatHERS rating and had limited features of solar passive design, though not assisted by the NW-SE orientation and our rather ambitious design.

Energy parameters of our home as built:

Mains gas for heating, cooktop, instant hot water and BBQ
Electricity for all other appliances.
The biggest demand for electricity was probably the pool pump, at 1.5-2.5KW for several hours per day.
For the first 12 years we had no air conditioner as we felt there were too few days each year hot enough to warrant it.
Our cars were both petrol driven with efficiencies of 7 to 10 li/100km

Our 2008 energy usage and CO2 production:

CO2-e (1)
12,500 kWh electricity from the grid
10,375 kg
11,800 MJ gas
608 kg
2,600 li petrol
6,032 kg
Total CO2-e contribution
17,015 kg
 ie  17 tonnes


Current Electricity situation

12 months to July 2019
kWh / year
Home usage
Solar generation
From Grid
To Grid
From battery
To battery
Battery efficiency


Our 2019* energy usage (*12 months to July 2019) was:

10,450 kWh electricity used
but 13,600 kWh produced
so net electricity consumed – 3,150 kWh -2,615 kg
Zero gas
280 li petrol (zero since April) 650 kg
Net CO2-e contribution -1,965 kg
ie net negative 2 tonnes


So, over 11 years, we have gone from adding 17 tonnes of CO2 to the sky per year, to offsetting 2 tonnes from elsewhere.

How did we get there?

In 2009

We bought a 2nd hand Prius – hybrid car and halved the petrol use in our main car.

In 2010

Installed 4.5kW of solar panels to the roof. It cost us $15K at that time but thanks to the rebates and feed in tariff scheme, we have had no net cost for electricity since then.

In 2015

Installed a 7kWh battery pack in the Prius to convert it to a plug in hybrid and bring it’s average petrol consumption to below 2.5 li/100km.

Bought a 2nd hand Prius V hybrid car to replace our second car and further reduce our petrol use.

Switched from gas to an induction cooktop in the kitchen (that was to have an easier to clean fast heat source, but it also reduced our gas usage)

In 2016

Replaced our gas hot water system with a solar hot water system. Installed a reverse cycle air-conditioner to replace our gas heater. Bought an electric hot-plate griller to slip into the gas bbq cabinet. And disconnected the gas.

In 2018

Bought a 2nd hand Holden Volt electric car to replace one of the Prii – so local commutes could be petrol free.
Added 4.5KW of solar panels – to cover additional power needs – air con and cars
and a 13.5KWH home battery – to reduce need for grid power, especially at peak times

In 2019

We bit the bullet and bought a new Kona long-range electric car – bringing our need for petrol to almost zero (alas the Volt has to burn a bit in it’s backup motor occasionally)

Also over the last 10 years we have

Reduced the meat that we eat by more than half, and red meat by around 90%. I haven’t done detailed figures, but it seriously looks like this can be as effective as reducing our energy emissions.

Changed our energy use habits to reduce waste (eg. Fewer and more appropriate pump hours for the swimming pool)

Reduced transport overhead & plastic packaging in some of our foods by buying local, in bulk, and avoiding imported foods where possible.

Setup up a veggie patch and grown a little food at home.

Recycled and composted as much as possible (with Council’s helpful FOGO), and reduced our landfill waste enormously. We’re now saving money with a 1/2 size red bin.

What we are still changing

While our home was built to reasonable standards at the time (NatHERs 3.5 stars) it still leaves a lot to be desired regarding it’s thermal efficiency. We are currently improving the ceiling insulation, ceiling and door sealing and window treatments to reduce our heating and cooling needs.

The home was fitted with low energy lighting from the start (mostly compact fluorescents), but the downlights provided unwanted ventilation, and technology has moved on. We have replaced many of these lights with LEDs which further lower the energy need and enable us to seal the ceilings.

When the pool pump dies (and it is on it’s last legs) we will replace it with a low energy variable speed pump.

Why do we still need the Grid ?

Well, we do get days that there is too little sun, and there are often times when we need more power than our solar panels and battery can provide. At these times we know that somewhere on the grid are some other solar panels, wind or hydro turbines generating. We pay our electricity provider a green energy levy to ensure that the power we use is effectively produced by renewable sources.

Then of course there are times when we are producing more electricity than we can use or store. Having the grid connected means we can sell that power to someone else.

We can also help flatten the Demand Curve !

An added benefit comes from the battery and our discretionary power uses. Our supplier (AGL) will buy our excess electricity at 20c/kWh while selling power to us at different rates according to the time of use (an option we have chosen). The off-peak rate between 10pm and 7am attracts a fee of only 17.5c/kWh. And so it helps us financially if we charge our cars at night and sell spare power from the solar panels during the day. It’s cheaper then because it helps AGL flatten out the demand.

The setup we have also helps in the daytime since the solar power we generate gets used to charge our battery up early in the day but by the afternoon (when the peak demand on the grid builds up) we are exporting power to support it. Then when the sun sets and the peak demand grows, our battery powers the house so we do not add to the load on the grid. It would be rare that the battery doesn’t keep us going until 10pm when demand on the grid begins to get to its lowest and the off peak rate starts.

And a word on charging electric cars.

It’s as simple as plugging your car in each time you park and theres no need to ever visit a local service station. You would very rarely need to fast charge your car at home, so just a standard power point could be enough, but faster options are possible. We have installed a 30amp power point so we can charge at a rate of ~50km range per hour of charging (the dc fast chargers on the highway are able to do it about 10 times quicker than this, but a bit expensive for this option at home).

Yes the cars do add to the amount of electricity we use, but only about the same as the swimming pool, and they can be set to charge at the most cost effective time of the day.

(1) For calculation of CO2 equivalent for each energy source : system/files/resources/5a169bfb-f417-4b00-9b70-6ba328ea8671/files/national-greenhouse- accounts-factors-july-2017.pdf : 0.83 CO2/kWh for electricity produced in NSW 2008 page 20; Natural gas distributed in a pipeline – total of 51.53 kg CO2-e per GJ page 13; petrol(gasoline) – 34.2×67.8/1000 = 2.32 kg CO2-e/li – page 15.