In July, our home maintained a comfortable indoor temperature within the target range of 18 to 21oC. The energy bill for the month was $79 or approximately $2.50 per day.
Describing our home.
The home is a single storey, 2 bedroom, 2 bathroom home occupied by two retired adults.
Based on passive solar design principles, the home has large north-facing double-glazed windows & doors, thermal mass from a concrete slab and reverse brick veneer Timbercrete walls and excellent insulation all round.
This is a grid-connected all-electric home. Solar power is provided by an 8kW SolarEdge array coupled with an RESU10H 9.8kWh LGChem battery. Excess electricity is exported to the grid. If there is insufficient electricity available from the panels or battery, GreenPower is imported from the grid.
July comfort data
For winter, the Sustainable Energy Authority Victoria recommends setting heating thermostats between 18 and 21oC. This is the lowest comfortable temperature range, a balance between comfort for the occupants and energy used to maintain that temperature.
We chose to use this range as our target indoor temperature.
The target was much higher than the ambient temperature in July. BOM records showed the average daily maximum was 13.9oC with the average night-time minimum dropping to 6.4oC.
This chart of temperatures recorded on the north and south sides of the house show the variations. Several days the external temperature struggled to reach a 10oC maximum.
Compare the ambient temperature with the temperatures recorded inside the house.
After a shaky start, the kitchen temperature generally stayed within the target range, showing 18+oC during the day.
Kitchen temperatures represent the living area in our home. This is where we spend most of our waking hours. At night, a drop in the indoor temperature is fine because we are tucked up in bed. Warming the living area is unnecessary if there’s nobody there.
The low indoor temperatures at the start of July were just me learning how to optimise the two heating methods available – passive solar and the air conditioner (heat pump). More about that experience later in this post.
Notice the difference in second bedroom temperatures. We closed the door to this room as it’s currently unoccupied. Being located on the southern side, the room isn’t directly heated by the Sun and doesn’t have any mechanical heating or cooling. However, this isn’t a dark room. A north facing clerestory window helps the south facing wall window keep the room illuminated during the day.
Where does the electricity go?
There are four main electricity users in our home.
A Fujitsu split system (heat pump) heats the living area. When needed, this system uses 1.5kWh of electricity to pump in 6kWh of renewable energy extracted from the air outdoors.
The Sanden Eco Heat Pump needs 0.8kWh of electricity to put 5kWh of heat into the 300L hot water storage pump.
Our electric vehicles are charged at home. How often we charge the total battery capacity of 64kWh and how much electricity they take depends on our movements. Being retired and occasionally in COVID lockdown means we don’t have a daily commute to frequently run down the car batteries.
Lastly, there is everything else in the house. This includes devices like the induction cooktop, fridge & freezer, heat pump dryer, washing machine and LED lighting.
July electricity cost.
In July, our home used 581kWh of electricity, just under an average of 19kWh per day. However, that’s not the number on our electricity bill.
Although July is the year’s low point as far as solar power production is concerned, our system still produced an average of 11kWh per day.
Hence, the amount of GreenPower imported from the grid to fill the gap between solar power production and house demand was a much lower figure of 307kWh, costing $85.
But wait, there’s more. On some days, we exported excess solar power to the grid. Our 62kWh export earned us $6.
The net energy cost for the house in July was $79.
Remember, that’s it for energy cost because we don’t have a gas connection or use gas appliances.
When is electricity used for home heating?
Heating our 8.4 NatHERS Star rated, passive solar home is a combination of free energy from the Sun on sunny days and mechanical heating from the heat pump if there is insufficient passive heating.
Obviously, we’d like to use as much free, passive heating as possible. During July, the Sun’s midday elevation is about 32o, a bit higher than the June low point of 28o. That’s good news because any sunlight streams through the large, north facing windows, warming the concrete slab and Timbercrete blocks.
But July wasn’t a great month for sunshine. Based on the daily BOM solar exposure data and my weather observations, only 42% of the days were what we might call “sunny”. Sunny means extended periods of sunshine during the day, rather than mostly cloud cover and/or rain. This correlated with BOM solar exposure numbers of 7.5 MJ/m2 or higher.
Still, on the bright side, even on a cloudy day, light fills the house. There’s no need to turn on LEDs because any sunlight finds its way into all the rooms. South facing rooms have north facing clerestory windows which channel light into the space below.
So, in July, sunshine helped with passive heating and the heat pump picked up the slack.
Minimising electricity consumption for heating in July.
Our heating strategy is simple. Use the heat pump when necessary.
After some experimentation, I settled into a routine of setting the air conditioner at 20oC at breakfast, then dropping it to 16oC overnight. The lower setpoint ensured the thermal mass temperature never fell too low. That made it easier for the heat pump to bring the interior temperature back up to the target range in the morning. It’s similar to banking a wood fire overnight.
Of course, if there was sunshine, the passive heating kicked in so we switched off the air conditioner for most of the day. It was turned on during late afternoon as the Sun’s energy waned.
Temperature spikes above 20oC at the beginning and end of July show the power of passive heating. Sunshine pushed the interior temperature up this high, without any assistance from the heat pump.
Minimising electricity cost for hot water.
The hot water heat pump program allows it to operate between 10am and 3pm.
During that time, there is usually sufficient solar power production or battery capacity to meet demand.
We found the 300L storage tank holds enough hot water for the two of us for 24 hours. A daily heating is all that’s required to keep the water hot.
Minimising electricity cost for EV charging.
The lowest cost option for EV charging is using excess solar power from the array on our roof. In July, that didn’t happen often.
Next best option, particularly if we are going on a longer drive, is to visit a 50kW rapid charger on the way home. Many of the rapid chargers in Melbourne are kindly subsidised by the local Council so a charging session is free. Another alternative is to visit a shopping centre with a 7kW fast charger. Often it’s a win-win situation with the EV charging station close to the entrance and power provided while we shop.
Finally, we trickle charge the EV in the middle of the night, using off-peak GreenPower.
Minimising electricity cost for other appliances.
Installing efficient devices (eg LED lights) and only using them when necessary goes a long way to minimising the cost of running the other appliances.
We also consider the solar production or battery charge when deciding to use an appliance. If there’s sunshine and the solar panels are pumping, we run the dishwasher or put on a few loads of washing. During cloudy weather, unless the need is urgent, we postpone until a sunny day.
Squeezing the most out of the battery.
The SolarEdge inverter has a clever function that allows me to tell the battery what to do and when to do it.
Normally, the inverter maximises self-consumption. Excess solar power goes to the battery and then to export when the battery reaches 100%. If there isn’t enough solar power being produced the inverter brings the battery online to avoid importing from the grid.
That’s fine but short, cloudy Winter days can leave the battery flat at the end of the day. That’s when the inverter switches to importing GreenPower at peak rates.
If a peak power import is likely the next day, I swap the inverter to an “import” strategy. This tops up the battery with off-peak power in the middle of the night. Lower cost energy is then available for use later that day, instead of importing peak rate power.
I use another inverter strategy to charge an EV overnight without draining the battery. The strategy hibernates the battery so it doesn’t discharge, leaving us with a flat battery at the start of the day. The EV charges from off-peak GreenPower instead.
July is first of 12 reports.
This is the first comfort and cost report. I plan to release one each month for the next year.
I’m looking for insights to share with others.
Let’s watch how the house performs as the seasons change.