Living area image by Dylan James showing the clerestory window above the sliding door
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December comfort and cost report

During December, the indoor temperature stayed below the recommended maximum of 27oC because the passive solar design protected the indoor thermal mass from rising outdoor temperatures.

Our energy bill flipped to a credit of $8, down from a cost of $2 in November.  Average daily electricity usage dropped slightly to 12kWh, but that hasn’t changed dramatically since September..

Long, sunny December days maximised solar power production so there was plenty of electricity for the three major users – recharging the battery for overnight consumption, powering the hot water system and charging EVs.  The reverse cycle air conditioner is wondering if we’ve lost the remote control.  Natural air circulation was all the cooling we required.

December comfort data

Summer arrived in December with the BOM recording an average daily maximum of 25.1oC, well above the 20.9oC recorded in November. The nighttime average also rose, going from 10.3oC to 11.6oC.

This temperature chart compares conditions in our living area (kitchen) with the external temperatures on the north and south sides of the house.  Compare the outdoor temperature fluctuations with the relatively flat green line for indoor temperature.

Comparing indoor and outdoor temperatures for December days
Green – living area; Dark blue – outdoors north wall; Light blue – outdoors south wall

Response to December temperature data

After swapping between Summer, Winter and Spring last month, Melbourne settled into early Summer.  Hot 30+oC days were followed by cooler nights with temperatures in the low 20’s or below.

Cooler nights allowed us to continue in “passive cooling” mode.  First thing in the morning, we closed all the windows.  The cool thermal mass kept the indoor temperature below the recommended 27oC summertime limit.  At night, opening windows vented warm air using natural ventilation.  Clerestory windows located high on the walls of the bedrooms and living area are essential for purging any warm air at ceiling height.

In the kitchen, the clerestory window is sited above the large sliding door on the north wall.  This is an ideal location because the ceiling rises up to this point from the southern wall and the east/west walls.

Living area image by Dylan James showing the clerestory window above the sliding door
Clerestory window sited at the peak of the ceiling

Nighttime cooling was so effective we didn’t need ceiling fans or air con to feel comfortable indoors.

This temperature snapshot from late-December shows how passive solar design and natural air circulation work together.

Steady increase in daily maximum for four days late in December
Green – living area; Dark blue – outdoors north wall; Light blue – outdoors south wall

Excellent insulation and thermal mass (concrete slab and reverse brick veneer Timbercrete blocks) limit the temperature increase during the day.  Suitably placed windows are opened at night to flush out the warm air, replacing it with cooler air from outside.

Living this way does require a bit more time and effort than simply turning mechanical cooling on and off.  However, there are benefits.  Natural air circulation feels more comfortable than an intermittent stream of cold air from an air conditioner.  Taking advantage of nighttime breezes also avoids using, and possibly paying for, energy to run the cooler.

Major electricity users

Hot water and EV charging remain the major consumers.

The hot water heat pump runs during the middle of the day so it’s powered by electricity from our solar panels.  When it’s cloudy, the battery cuts in to make up the difference.

I continue to trickle charge the MG or LEAF electric vehicles during peak solar power production in the middle of the day.

On average, we used 12.3kWh per day for all our energy needs – hot water, EVs, cooking, lighting, washing, etc.

Solar power production observations

This chart shows the solar panels are regularly producing 40+kWh per day.  Things are pumping away on the roof.

Daily solar power production in December

A weekly production chart indicates this is peak solar production season.  The trend line has flattened around the 240kWh per week mark.

Weekly solar power production since July 2021

Let’s review a typical clear, sunny day to see how solar power production and household consumption work. December 29th is a good example.

Solar power production and consumption during a sunny day in December 2021
Green – solar power production; Blue – consumption

I aim to have our big electricity users consuming solar power during peak production, from 11am to 4pm.

Between 11:00am and 1:30pm, the hot water heat pump warmed the water in the 300l storage tank. Once that finished, I charged the LEAF, then the MG.

In the background, the inverter is exporting excess solar power to the grid, up to the 5kW maximum allowed.

6kW production limit confirmed

The chart confirms my suspicion about this being a high water mark for solar power production. Our 8kW system is limited to 6kW production. A 2kW household demand plus a 5kW export limit should, in an ideal world, have the array producing 7kW. But it doesn’t. If we’re using 2kW, the inverter exports only 4kW because the best the panels can do is make a total of 6kW. The reasons are shading from our neighbour’s trees and the panel locations. All 26 aren’t exposed to direct sunshine simultaneously.

Daily solar power production showing consistent peaks at 6kW

The ten panels on the south roof are definitely taking advantage of the Sun being high in the sky.  As you can see, they are making similar amounts of electricity to the raised panels on the north roof.

Total production from each solar panel for December

However, no matter what I do, total production won’t go above 6kW.

Still, that’s not an issue.  Even with the production limit, we have more than enough solar power to meet our needs.

In December, the panels produced an average of 33kWh per day but we only used 12kWh.  The rest went into the grid to back out fossil fuel power.

December electricity cost

Imported GreenPower dropped to a new low of 8kWh for the month.

So, with excess solar power going to the grid during the day and a battery supplying electricity at night, why are we still importing?  Importing occurs when the inverter can’t keep up with a sudden change in household load such as using the oven or induction cooktop.  They suddenly require a high load which comes from the grid until the inverter catches up with electricity from the panels or battery.

In the scheme of things, it’s a tiny amount at an average of 0.3kWh per day.  Also, I’m buying GreenPower so the power is sourced from a wind turbine or solar farm.

Exports for the month reached a personal best of 650kWh.

The exported electricity earned enough income to more than pay for the $1.10 per day grid connection fee.  As a result, we finished December with an $8 credit to put towards the January connection fee.

Energy cost since July 1

Let’s review the energy balance and costs for the last six months.

  • Electricity imported = 503kWh or 2.7kWh per day (and falling)
  • Electricity exported = 2 206kWh or 12kWh per day (and rising)
  • Energy cost = $141 or $0.77 per day (and falling)

Looking ahead to January

Those few days at the end of December are an indicator of the weather ahead.

Temperatures will be higher for longer, both during the day and at night.    Chances are we might have to, pause for dramatic effect, turn on the air conditioner after an extended period of hot days.  That won’t be often.  The house has already demonstrated it can stay below 27oC for a few days of hot weather, relying only on natural air circulation for cooling.

My forecast is passive cooling with occasional mechanical cooling for some light relief.  Note to self though.  Where did I put that air conditioner remote?

Now for the electricity bill.  Our daily consumption could rise from using the air conditioner and, possibly, driving longer distances in the EVs if Omicron allows.  These changes probably won’t increase grid imports since there should still be plenty of excess solar power available.

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