Outdoor temperatures quickly changing from cold to hot in November didn't significantly affect indoors
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November cost and comfort report

During November, indoor thermal mass plus waning passive solar heating maintained a comfortable indoor temperature in the target range of 18 to 21oC.

The total energy bill was an all-time low of $1.80, down from $5 in October.  Average electricity usage rose slightly to 14kWh per day.  That figure hasn’t changed much since September.

November continued the trend of enough solar energy to fill the battery for overnight consumption and providing excess for trickle charging EVs during the day.  Apart from a few cool days in mid-November, the reverse cycle air conditioner stayed offline.

November comfort data

BOM recorded an average November daily maximum of 20.9oC, up from 19.1oC in October. The nighttime average jumped from 8.7oC to 10.3oC.  These averages conceal a mid-month cold snap.  Melbourne dropped back into Winter with nighttime temperatures plunging to 5oC and the days struggling to get much above 10oC.

This temperature chart compares conditions in our living area (kitchen) with the ambient temperature on the north and south sides of the house.  Note how the indoor temperature sails along within the desired range despite the outdoor fluctuations.

November indoor and outdoor temperature comparison
Green – living area temperature, Light blue – outdoors north wall, Dark blue – outdoors south wall

Response to November temperature data

Melbourne jumped from Summer to Winter to Spring and then back to Summer in one month.  However, our passive design features coped with the changes.

At the beginning and end of November, Summer had the upper hand with daytime temperatures climbing to 30oC.  We ran the house in “passive cooling” mode.  In the day, all windows are closed so the cool thermal mass keeps the indoor temperature at or below the recommended 24 to 27oC summertime range.  If necessary, opening windows at night vents warm air by natural circulation.  We only needed to vent a couple of times at night and never felt the need to use the air conditioner during the day.

Then Melbourne plunged back into Winter with cold, cloud and rain.  We swapped back to “heating” mode, using a combination of whatever watery sunshine made it through the north-facing windows plus the air conditioner.  This held the indoor temperature in the wintertime daytime range of 18 to 21oC.  Since the cold snap only lasted a few days, our thermal mass didn’t cool off enough to require the air conditioner at night.

The reminder of Winter was followed by a few Spring days so we swapped to “neutral” mode.  Neutral is just letting the house do its thing.  We don’t need air conditioning for heating or cooling and the temperatures aren’t high enough to warrant venting at night.

This chart shows how well the passive solar design shrugs off weather changing from cold to hot, without any mechanical heating/cooling to maintain a comfortable indoor temperature.

Outdoor temperatures quickly changing from cold to hot in November didn't significantly affect indoors
Green – living area, Light blue – outdoors north, Dark blue – outdoors south

Major electricity users

At this time of the year, there are two major electricity consumers – hot water and EV charging.

The hot water heat pump is a super efficient consumer of solar power during the middle of the day.

I trickle charge the EVs whenever there is excess solar power.  Since we aren’t travelling very far or too often, that’s all we need to top up the MG and LEAF.

Solar power production observations

This chart shows the solar panels are producing up to 40kWh per day in the right conditions.

Daily solar power production for November

As you can see below, the weekly production is climbing as the weather moves into the longer, generally sunnier Summer days.

Weekly solar power production since July 1

However, I can see the 8kW system is about to be caught between a rock and a hard place.

Two constraints limiting solar power production

The rock is the grid export limit of 5kW.  On paper, the panels can pump out a maximum of 8kW.  However, that’s difficult to achieve.  We don’t have big continuous daytime users.  Getting to 8kW for any length of time means using 3kW in the house and exporting the 5kW maximum.  Unlikely.  Still, we give it our best shot by looking for consumers whenever we hit the export limit.  Hence, the focus on charging an EV, doing the washing or cleaning the oven during peak production from midday to late-afternoon.

The hard place is shading from the neighbour’s canopy trees located on our eastern boundary.  November data shows the panels peaked at 6kW.

Daily solar power production peaked at 6kW in November

Although each panel is optimised, the monthly production figures show the eastern panels (right hand side) are down in production compared to the ones further away from the trees.

November production totals for each solar panel

It’s interesting to note the ten panels lying on the southern roof are only slightly lower than the 12 panels angled upwards on the northern roof.  The Sun is passing high in the sky over these panels so the southern orientation is less of an issue.

I’m looking forward to seeing if the panels can exceed the current 6kW PB in December.

November electricity cost

In November, average consumption rose marginally to 14kWh per day.  Our consumption patterns didn’t change from October, with the big users being hot water and EV charging.

Average daily solar production jumped to 30kWh.

Imported GreenPower remained low at 14kWh for the month. The home battery had enough capacity to get us through the night (and then some).  Even on cloudy days, we managed to recharge it and avoid importing from the grid.

With imports almost non-existent and good solar production, we exported 500kWh to the grid, up from 460kWh in October.

Payments for our solar exports almost offset the $1.10 per day grid connection fee.  Our energy bill dropped below $2.

Energy cost since July 1

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

  • Electricity imported = 495kWh or 3.2kWh per day (and falling)
  • Electricity exported = 1559kWh or 10.2kWh per day (and rising)
  • Energy cost = $149 or $1.00 per day (and falling)

Looking ahead to December

It’s likely Summer will decide to stay in December so we’ll move into cooling mode.

Our first response is using passive cooling.  Keep the thermal mass cool by venting warm air at night using natural circulation.  If that’s not enough, use the ceiling fans to create a cooling breeze.  Switching on the air conditioner is the last resort if we hit an extended period of hot days.

Summer, here we come.

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