Compare indoor and outdoor temperatures during the January hot spell
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January comfort and cost report

A record breaking run of hot weather in January highlighted the benefits of a high performance home.  At night, opening windows vents warm air and replaces it with fresh, cool air.  Shaded windows, excellent insulation and a cooled thermal mass ensures the interior stays comfortable without air conditioning.

Our energy bill was another credit, up slightly to $11.  As in the previous four months, average daily electricity usage hovered around the 12kWh mark.

Sunny January days helped the solar panels pump out 1 MWh.  That’s a lot of electricity and far more than we need for hot water, EV charging, cooking, washing, etc.  Almost 700 kWh went straight into the grid.

January comfort data

Melbourne’s weather broke or equalled a few records in January.  For example, we broke the record for the warmest average minimum since 1855 and, at 17 days, equalled the record for most days above 30oC.

At our local BOM weather station, the average daily maximum was 29.8oC, significantly higher than December’s 25.1oC. The nighttime average jumped too, going from 11.6oC to 16.5oC.

The heat was definitely on.

Response to January temperatures

This temperature chart compares conditions in our living area (kitchen) with the ambient temperature on the north and south sides of the house.

Indoor versus outdoor temperature for January
Black – kitchen; Blue – outside south side; Green – outside north side

The red lines show the recommended indoor summer temperature range of 24 to 27oC.  This range is based on balancing air conditioning costs with personal comfort.  The lower the setpoint on the air con, the more electricity (and usually dollars) it needs.

Despite the outside temperature frequently rising above 34oC, the indoor temperature stayed within (or below) the recommended range.  I’m sure that many homes can do the same thing, but we managed this feat without using the air conditioner.  We did use ceiling fans to create a gentle breeze in the living area during the day and for a few hours in the bedroom at night.

Living area image by Dylan James showing the clerestory window above the sliding door
Ceiling fan in the dining area

The blue line is important

The blue line at 20oC helps to explain why air conditioning wasn’t necessary.

Quite often, Melbourne kindly follows a hot day with a cooler night.  The blue line shows the overnight minimum consistently dropping to 20oC or below.  This natural supply of cool air is the basis for “passive cooling” mode.  The black line shows the indoor (kitchen) temperature gradually climbing during the day.  As soon as the outdoor temperature fell back towards the indoor temperature at night, we opened all the windows and vented warm air.

Next day, I closed all the windows when the outdoor and indoor temperatures equalised. This trapped cool air inside.  Having a cool interior thermal mass and excellent insulation prevents heat transfer from the outside rapidly warming the indoor air. The eave on the north facing windows stopped any sunlight hitting the concrete slab thermal mass, preventing another way heat sneaks into a house.

Passive cooling under pressure

This chart of a 9 day hot streak in late January shows exactly how the house works during a run of hot weather.

Compare indoor and outdoor temperatures during the January hot spell
Black – kitchen; Blue – outside south side; Green – outside north side

The outdoor temperature climbed steadily to 34+oC but the indoor temperature rose by only a couple of degrees, staying within the target range.  When the outdoor temperature fell at night, venting drops the indoor temperature by a few degrees so we’re back to 24oC.  And around we go again.

On the last day, I did find the air conditioner remote and switched it on for a couple of hours.  We wanted to check it was working and, hey, given the electricity was coming from the solar panels, the only cost was giving up the 6c/hWh feed-in credit.

Will we ever use the air con?

Could we continue this way forever and let cobwebs gather on the air conditioner?

Unlikely, because Melbourne weather hasn’t unleashed its worst case scenario yet.  We can get extended periods of high 30’s and low 40’s for daytime maximums, followed by nighttime minimums in the high 20’s.  In that situation, venting at night just lets in hot air and makes it uncomfortable inside.

With extreme hot weather events, an air conditioner is necessary to maintain a comfortable interior.  However, the passive solar design ensures it doesn’t have to work too hard.  Heat transfer from the hot air outside is limited by ceiling and wall insulation.  Also, the air con cost is minimal because the electricity comes from the solar panels during the day and the battery at night.

Based on our experience so far, the air conditioner gets more of a workout in Winter during cold, cloudy days.

Major electricity users

No change to our major users being hot water and EV charging.  But Libby’s beloved Nissan LEAF is gone.  We sold it to the local Nissan dealer who wanted to increase their second hand car stock.  Libby was sad to see the LEAF go, but we can’t justify two vehicles with our limited travel.  To get her over the line, the dealer upped the sale price by $500 and I promised to share the MG ZS EV without favouring my need for the car.

I trickle charge the MG during the day and soak up some of the excess solar panel production.  Contrary to what some commentators say, we aren’t loading the electricity grid with EV charging demand.

On average, we used 11.7 kWh per day for all our energy needs – hot water, EV, cooking, lighting, washing, etc.

Solar power production observations

This chart shows the solar panels continue to regularly produce 40+kWh per day.  The daily average for January was a healthy 34kWh.

Daily solar power production for January

Weekly production has probably plateaued around the 250 kWh mark.  As I explained in the last post, this is the maximum we’ll reach during long, sunny Summer days.

Weekly solar power production since July 1 2021

January electricity cost

Imported GreenPower increased fractionally to 9kWh for the month.  The number is so small that slight variations are meaningless.  The solar panels and battery are providing almost all the electricity we need.  Importing only happens for brief periods when the inverter is catching up to a sudden load change like the induction cooktop. 

Speaking of record breaking months, we set another personal best for electricity exports.  In January, exporting 684kWh smashed the December record of 650kWh.

The exported electricity earned enough income to more than offset the $1.10 per day grid connection fee, giving us an $11 credit.

Energy cost since July 1

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

  • Electricity imported = 512kWh or 2.4kWh per day (and falling)
  • Electricity exported = 2 890kWh or 13.4kWh per day (and rising)
  • Energy cost = $130 or $0.60 per day (and falling)

Looking ahead to February

January showed our 8.4 Star design doesn’t need air conditioning to keep indoors comfortable as long as we do our bit with nighttime venting/cooling.

The big question is whether Melbourne is going to further test the house design with a run of 40+oC days and hot nights.

I’m confident this won’t significantly affect our comfort or cost.  We can give the air conditioner a surprise by switching it on when required.  Also, since the power is supplied by the solar panels or battery, electricity imports should stay low.

If Summer does turn the heat up, we will simply move from “passive cooling” mode to “active cooling” and sail on.

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