How to power through a heatwave – in comfort, on 100% solar energy

One Step Off The Grid
Last week I received a phone call from an ABC journalist. Unfortunately he didn't want to invite me on Gruen to talk about the joys of solar advertising, or book Ronald & Tonto23 to perform on Mad As Hell. Rather, he'd been told that solar panels lose efficiency when it gets hot, and was keen to learn more.
Specifically, he wondered how much solar panel efficiency would be lost if the temperature hit 45ºC during the heatwave – as forecast for the following day.
I spent the next 20 minutes going through the 3 basics of solar power and temperature that, in my experience, cause confusion to non solar-nerds:
How solar panels
love light, but dislike heat.
The difference between solar thermal and solar PV panels.
Why the temperature used to determine the efficiency loss in a solar panel is the panel temperature, not the ambient air temperature.
After the call I worried I may have inadvertently given the impression solar panels cannot be relied upon to power your home (or help power the nation) through extreme heatwaves. I really don't want the anti-renewables brigade to think they've discovered solar power's kryptonite: extreme heat.
Hence this blog post showing real data from two rooftop solar systems in Adelaide that reliably soldiered on through the oppressive heat.
The next day, as forecast, we experienced the extreme heatwave in my hometown of Adelaide, with the top temperature of 46.6ºC breaking all records. I have a relatively efficient solar and battery powered home.
I also have monitoring
on a less thermally-efficient, solar powered (no battery) home down the road, so I thought I'd share how both houses performed on the hottest day Adelaide has ever seen.
And it really was pretty fricking hot. I rode to the shops (to get a red onion) at 3pm and it was like pedaling through a wall of hairdryers. I didn't see any other bicyclists (or pedestrians) on my 400m round trip.
For the record, these were the half-hourly temperatures in my part of Adelaide on the day in question (24 Jan 2019):
House #1 – My home: 6kW of solar, 13.5kWh of battery, split aircon, good thermal envelope.
Q. How much energy did I need to run the house and keep everyone cool?
A. 39.5kWh
This is the 5 minute power consumption of my house on that day. The total energy used1
is a whopping 39.5kWh.
My home's typical daily consumption is around 15kWh. That powers a family of 5 and a small business. So you can see that keeping cool on very hot days takes an exceptional amount of energy. Even in a house with a good thermal envelope and a modern, efficient air conditioner.
Q. How much solar energy did I generate?
A. 34.8kWh
A typical solar panel loses 0.5% of its efficiency for every degree over 25ºC that the panel's temperature is at. In 45ºC I'd expect the panel temperature to be close to 70ºC. So I'll be losing about (70-25) x 0.5  = 22.5% of my power from the crazy-high temperature.
I have a 6kW system
so I'd expect to lose about 1.3kW of power due to the heat and perhaps another 5% (0.3kW) due to other system losses.  i.e I'd expect the peak power of my 6kW system to be about 4.4kW on a 45º day.
Here's the solar electricity generation laid over the consumption:
Two things to note:
My 6 kW system peaked at 4.4 kW as predicted.
Even if I didn't have a battery
, the solar alone covers most of my consumption over 24 hours. Read on to discover how much.
Q. How much of my solar electricity was self-consumed (ignoring the battery)
A. 22kWh
The red area here shows the directly self consumed solar energy:

So from 39.5kWh consumed, 22 was supplied directly from solar electricity.
Q. How much money did my solar save me over 24 hours?
A. $10.92
The red, self-consumed solar electricity saved me (22 x $0.38) = $8.36
because I didn't have to import that energy from the grid.
If I didn't have a battery, the remaining 17.5kWh would be simply topped up from the grid.
The cost of that imported electricity would have been $6.65. But that would be reduced by the $2.56 earned from exporting the excess solar electricity at 20c per kWh.
So, with solar power only (no battery) my energy bill for the 24 hours would have been $4.09, compared to $15.01 if I had no solar panels.
My solar alone saved me $10.92 over the 24 hour period.
But, I have a Powerwall 2, 13.5kWh battery on this house.
Q. How much of the remaining consumption was supplied by the battery?
A. All of it.
The battery was happily discharging before sunrise and started to recharge from the excess solar electricity at about 8:30am. The battery didn't run out until 1am the following morning.
The battery allowed me to self-consume 17.5kWh of solar energy instead of exporting it to the grid. The net savings per kWh discharged from the battery are calculated as the cost of grid electricity minus my feed in tariff ($0.38 – $0.20) = $0.18 per kWh.
The battery saved me (22kWh x $0.18) = $3.96
Compare that to the solar-only savings of $10.92. And consider that a really good 6kW solar system costs about $7,000 installed whereas a Powerwall 2 costs $15,300 fully installed.
Some things to note:
This was an exceptional day. The battery was used at 130% capacity. Normally I use about 5kWh (36% of battery capacity) giving me typical daily savings of $0.90 from the Powerwall.
I didn't use any grid energy that day. That's pretty cool. I didn't place any load on a very stressed grid2
.
Here's the full energy profile for the 24 hours courtesy of Solar Analytics
:
House #2: 6.6KW solar (east/west split), ducted aircon, poor thermal envelope.
The guys in this house (AirBnB guests) used a phenomenal 70.5 kWh of energy, of which 65.3 kWh was the air conditioner. The air conditioner is about 13 years old and ducted.
The house generated 35.9 kWh of solar electricity with its 6.6kW of Winaico solar panels
– half east and half west.
The residents self consumed 30kWh of solar energy – saving $11.40 and reducing the electricity bill for the day from $26.80 to $15.40.
Adding a 13.5 kWh battery would have saved between $1.06 and $3.33 depending on how much charge it had at the start of the 24h period (0%-100%) and assuming only solar electricity was used to recharge the battery3
.
Can a modest solar system plus battery power you through a heatwave?
If you have a home with good thermal performance, yes. You can get through a heatwave without spending a penny on grid electricity4
. The battery is a luxury though – as the vast majority of the savings will be from the solar panels.
If you live in a typical Australian home with ducted air conditioning and a poor thermal envelope, you are going to need a huge battery and big solar panel system
to be 100% solar powered in a heatwave.
House #2 above would need 3 Powerwalls and a 15kW solar system. That's at least $50,000 of gear.
If you want to reduce such a home's impact on the grid on extreme days, then the most cost effective way to do that is to improve the thermal efficiency of your home (gaps, glazing, insulation) and get the most efficient air conditioner you can
.
It's not as sexy as a battery, but it will make your home more comfortable all year round and save more money.
If you combine that with a reasonably sized
solar power system, your home can be transformed from being part of the problem to becoming part of the solution when these increasingly common heatwaves hit. And I think that's a noble endeavour.

This article was originally published on RenewEconomy's sister site, One Step Off The Grid
, which focuses on customer experience with distributed generation. To sign up to One Step's free weekly newsletter, please click here
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