A run of cold, cloudy Winter days in Melbourne was the ideal opportunity to try another operating mode for my SolarEdge inverter. During Summer, I regularly exported excess electricity from the battery at night. Could I do the reverse in Winter and import power?
The short answer is yes. But why bother? Isn’t this the opposite of what a sustainable home is all about? What’s the point of importing GreenPower electricity from the grid on purpose?
Let me explain how importing off-peak power can help the old hip pocket when the temperature plummets and the Sun isn’t shining.
Unusual for us to use electricity for heating and cooling.
With an 8.2 Star NatHERS rating, our home only requires a small amount of energy per square metre to keep it comfortable throughout the year. As the following chart shows, a new home built to the 6 Star minimum requirement consumes twice the energy. Since many of Melbourne’s homes were built before the 6 Star requirement, energy consumption can be much higher. The average pre-NatHERS home has a 1.5 Star rating so it requires at least 8 times more energy (gas/electricity) to keep the occupants comfortable.
Most of the time, a passive solar design doesn’t need any mechanical help with temperature control.
In Summer, eaves shade the windows and stop sunlight warming the thermal mass in the concrete floor and Timbercrete walls. The reverse brick veneer construction and good insulation in the ceiling/walls prevents hot air outside warming the inside of the home. Natural air circulation by opening clerestory windows at night flushes out any warm air that builds up during the day.
During colder months, the Sun is lower in the sky so sunlight streams through the large north-facing windows and heats the interior thermal mass. Insulation minimises heat loss at night so indoors stays pleasant after a day of free solar heating from the Sun.
This temperature chart shows inside/outside temperatures for the second half of April. Our home was doing a good job of keeping us comfortable without air conditioning.
There are times when we use the air conditioner.
We do need some electricity for air conditioning to keep it comfortable during extremely hot or cold days. Only a 10 Star home is designed to ride through temperature extremes without the need for mechanical heating or cooling.
If extra heating or cooling is necessary, we switch on the Panasonic 3.5kW reverse cycle air conditioner (heat pump).
A small, well-insulated home doesn’t need massive units to keep it comfortable indoors. We’re gently nudging the interior temperature back within a comfortable range. Non-passive solar homes often require much larger air conditioners, and more of them, to overcome issues such as poor orientation (eg west facing windows), large spaces and lack of insulation.
Where does the electricity come from?
For much of the year, our 7.8 kW solar panel array supplies electricity for the air conditioner. The split system is quite the energy miser though, drawing a maximum of 0.8 kW when running flat out. At night, it’s supplied with electricity from the LG Chem RESU10 battery. The Sun might not be shining but we’re still using solar power.
The other significant energy user in our all-electric home, the hot water system, is also solar powered. I’ve set the timer on the Sanden heat pump to run in the middle of the day. The combination of solar panels and a battery means solar power usually meets our hot water needs.
Energy numbers from the fortnight leading up to Melbourne’s cold snap are an example of what I’m talking about. On average, we imported 1.6 kWh of GreenPower. That’s because the inverter lags on sudden changes in household electricity demand (eg induction cooktop cycles). But, the Sun was shining enough for us to fill the battery and export an average of 6.7 kWh per day. Our electricity ledger was well and truly in the black.
What’s the problem with Melbourne’s cold snap?
Four days of cold, rainy weather arrived in early May. Low temperatures by themselves aren’t a major issue for a passive solar home. It’s the combination of cold and thick clouds. Clouds cut off the supply of free heating from the Sun and reduce the amount of electricity produced by the solar panels.
Temperatures dropped to 6 deg C overnight and then struggled up to 10 – 12 deg C in the day.
I used the air conditioner to maintain a comfortable 20 deg C during the day. At night, I dropped the set-point to the minimum of 16 deg C to stop the home’s thermal mass getting too cold. This is a trick we discovered last Winter. Using a bit of energy overnight to stop the concrete slab/walls falling below 16 deg C delivers a more pleasant start to the day. The air conditioner doesn’t have to work hard to bring room temperature back to 20 deg C when we’re up and about.
Why do we have to import power?
When the cloud and rain moved in, solar power production fell off the proverbial cliff. Leading up to the cold snap, the 7.8 kW array produced an average of 18 kWh per day. Putting a layer of dark, rain filled clouds between the panels and the Sun cut daily production in half.
The bad weather put our energy production and demand out of whack. Demand rose because we needed the air conditioner to keep indoors comfortable. Production fell so there wasn’t enough to charge the battery for nighttime supply.
There was never a question of our house running out of electricity. After all, we’re connected to the grid. When the solar panels and battery can’t meet demand, imported power from the grid does the job.
What attracted my attention was the cost of importing electricity. It was likely we’d import grid power during peak time (late afternoon and evening) as solar production tapered off. Without excess solar production during the day, our battery would be unable to contribute much when we needed it most. As a result, we’d purchase electricity at the most expensive rate.
Or, would we?
How to import power at lowest cost?
One advantage of the SolarEdge SE6000 inverter is the ability to be programmed. I tell it to do specific tasks at particular times using the StorEdge Energy Manager function on the website.
In Summer, I used a “discharge to maximise export” program to send electricity from the battery to the grid in the early morning. Whenever I predicted the battery had excess to our overnight requirements AND the next day was going to be sunny, I exported from the battery. I felt better about using it to back out a little bit of generation from a brown coal fired power station. Next morning, the solar panels recharged the battery.
Well, if the inverter can be trained to export, I was willing to bet the reverse was true. Diving back into the instruction manual I found the “charge from solar power and grid” option. If there isn’t any solar production, the inverter goes to import power from the grid and charges the battery.
Import power in action.
This StorEdge feature is exactly what I needed for the May cold snap. Instead of peak power import, I could top up the battery from off-peak GreenPower in the middle of the night. During the four cold, cloudy days, our home was powered by a combination of solar production (whatever the panels could produce) and off-peak power imported the night before.
Data from May 1 shows what I was doing. With the import power option, I recharged the battery during the early morning. This was enough to get us through the day without the need for buying peak power. On May 2, I recharged the battery with off-peak electricity and away we went again.
Power import during off-peak times minimised the energy cost. In late-April, the average power import was 1.6 kWh per day at a cost of $1.67. When the wintry blast hit, imports jumped to an average of 8 kWh per day. Using off-peak power did add $1/day to my bill. However, buying electricity at peak rates would have more than doubled the cost.
SolarEdge gives me choices.
Having a few extra dollars in my pocket is better than a poke in the eye with a sharp stick, but not the main reason for this experiment.
Using savvy inverter/battery technology helps me transition from a passive grid connection to a more active participant.
In Winter, I decide when I buy electricity to make up the difference between my household demand and solar power production. At the moment, this is a bit fiddly. After checking the weather forecast and the battery status, I decide if power import is required. If there’s likely to be a shortfall, I log into the SolarEdge website and select one of three import options I’ve created – 30, 60 or 90 minutes. Sixty minutes gives me 5.5 kWh import power If I’m not predicting a shortfall, I reset the inverter to “maximise self consumption”. In this default option, solar power goes first to meet household demand, then to charging the battery and finally to export.
At times of good solar production, I can choose to help the grid by exporting excess power from the battery.
Wouldn’t it be nice if, in the future, some clever person developed an app that could help me do all this just by pressing a few buttons?
Towards a comfortable, lower energy cost Winter.
Last Winter confirmed that living in a passive solar house is a comfortable, light-filled experience. This Winter, I have the bonus of being able to shave a few dollars off the electricity bill using my power import option.
Next time I talk about solar power, I’ll bring you up to date with the neighbourhood’s high grid voltage issue. Why was it putting a damper on my enjoyment of sunny Summer days? Why is there cause for hope for better days later this year?