In the process of moving into the Greeny Flat, Cintia and I decided that we needed to get a new fridge. The one we had been using was an older Fisher and Paykel that we borrowed from a friend and it had icing-up problems. But it also had a 4-Star Energy Rating sticker on the door and an estimated annual energy usage of 710kWh. This sounded pretty good until I went looking for a new fridge and found one of a comparable size with a 3.5-Star Energy Rating that only uses 300kWh per year. That didn’t make much sense to me until I dug around on the internet and discovered that, a few years ago, the Energy Star Rating system was changed. So newer fridges can be much more energy efficient but have fewer stars than older fridges. This is important information for anyone looking to buy a fridge. You can’t just look at the number of stars, you have to compare the estimated annual energy consumption which is the number written on the label underneath the stars.

How can our old fridge have 4 stars and use 710kWh but our new fridge has 3.5 stars but only uses 300kWh?

The fridge we ended up getting is a Samsung SR319MW Digital Inverter model and it is wonderfully energy efficient. In fact it is one of the most efficient fridges on the market and yet it only has a 3.5-Star rating. It makes you wonder what you would have to do to get six stars. Our energy monitoring system shows us that when this fridge kicks on it only uses about 60 watts and only for brief periods at a time. I did quite a lot of research before I bit the bullet and bought this baby and I learned some interesting things about inverter technology. Fridges like this one have a DC motor (hence they need an inverter to change the power from AC to DC) which allows the motor to be ramped up and down in stages. Unlike a standard AC fridge which is either full on or full off, this fridge can just cruise along and only add a little bit of coolth when it is needed. I noticed when I first turned it on that it was drawing about 140 watts while it was cooling the whole inside of the cabinet, but now it is using much less power and only in short bursts. I also learned that, whereas the temperature inside an AC fridge varies by about a 4˚C, the temperature inside a Digital Inverter fridge should stay within about a 1˚C range. Apparently this is much better for keeping food fresh for a longer period of time and I have certainly noticed that our food stays fresher for longer in this fridge. It also has an LED light which only uses a tiny bit of power when the door is open. (Click on the photo below for more information and specs on the SR319MW).

Cool Fridge! The Samsung SR319MW Digital Inverter Fridge with LED lighting only uses about 60W for brief periods to keep our food perfectly fresh.

There was one potential drawback I had read about in various internet forums. Some people complained that their Digital Inverter fridge made an awful electronic buzzing noise. I asked the salesman at Harvey Norman to plug in the display model so I could listen to it and it seemed fine to me. But that was in a noisy showroom so I was still pretty nervous about the noise issue when I brought the fridge home. Having lived with it for a month now we are perfectly happy with it and haven’t noticed any undue amount of noise. Sure, it’s not silent (and neither of us have the most acute hearing in the world), but it’s no more annoying or obtrusive than any other fridge and we very seldom even notice it.

In general I can highly recommend this fridge. Only time will tell whether it is durable and long-lasting, but for the moment I can say that it is extremely energy efficient, reasonably quiet, well designed, and at about $750, decently priced so we’re very happy with it.

Well we’ve been monitoring the Greeny Flat for almost a month now and the early results are very promising for us to reach the goal of being Energy Positive over a full year. As you’ll see from the following graphs, we’ve already made significantly more power than we’ve used in spite of the fact that for about three weeks we were heating all our water using the electric booster on the solar hot water (SHW) tank. This was due to a couple of key sensors being missing when the SHW system was first installed and taking a long time to get here. The good thing about this is that it has given us some interesting comparisons between our energy use with no hot water at all, with electric hot water, and now (thankfully) with solar hot water.

We have a wonderful little energy monitoring device called a Wattson Meter which monitors and records both the energy that we use and the energy that we produce. We can connect it to a PC and, using the “Holmes” software, see graphs of the various recordings from the Wattson Meter. One of the available graphs shows the total amount of energy we have produced each day (the green columns) and the total amount we have used each day (the red columns) in Kilowatt Hours (kWh). In the graph below we can see the energy that we used for the first six days that the Wattson Meter was installed (on the left hand side) during which time we had no hot water in the house (great energy performance and very quick showers). Our average energy use during this period was 2.71kWh per day.

Wattson Meter recordings showing daily energy usage (red) and production (green).

For the following three weeks, while we waited for the parts for the SHW system, we used more than twice as much power (7.76kWh) as we can see in the graph below. It’s a telling reminder of just how much energy it takes to heat anything with electricity… mind you it takes just as much energy to heat things with gas or solar, it just doesn’t cost as much. Notice how much energy we used on the first day the SHW tank was installed. This was because it had to heat the whole tank full of hot water. On subsequent days it only had to heat as much water as we used and the lowest energy use days were ones that we were away and didn’t use any hot water. The variations in production (green columns) are entirely due to how much cloud there was each day. The highest production days were the sunniest days and the lowest ones were the cloudiest. But the exciting thing for us is that there were very few days when we didn’t make more energy than we used.

Energy use during the time we had to use the electric booster to heat the hot water tank.

For the last two days we’ve finally had the SHW system fully operational and our energy use has dropped back to an average of 4.05kWh per day. This should be somewhere close to our average energy usage for the rest of the year. This compares to an average production from the solar power panels of 9.76kWh. So we’re making a bit more than twice the amount of energy that we’re using. If this continues for the rest of the monitoring period we’ll have no trouble at all meeting the Energy Positive goal and our electricity bills should be pretty low too.

Energy use now that the SHW system is working

But we will still have electricity bills to pay. This is due to a number of factors. For one thing we pay 24.66 cents for each kWh we draw from the grid and only get 8c for each kWh we put into the grid. We also have to pay a service charge of about 70c per day regardless of how much energy we use or make. So we’d have to make about 20kWh per day in order to have no bills at all. As it is, the way we can minimise our bills is to use as much power during the day as possible. That way, the power that we are making goes straight to our use and only the excess production goes to the grid. The following graphs show that we only used 26.5% of the energy that we made from solar (the rest was exported to the grid) but we were able to make 42.3% of the total amount of energy that we used (the rest came from the grid). In order to keep our bills as low as possible we need to find ways to make the blue and green portions of the charts below as big as possible (I’ll write more about how we might do that in another post).

Proportions of energy used and produced

The last graph I want to share today is one that shows when power was used and produced during an average sort of day. Once again the green line shows power production from the solar system which, predictably, peaks right at noon. The red line shows when power was being used and how much (in Watts). Starting on the left we can see that our baseline power usage is about 56W. This is from the monitoring equipment that we have installed and a continuous ventilation fan (necessary to maintain good indoor air quality in this well-sealed house). The three bumps on the left are when the fridge motor kicked in. Thankfully we have a VERY efficient Samsung Digital Inverter fridge so these bumps only add about 60W for a brief period of time. Then we come to the three major peaks for breakfast, lunch and dinner. Once again, the height of these peaks is due to the fact that we are heating with electricity. The good news is that each one is for a relatively short time. We can also see that during the day the baseline power use went up a bit. This is due to the pump on the solar hot water system being activated when the sun is heating the panels. Finally, on the RHS, there is a bit of a mound while we watched a movie before the power drops back down towards the nighttime baseline.

Graph showing when power was used and produced during a day.

The other thing that we’ve been keeping a close eye on is the comfort level inside the flat. I haven’t downloaded any data from the weather station yet but I can tell you that the temperature inside the Greeny Flat hasn’t dropped below 16degC even though the outside temperature has consistently dropped to between 2 and 4degC during the nights. This is a very good indication that it should stay comfortable inside through the winter particularly because we don’t have the insulated blinds installed on the windows yet. Once they are up we will have a lot less heat loss during the night so we should stay nice and cosy. Which all goes to show that Passive Solar Design really does work when it is done right.

In short, we’re thrilled with how things are going and looking forward to the rest of the year.