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Updated to 31st December 2018
As a retired electrical engineer, I had always assumed that adding solar battery storage would result in an increase in what we save, over and above what we saved by having solar panels only, so I was surprised to find that this didn’t happen with the AGL Virtual Power Plant (VPP) Sunverge system, no matter which way the data was examined.
In this article I have compared 3.50 years of data from 5.2 kW of 260 watt REC solar panels and an SMA 5000TL-21 solar inverter, for the period from 17th November 2013 to 21st May 2017, against 1.42 years of the same panels with an 11.6 kWh Sunverge SIS-7048 DC coupled solar battery, for the period from 26th May 2017 to 31st October 2018, as a part of the AGL VPP.
On 1st November 2018, the Sunverge system was replaced by an AC coupled Tesla Powerwall 2 battery and a Fronius Primo inverter with a Fronius Smart Meter, so this article now includes 61 days of the Tesla Powerwall 2 system.
A detailed analysis revealed two main reasons why we didn’t save money by adding the Sunverge battery:
Of the total of 1,811 kWh of peak energy imported for the entire period, 1,305 kWh, or 72%, of that peak energy should have been taken from the battery. The system also exported battery energy to the grid at night for unknown reasons.
The net result of importing all this unwanted peak energy, instead of using the stored energy from the battery, was an increase of $455.15 in our peak import bill, or $26.77 per month.
You may wish to read An Introduction to AC and DC Coupled Solar Battery Systems for an understanding of some terms used in this document.
Our panels face directly north, they are not shaded, and they are at a pitch of 22.5 degrees above the horizontal.
Solar production data prior to the battery installation was taken from the SMA inverter via Bluetooth, and after the battery was installed it was taken from AGL’s Solar Command platform.
I have maintained a very detailed spreadsheet of all our monthly electricity costs and energy flows since November 17th 2013, with the earlier quarterly bills converted on a pro rata basis into monthly data, and all prices include GST, unless otherwise specified. All spreadsheet data has been reconciled to the cent and to the watt, and are based entirely on actual data except for January 2018, when a Sunverge software update failed and the system was disabled for 10 days. This meant we imported a lot more peak energy than normal, and exported a lot less feed-in. For January 2018 therefore, I have used the daily average of the 21 days for which we had solar energy scaled up to 31 days.
The gross peak tariffs quoted in this document are the full retail tariff including GST. The net peak tariffs are calculated by first discounting the ex-GST rate at the offered discount rate, and then adding 10% GST. Government feed-in tariffs are always ex-GST. I have excluded state government pensioner discounts and other minor account-based charges, and I have omitted controlled load in this document because it is not pertinent to solar energy generation and storage.
The data used for no solar panels is as derived in The Significant Cost Benefits of Solar Panels.
SUNVERGE BATTERY MANAGEMENT LOSSES
Sunverge decided, after some experimentation early on, that the battery couldn’t be discharged below 24%, nor charged above 94%. This meant that we got only 70% of our supposed 11.6 kWh, or 8.12 kWh.
A detailed examination below of the way Sunverge managed the importation of peak energy showed that we imported more peak energy than we required in the 518 days from 1st June 2017 to 31st October 2018, at an extra cost of $455.15 at the then prevailing AGL discounted tariffs, or $466.73 at the current discounted peak tariff. That is, of the total of 1,811 kWh of imported peak energy, 1,305.2 kWh, or 72%, were excess to requirements.
Using December 2017 as a graphical example to illustrate the point, I have graphed the daily minimum and maximum battery status percentages from Solar Command, and the current battery discharge limit. We were charged for 31.328 kWh of imported peak energy even though, as you can see from the graph below, the battery was fully charged each day, and never discharged to the discharge limit of 24%. In fact there was only one day where the minimum daily discharge was even close to the limit. On that basis, there would appear to have been no need to import any peak energy during December 2017.
The starkest example of this problem is illustrated in the histogram below, in that for 55 days in September and October 2018, our house was vacant, with just the refrigerator running, yet we imported 124.695 kWh of grid energy, or 2.267 KwH per day, at a total cost of $44.59 at the discounted tariff of $0.3576. During the night-time periods of those 55 days, from 8:30pm to 6:00am, we consumed an average of 1.794 kWh per day, imported an average of 1.368 kWh per day, and even exported 0.051 kWh per day. In other words, the Sunverge battery provided only 23.7% of our night-time energy requirements, with the rest being drawn from the grid. These results are not a direct result of the house being vacant, but the absence of any other energy flows makes it stand out.
The next histogram shows the overall nightly grid imports and exports for the life of our Sunverge system. The significant amounts of battery energy exported to the grid during the night are a surprising feature of the Sunverge system, as are the large amounts of grid energy imported after midnight, in lieu of using stored battery energy. Energy imported before midnight is to be expected, and is a result of the battery energy being used up in the normal course of events.
The next histogram shows the significant difference between the Sunverge and Tesla night-time grid imports. Again, energy imported before midnight is to be expected, and is a result of the battery energy being used up in the normal course of events.
The next histogram shows the significant difference between the Sunverge and Tesla night-time grid exports. The heavier exports before midnight cannot be put down to normal battery draining, and make no apparent sense.
In order to provide a detailed numerical analysis of the problem, I’ve analysed the data from 1st June 2017, our first full month with the battery installed, until 31st October 2018.
I have examined, on a monthly basis, all the days in each month in which our consumption exceeded net production.
Table 1 below illustrates this process for May 2018. Columns 2 and 3 are taken from Solar Command csv data. Column 4 is net solar production using an estimated round turn efficiency of 90%, and Column 5 is net production minus consumption. Column 6 lists the daily amounts of energy for when our consumption exceeded net production. The total of 79.814 kWh is what we really needed to import for May, but the actual imported grid energy for May, as taken from our calendar month AGL bills, was 173.910 kWh.
Table 2 below is a monthly summary of the process detailed in Table 1, since 1st June 2017.
Column 4 shows the monthly totals as derived in Column 6 of Table 1. Column 6 shows the excess energy imported, being the metered and billed imported energy minus the energy we actually needed from Column 4. Column 7 is the cost of the values in Column 6 at the prevailing discounted tariff for that month.
SUNVERGE ENERGY FLOW RATIO LOSSES
This is a comparative study of what actually happened over the period for which we had solar panels only, compared to the period when we had the Sunverge battery as well.
Table 3 below shows the costs, exactly as they occurred, based on having solar panels installed, but no solar battery. The PER ANNUM data at the bottom of Table 3 is derived by dividing the TOTAL values by 1278 to give a daily average, and then multiplying by 365 to give an annualised average.
Table 4 below shows the costs, exactly as they occurred, based on having solar panels and a Sunverge solar battery. The PER ANNUM data at the bottom of Table 4 is derived by dividing the TOTAL values by 518 to give a daily average, and then multiplying by 365 to give an annualised average.
Table 5 above is a summary of annualised actual costs/revenues, taken from Table 1 in The Significant Cost Benefits of Solar Panels, and Tables 3 and 4 above, and also annualised costs/revenues adjusted to reflect current tariffs.
Columns 1 to 4 summarise what we actually paid, or were paid, and what the average tariffs were.
Columns 7 and 8 show the percentage changes from the average historical tariffs to current tariffs, and when the costs in Columns 1 and 2 are scaled by these changes, we get the values in Columns 9 and 10.
Rows 4 and 6 show the decrease in imported energy costs and the increase in exported energy revenue, while Rows 5 and 7 show the total net savings resulting from adding both these changes together.
Row 8 is the difference between the total net savings in Row 7 and the total net savings in Row 5; that is, the losses resulting from adding the Sunverge battery to the solar panels.
The loss shown in Row 8 for Columns 1 and 2, as compared to Columns 9 and 10, is a result of much lower import tariffs and much higher export tariffs for the period when we had panels only, compared to the period when we had the Sunverge battery as well, as clearly detailed in Tables 3 and 4.
Column 11 shows net revenues; the difference between revenue earned from exporting energy and the costs incurred in importing energy. Clearly, adding the Sunverge battery has reduced that net revenue by $72.40 per annum.
The difference between actual and current values in Table 5 highlights the difficulty of a simple price comparison over long time frames as a method of measuring any cost benefit from adding the Sunverge battery.
A surprising contribution to the drop in savings after adding the Sunverge battery turns out to be based on the way our imported and exported energy flows changed with the addition of the battery, as Table 6 below shows.
Row 1 shows the current tariffs, Row 2 shows the annualised peak imported and feed-in exported energies from December 2013 to May 2017 for panels only, and Row 3 shows the annualised peak imported and feed-in exported energies from June 2017 to October 2018 for panels and Sunverge battery.
Row 4 shows the ratio of how much annualised imports and exports have changed as a result of adding the Sunverge battery, and Row 5 shows the annualised reductions in kWh.
Applying the tariffs in Row 1, to the energy changes in Row 5, results in the annualised cash flows in Row 6.
Row 7, net revenue change, is Row 6 export revenue changes minus Row 6 import cost changes, and shows how much per annum the net revenues have decreased as a result of adding the Sunverge battery. Note that this result is very similar to the results in Row 8, Columns 9 and 10, from Table 5.
Row 6 clearly shows that, as a result of adding the Sunverge battery, we have lost more in export revenue than we have gained in reduced import costs, as reflected in the ratio of 1.253. If we are to increase net revenues, we need the ratio in Row 6 to be less than 1.000. That is, we need to save more in imported energy costs than we lose in exported revenue.
A simple way to illustrate the problem is to assume ratios of 3 and 0.5, and start with 6 units of exported energy and 2 units of imported energy for panels only. Adding a battery halved those numbers to 3 units of exported energy and 1 unit of imported energy. In other words, at AGL’s current discounted tariffs of $0.16300 for exports and $0.35759 for imports we have gone from a profit of 6 x $0.16300 – 3 X $0.35759 = $0.26282 with panels only, to a profit of 3 x $0.16300 – 1 X $0.35759 = $0.13141 with the addition of the battery. With those ratios, adding the Sunverge battery has halved our net revenues, and it will now take much longer to pay off the battery.
Another way of looking at this is that the Sunverge battery did not contribute to paying for itself; the savings from the solar panels would be what would have eventually paid off the Sunverge battery, with the losses in revenue from adding the Sunverge battery being a considerable drag on the payoff period.
TESLA POWERWALL 2 ENERGY FLOW PROFITS
This analysis covers only 0.17 years from 1st November 2018, so at this stage it is indicative only, given that it involves annualising just 61 days of data. Note also that of the 18.984 kWh imported in November, 6.112 Kwh were imported on the first day to initially charge the battery. Tables 7 and 8 below are repeats of Tables 4 and 5 above, except they only include data for the Tesla Powerwall 2 battery.
The results at this early stage show that the Powerwall 2 will contribute substantially to paying itself off, unlike the Sunverge, in that the net savings over and above having just solar panels are very significant. Note that the projected savings from the Powerwall 2 are of a similar order to the losses we suffered from the Sunverge battery.
Again, while it is too early to make any confident predictions, 61 days of Tesla data suggests that the current subsidised Powerwall 2 VPP battery cost (with backup) of $8,389 could be paid off in 14.6 years. For those not in the VPP, the latest retail pricing for the installation of a Tesla Powerwall 2 is estimated currently at $15,275, with a time of 26.6 years to pay it off.
I will be updating these results on a monthly basis.
SUNVERGE SOLAR PRODUCTION/CONSUMPTION EFFICIENCY
Solar Command provides data on the total gross amount of solar energy produced by the solar panels, and the total gross amount of energy consumed in the home.
If we subtract the feed-in energy exported to the grid from the gross solar production, we get net solar production, which is the total amount of solar energy actually available for use in the home, and to charge the battery.
If we subtract the energy imported from the grid from the gross solar consumption, we get net solar consumption, which is the total amount of solar energy actually consumed in the home, and by the battery.
Remember that any energy required by the battery was energy that was earlier consumed in the home. In other words, energy consumed from the battery is equivalent to solar energy produced from the solar panels, but with some losses resulting from the process of charging, and subsequently discharging, the battery.
The efficiency of the solar battery system can then be defined as net consumption expressed as a percentage of net production, this being a measure of how well the system manages the household loads and the battery. Some energy is always lost in the charging/discharging/inverting process. All inverters will have a minimum threshold load below which the inverter can’t start, and it may need to go to the grid until that threshold is reached. Also there can be a small but finite delay before the inverter can ramp up to power large loads, during which time the grid may be used.
Table 9 above shows how the efficiency values were calculated, and shows quite dramatically how the excessive importation of grid energy by the Sunverge system grossly distorted the values.
The data in Columns 2 and 3 were taken directly from our AGL monthly bills. Columns 4 and 5 were downloaded in csv format from AGL’s Solar Command. Column 6 is the total solar energy available for in-house use, after allowing for the energy exported. Column 7 is the total solar energy used in-house, after allowing for peak energy imported. Column 8 is the difference between the total solar energy available and the solar energy actually used. Column 9 shows the losses in Column 8, expressed as a percentage of net production in Column 6, and Column 10 is 100% minus the losses, or the efficiency.
The very low efficiency numbers are actually a direct result of Sunverge importing so much excess grid energy, particularly at night, meaning the numbers in Column 7 are much smaller than they would have been had we not imported so much extra grid energy.
These numbers are therefore not a measure of the round turn efficiency of the Sunverge battery, but rather a sad comment on how the system was managed by Sunverge.
TESLA SOLAR PRODUCTION/CONSUMPTION EFFICIENCY
Unfortunately, neither Tesla nor AGL provide any detailed downloadable csv data for the Powerwall 2 system. However, there is a Windows app called Powerwall Companion which does facilitate downloading Tesla data. The app cannot be used other than in real-time, but by exporting the weekly data every 7 days, it is possible to gather comprehensive daily data. Unfortunately I didn’t discover this app early enough to gather daily data for November and December 2018, but I will be from January 2019 onwards.
Powerwall Companion’s csv export of Tesla’s own data for December 2018 showed the battery exported 154.63 kWh, and imported 180.10 kWh. The losses in the battery were therefore 25.47 kWh and the round turn efficiency of the battery was 85.9%. This is a small sample and could be affected by the possibility of more or less energy being stored and consumed on the first and last day of the month, but is reasonable indication nonetheless.
The Sunverge battery system turned out to have cost us quite a bit of money, rather than saving us money, and AGL and the VPP, to their credit, clearly recognised the need to offer alternatives. The early evidence suggests the Tesla Powerwall 2 system is vastly superior.
There is a strange paradox in solar energy in that it is not free. In South Australia, every kWh that we used directly during the day cost us $0.16300 in lost revenue, and every kWh we drew from the battery at night cost us $0.1811. The latter cost is $0.16300 increased by an assumed round turn loss of 10.0%, because the energy used at night has to be replaced the next day, and that energy goes through the charge/discharge/invert cycle with its attendant losses. In other words, every kWh used at night uses up 1.111 kWh of solar energy the next day to charge the battery, instead of being exported.
There are two other aspects of solar batteries worth considering; warranties and repair or service costs.
The REC solar panels we have used have a 10 year warranty and a 25 year linearity warranty. This means they are warranted not to degrade by more than 0.7% per annum, which means that after 25 years they will still perform at 84% of their original specification. Most solar batteries seem to have a 5 to 10 year warranty, but often with no linearity warranty. The Tesla Powerwall 2 is, at least, warranted to have 70% of its initial 13.5 kWh usable after 10 years. If the batteries have to be replaced after 10 years or so, this could greatly affect the economic viability of solar battery storage.
Replacement costs and repair costs for solar battery systems will clearly be much greater than for a simple solar panel array without a battery. We had two installation faults and at least three run-time failures with our Sunverge battery, so if we were responsible for the repair and maintenance costs, that could have been a significant additional cost.