Photovoltaic Generator and Heat Pump: Daily Power Generation and Consumption

You can generate electrical power at home but you cannot manufacture your own natural gas, oil, or wood. (I exempt the minority of people owning forestry). This is often an argument for the combination of heat pump and photovoltaic generator.

Last year I blogged in detail about economics of solar power and batteries and on typical power consumption and usage patterns – and my obsession with tracking down every sucker for electrical energy. Bottom line: Despite related tinkering with control and my own ‘user behaviour’ it is hard to raise self-consumption and self-sufficiency above statistical averages for homes without heat pumps.

In this post I will focus on load profiles and power generation during several selected days to illustrate these points, comparing…

  • electrical power provided by the PV generator (logged at Fronius Symo inverter).
  • input power needed by the heat pump (logged with energy meter connected to our control unit).
  • … power balanced provided by the smart meter: Power is considered positive when fed into the grid is counted  (This meter is installed directly behind the utility’s meter)

A non-modulating, typical brine-water heat pump is always operating at full rated power: We have a 7kW heat pump – 7kW is about the design heat load of the building, as worst case estimate for the coldest day in years. On the coldest day in the last winter the heat pump was on 75% of the time.

Given a typical performance factor of 4 kWh/kWh), the heat pump needs 1/4 of its rated power as input. Thus the PV generator needs to provide about 1-2 kW when the heat pump is on. The rated power of our 18 panels is about 5kW – this is the output under optimum conditions.

Best result near winter solstice

If it is perfectly sunny in winter, the generator can produce enough energy to power the heat pump between 10:00 and 14:00 in the best case.

2015-12-31: Photovoltaics and Power Consumption, Heat Pump's Compressor

But such cloudless days are rare, and in the cold and long nights considerable electrical energy is needed, too.

Too much energy in summer

On a perfect summer day hot water could even be heated twice a day by solar power:

2015-07-01: Photovoltaics and Power Consumption, Heat Pump's Compressor

These peaks look more impressive than they are compared to the base load: The heat pump needs only 1-2kWh per day compared to 10-11kWh total consumption.

Harvesting energy in spring

On a sunny day in spring the PV output is higher than in summer due to lower ambient temperatures. As we still need space heating energy this energy can also be utilized better:

2016-04-29: Photovoltaics and Power Consumption, Heat Pump's Compressor

The heat pump’s input power is similar to the power of a water heater or an electrical stoves. At noon on a perfect day both the heat pump and one appliance could be run on solar power only.

The typical day: Bad timing

On typical days clouds pass and power output changes quickly. This is an example of a day when sunshine and hot water cycle did not overlap much:

2016-03-29: Photovoltaics and Power Consumption, Heat Pump's Compressor

At noon the negative peak (power consumption, blue) was about 3,5kW. Obviously craving coffee or tea was string than the obsession with energy efficiency. Even the smartest control system would not be able to predict such peaks in both solar radiation and in erratic user behavior. Therefore I am also a bit sceptical when it comes to triggering the heat pump’s heating cycle by a signal from the PV generator, based on current and ‘expected’ sunshine and weather data from internet services (unless you track individual clouds).

Half a Year of Solar Power and Smart Metering

Our PV generator and new metering setup is now operational for half a year; this is my next wall of figures. For the first time I am combining data from all our loggers (PV inverter, smart meter for consumption, and heat pump system’s monitoring), and I give a summary on our scrutinizing the building’s electrical power base load.

For comparison: These are data for Eastern Austria (in sunny Burgenland). Our PV generator has 4.77kWp, 10 panels oriented south-east and 8 south-west. Typical yearly energy production in our place, about 48° latitude: ~ 5.300 kWh. In the first 6 months – May to November 2015 – we harvested about 4.000kWh.
Our house (private home and office) meets the statistical average of an Austrian private home, that is about 3.500 kWh/year for appliances (excl. heating, and cooling is negligible here). We heat with a heat pump and need about 7.200kWh electrical energy per year in total.

In the following plots daily and monthly energy balances are presented in three ways:

  1. Total consumption of the building as the sum of the PV energy used immediately, and the energy from the utility.
  2. The same total consumption as the sum of the heat pump compressor’s input energy and the remaining energy for appliances, computers, control etc.
  3. Total energy generated by PV panels as the sum of energy used (same amount as contributing to 1) and the energy sold to the utility.

Monthly energy balances: PV generation and consumption, May-Nov 2015

Monthly electrical energy consumption - heat pump and appliances, May-Nov 2015

In summer there is more PV  energy available than needed and – even with a battery – the rest would needed to be fed into the grid. In October, heating season starts and more energy is needed by the heat pump that can be provided by solar energy.

This is maybe demonstrated best by comparing the self-sufficiency quota (ratio of PV energy and energy consumed) and the self-consumption quota (ratio of PV energy consumed and PV production). Number ‘flip’ in October:


In November we had some unusually hot record-breaking days while the weather became more typical at the end of the month:


This is reflected in energy consumption: November 10 was nearly like a summer day, when the heat pump only had to heat hot water, but on the colder day it needed about 20kWh (resulting in 80-100kWh heating energy).

Daily energy balances: PV generation and consumption, Nov 2015

Daily electrical energy consumption - heat pump and appliances, Nov 2015

In July, we had the chance to measure what the building without life-forms needs per day – the absolute minimum baseline. On July 10, 11, and 12 we were away and about 4kWh were consumed per day160W on average.

Daily energy balances: PV generation and consumption, July 2015

Note that the 4kWh baseline is 2-3 times the energy the heat pump’s compressor needs for hot water heating every day:

Daily electrical energy consumption - heat pump and appliances, July 2015

We catalogued all devices, googled for data sheets and measured power consumption, flipped a lot of switches, and watched the smart meter tracking the current consumption of each device.


Consumption minus production: Current values when I started to write this post, the sun was about to set. In order to measure the consumption of individual devices they have been switched an of off one after the other, after sunset.

We abandoned some gadgets and re-considered usage. But in this post I want to focus on the base load only – on all devices that contribute to the 160W baseline.

As we know from quantum physics, the observing changes the result of the measurement. It was not a surprise that the devices used for measuring, monitoring and metering plus required IT infrastructure make up the main part of the base load.

Control & IT base load – 79W

  • Network infrastructure, telephone, and data loggers – 35W: Internet provider’s DSL modem / router, our router + WLAN access point, switch, ISDN phone network termination, data loggers / ethernet gateways for our control unit, Uninterruptible Power Supply (UPS).
  • Control and monitoring unit for the heat pump system, controlling various valves and pumps: 12W.
  • The heat pump’s internal control: 10W
  • Three different power meters: 22W: 1) Siemens smart meter of the utility, 2) our own smart meter with data logger and WLAN, 3) dumb meter for overall electrical input energy of the heat pump (compressor plus auxiliary energy). The latter needs 8W despite its dumbness.

Other household base load – 39W

Electrical toothbrush, at least no bluetooth.

  • Unobtrusive small gadgets – 12W: Electrical toothbrush, motion detectors, door bell, water softener, that obnoxious clock at the stove which is always wrong and can’t be turned off either, standby energy of microwave oven and of the PV generator’s inverter.
  • Refrigerator – 27W: 0,65 kWh per day.

Non-essential IT networking infrastructure – 10W

  • WLAN access point and router for the base floor – for connecting the PV inverter and the smart meter and providing WLAN to all rooms.

These are not required 24/7; you don’t lose data by turning them off. Remembering to turn off daily might be a challenge:

Boldly going where no one has gone before!

Non-24/7 office devices – 21W. Now turned off with a flip switch every evening, and only turned on when needed.

  • Phones and headsets: 9W.
  • Scanner/Printer/Fax: 8W. Surprisingly, there was no difference between ‘standby’ and ‘turned off’ using the soft button – it always needs 8W unless you really disconnect it.
  • Server in hibernated state 4W. Note that it took a small hack of the operating system already to hibernate the server operating system at all. Years ago the server was on 24/7 and its energy consumption amounted to 500kWh a year.

Stuff retired after this ‘project’ – 16W.

  • Radio alarm clock – 5W. Most useless consumption of energy ever. But this post is not meant as bragging about the smartest use of energy ever, but about providing a realistic account backed up by data.
  • Test and backup devices – 7W. Backup notebooks, charging all the time, backup router for playground subnet not really required 24/7, timer switch most likely needing more energy than it saved by switching something else.
  • Second old Uninterruptable Power Supply – 4W. used for one connected device only, in addition to the main one. It was purchased in the last century when peculiarities of the local power grid had rebooted  computers every day at 4:00 PM.

Historical UPS from last century

In total, we were able to reduce the base load by about 40W, 25% of the original value. This does not sound much – it is equivalent to a small light bulb. But on the other hand, it amounts to 350kWh / year, that is 10% of the yearly energy consumption!


Logging setup:

  • Temperature / compressor’s electrical power: Universal control UVR1611 and C.M.I. as data logger, logging interval 90 seconds. Temperature sensor: PT1000. Power meter:  CAN Energy Meter. Log files are exported daily to CSV files using Winsol. Logging interval: 90 seconds.
  • PV output power: Datamanager 2.0 included with PV inverter Fronius Symo 4.5-3-M, logging interval 5 minutes.
  • Consumed energy: Smart meter EM-210, logging interval 15 minutes.
  • CSV log files are imported into Microsoft SQL Server 2014 for analysis and consolidation. Plots are created with Microsoft Excel as front end to SQL Server, from daily and monthly views on joined UVR1611 / Fronius Symo / EM-210 tables.

Solar Power: Some Data for the First Month.

On May 4, 2015, we started up our photovoltaic generator. Here are some numbers and plots for the first month – and what I plan to do next.

Our generator has a rated power of 4,77 kWp (kilowatt peak), one module has 265 Wp. The generator would deliver 4,77 kW of electrical power under so-called standard testing conditions: An irradiance of 1000 W/m2 of light from the sun, a module temperature of 25%, and a standard spectrum of wavelengths determined by the thickness of the atmosphere light has to traverse (Air mass – AM 1,5, equivalent to sunlight hitting the earth at an angle of about 48° from the zenith).

Our 18 panels are mounted on two different roof areas, 10 of them (2,65 kWp) oriented south-east and 8 modules (2,12 kWp) south-west. The inclination relative to the surface of the earth is 30°, the optimum angle for PV at our latitude:

Plan of our house with PV modules.

Positions of our PV panels on the roof.

We aimed at using our 30° upper roof spaces most efficiently while staying below the ‘legal threshold’ of 5 kW, avoiding a more complicated procedure for obtaining a permit to install them.

The standard conditions are typically met in spring here – not in summer – as the efficiency of solar panels gets worse with increasing temperature: for our panels -0,44% of rated power per °C in temperature difference. If the temperature is 60°C, peak power (for otherwise same irradiance and spectrum) would drop by 15% . We can already see this effect, when comparing two nearly cloudless days in May and in June. The peak power is lower in the first days of June when maximum daily air temperatures were already about 30°C:

PV power over time, for a day in May versus a day in June

Total output power (AC) of the PV generator and input power (DC) for each string as a function of time for two days. 1) May 11 – maximum ambient air temperature 23°C. 2) June 5 – maximum ambient air temperature 30,5°C.

The temperature-dependence of performance might in part explain impressive spikes in power you see after clouds have passed: The modules have a chance to cool off, and immediately after the cloud has gone away the output power is then much higher than in case of constant irradiance. Here is a typical example of very volatile output:

PV power over time, data points taken every few seconds.

Output power of our PV generator when clouds are passing. The spikes (clear sky) show a peak power much higher than the constant value on a cloudless day in May; the troughs correspond to clouds shadowing the panels. The data logger included with the inverters only logs a data point every 5 minutes, so I parsed the inverter’s website instead to grab the current power displayed there every second (Using the inverter’s Modbus TCP interface would be the more professional solution, but parsing HTTP after reverse engineering the HTML structure is usually a quick and dirty ‘universal logging interface’.)

The maximum intermittent power here was about 4,4 kW!

Another explanation for the difference is local ‘focussing’ of radiation by specific configuration of clouds reflecting more radiation into one direction: Consider a cloudless region surrounded by clouds – a hole in the clouds so to speak. Then radiation from above might be reflected at the edges of that hole at a very shallow angle, so that at some place in the sunny spot below the power might be higher than if there were no clouds at all. Here is another article about this phenomenon.

A PV expert told me that awareness of this effect made recommendations for sizing the inverter change: From using one with a maximum power about 20% lower than the generator’s power a few years ago (as you hardly ever reach the rated power level with constant radiation) to one with matches the PV peak output better.

The figures from May 11 and June 5  also show that the total power is distributed more evenly throughout the day as if we would have had a ‘perfect’ roof oriented to the south. In the latter case the total energy output in a year would be higher, but we would not be able to consume as much power directly. But every kWh we can use immediately is worth 3 times a kWh we have to sell to the utility.

The next step is to monitor the power we consume in the house with the same time resolution, in order to shift more loads to the sunny hours or to identify some suckers for energy. We use more than 7000 kWh per year; more than half of that is the heat pump’s input energy. Our remaining usage is below the statistical average in Austria (3700 kWh per 2-person household) as we already did detective work with simpler devices.

Smart meters are to be rolled out in Austria in the next years, by 2020 95% of utilities’ clients should be equipped with them. These devices measure energy consumption in 15-minute intervals; they send the data to the utility daily (which runs a web portal where clients can access their data) but must also have a local interface for real-time logging given to clients on request. As a freshly minted owner of a PV generator I got a new ‘smart’ meter by the utility; but this device is just a temporary solution, not connected to the utility’s central system. It will be replaced by a meter from another vendor in a few years. Actually, in the past years we could read off the old analogue Ferraris meter and submit the number at the utility’s website. This new dumb smart meter, in contrast, requires somebody to visit us and read off the stored data once a year again, using its infrared interface.

I did some research on all possible options we have to measure the power we consume, the winner was another smart meter plus integrated data logger and WLAN and LAN interfaces. It has been installed yesterday ‘behind’ the official meter:

Our power meters in the distribution cabinet

Our power distribution cabinet. The official (Siemens) smart meter is the rather large box to the left; our own smart meter with integrated data logger is is the small black one above it – the one with the wireless LAN antenna.

We will combine its data with the logging of ‘PV energy harvested’ provided by the inverter of the PV panels – an inverter we picked also because of the wealth of options and protocols for accessing it [*]

For the first month we can just have a look at daily energy balances from two perspectives (reading off the display of the dumb smart meter manually every day):

  1. The energy needed by appliances in the house and for hot water heating by the heat pump – 11 kWh per day: On average 56,5% in the first month come from the solar panels (self-sufficiency quota), and the rest was provided by the grid.
  2. The daily energy output of the solar generator was 23 kWh per day on average – either consumed in the house – this is the same cyan bar as in (1) – or fed into the grid. In this month we consumed 27% of the PV power directly (self-consumption quota).
Daily energy balance: 1) The energy we consume in the house - partly from PV, partly from the grid and 2) The energy harvested by the PV generator - party used directly, partly fed into the grid.

Daily energy balance: 1) The energy we consume in the house – partly from PV, partly from the grid (left axis) and 2) The energy harvested by the PV generator – party used directly, partly fed into the grid (right axis).


[*] For German-speaking readers: I wrote a summary about different solutions for metering and logging in this case in this German article called ‘The Art of Metering’ – options are to use the official meter’s IR interface with yet another monitoring ‘server’, your own unrelated meter (as we did), a smart meter integrated with the inverter and using the inverter’s own data logging capabilities), or building and programming your own smart meter from scratch.

Cyber Security Satire?

I am a science fiction fan. In particular, I am a fan of movies featuring Those Lonesome Nerds who are capable of controlling this planet’s critical infrastructure – from their gloomy basements.

But is it science fiction? In the year Die Hard 4.0 has been released a classified video has been recorded – showing an electrical generator dying from a cyber attack.

Fortunately, “Aurora” was just a test attack against a replica of a power plant:

Now some of you know that the Subversive El(k)ement calls herself a Dilettante Science Blogger on Twitter.

But here is an epic story to be unearthed, and it would take a novelist to do that. I can imagine the long-winded narrative unfolding – of people who cannot use their showers or toilets any more after the blackout. Of sinister hackers sending their evil commands into the command centers of the intricate blood circulation of our society we call The Power Grid. Of course they use smart meters to start their attack.

Unfortunately my feeble attempts of tipping my toes into novel writing have been crashed before I even got started: This novel does exist already – in German. I will inform you if is has been translated – either to a novel or directly into a Hollywood movie script.

As I am probably not capable of writing a serious thriller anyway I would rather go for dark satire.

Douglas Adams did cover so many technologies in The Hitchhiker’s Guide the Galaxy – existing and imagined ones – but he did not elaborate much on intergalactic power transmission. So here is room for satire.

What if our Most Critical Infrastructure would not be attacked by sinister hacker nerds but by our smart systems’ smartness dumbness? (Or their operators’.)

(To all you silent readers and idea grabbers out there: Don’t underestimate the cyber technology I had built into that mostly harmless blog: I know all of you who are reading this and if you are going to exploit this idea on behalf of me I will time-travel back and forth and ruin your online reputation.)

That being said I start crafting the plot:

As Adams probably drew his inspiration from his encounters with corporations and bureaucracy when describing the Vogons and InfiniDim enterprises I will extrapolate my cyber security nightmare from an anecdote:

Consider a programmer (a geek. Sorry for the redundant information!) trying to test his code. (Sorry for the gender stereotype. As a geekess I am allowed to do this. It could be female geek also!)

The geek’s code should send messages to other computers in a Windows domain. “Domain” is a technical term, not some geeky reference to Dominion or the like.  He is using net send. Generation Y-ers and other tablet and smartphone freak: This is like social media status message junk lacking images.

But our geek protagonist makes a small mistake: He does not send the test command to his test computer only – but to “EUROPE”. This does nearly refer to the whole continent, actually it addresses all computers in all European subsidiaries of a true Virtual Cyber Empire.

Fortunately modern IT networks are built on nearly AI powered devices called switches which make the cyber attack petering out at the borders of That Large City.

How could we turn this into a story about an attack on the power grid, adding your typical ignorant non-tech sensationalist writer’s cliched ideas:

  1. A humanoid life-form (or flawed android that tests his emotions chip) is tinkering with sort of a Hello World! command – sent to The Whole World literally.
  2. The attack that is just a glitch, an unfortunate concatenation of events, that is been launched in an unrelated part of the cyber space. E.g. by a command displayed on a hacker’s screen in a Youtube video. Or it was launched from the gas grid.
  3. The Command of Death spreads pandemically over the continent, replicating itself more efficiently than cute cat videos on social networks.
Circuit Breaker 115 kV

Any pop-sci article related to the power grid need to show-off some infrastructure like that (Circuit Breaker, Wikimedia)

I contacted my agent immediately.

Shattering my enthusiasm she told me:

This is not science-fiction – this is simply boring. Something like that happened recently in a small country in the middle of Europe.

According to this country’s news a major power blackout had barely been avoided in May 2013. Engineers needed to control the delicate balance of power supply and demand manually as the power grid’s control system has been flooded with gibberish – data that could not be interpreted.

The alleged originator of these commands was a gas transmission system operator in the neighboring country. This company tested a new control system and tried to poll all of its meters for a status update.  Somehow the command found its way from the gas grid to the European power grid and has been replicated.


Update –  Bonus material – making of: For the first time I felt the need to tell this story twice – in German and in English. This is not a translation, rather different versions in parallel universes. German-speaking readers – this is the German instance of the post.