PV phases and own use

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A washing machine with a connected load of 2 kilowatts still consumes 1 KW of mains electricity even with a PV output of 3 KW. And a waterbed with 600 watts still draws mains electricity in the morning instead of heating purely on solar power. Even if your own PV already supplies 800 watts.

Why is that?

Most solar inverters today supply 3-phase current, matching the 3 connections (phases) usually available from electricity suppliers. This is very convenient for the electricity suppliers, but also for the inverter manufacturers: a typical 10 KW system today can deliver its power on 3 phases with significantly cheaper components than if the 10 KW had to be fed in on a single phase.
These considerations all relate to the European interconnected electricity grid with 3 phases per household. There are also other grids with 2 or 1 phase where the following description does not apply or only partially applies - see digressions.

With 10 KW on one phase, we have approx. 43 A (10,000 watts / 230 volts) of current to be supplied,
with 3 phases, however, only 8.3 A (10,000 watts / 400 volts / 3 phases). With 3 phases, the current to be supplied is also distributed much more evenly over time. The inverter has to buffer less power in its capacitors. With one phase, there are 50 moments (50 Hz alternating current) in one second in which the inverter cannot transfer any energy to the power grid (zero crossing). However, the solar panels supply the energy they can generate (or better: convert) to the inverter at all times. The inverter must therefore buffer (or ignore, i.e. not use) the incoming power during the zero crossing. With 3 phases, however, there are always 1.5 phases that can absorb energy. With the ironless inverters commonly used today, this saves an enormous amount of expensive components. The above values have been simplified to make them easier to understand.

Excursus on ironless inverters

What is that, an ironless inverter? Of course, it still has a sheet metal casing, steel screw connections and all that stuff. "Ironless" refers to the former core element: the transformer, which converted the (then often still low) direct voltage into 220/230 volts alternating current by means of an alternating circuit consisting of transistors, thyristors or other semiconductors . This circuit technology, which prevailed until the 1990s, was considered to have "no alternative" and required very few semiconductors, which were still very expensive at the time, and massive, copper-wound transformers with an ironcore, which were still inexpensive (in comparison) at the time.

These were always the performance-limiting factor back then. And thus became more and more of a cost factor: copper in particular became more and more expensive, and it was not possible to build power transformers of any size (necessary for low primary voltages). Then there was the disruptive technology of the ironless inverter, a game changer from the Frauenhofer Institute & Mr. Ivan Riegel, perfected by SMA from Kassel.Suddenly, the elimination of the limiting transformer meant that battery and solar voltages could be increased almost at will, which reduced the cost of cables and regulators enormously. Series connections became the rule. And "more power" could now simply be provided by increasingly inexpensive power semiconductors with equally inexpensive high-capacity capacitors. Today's wind and solar energy would not exist in this form without these modern inverters. Modern electric motor control, whether for rail or car drives, would be inconceivable without these inverters with practically unlimited power (simply add another transistor and another capacitor and you're done). The (technically) same inverters that today turn the electricity from wind and solar systems into grid-compatible 50/60 Hz alternating current also drive electric cars and ICEs.

But 3 KW is still 3 KW, isn't it? 

Yes no
The difference is due to the typical household consumers. These are all pure alternating current consumers. They therefore only ever load a single phase of the mains current during operation. This is not of great significance for the energy producers, as a washing machine may load the 1st phase in your home, but it is purely coincidental that it loads the 3rd phase in your neighbor's home. On a statistical average, this means that all 3 phases of the power grid are loaded quite evenly.

Excursus - Why three-phase current?

In Germany, we are used to having 400V (formerly 380V) three-phasecurrent (three-phase grid) in every household. As written: Practically 100% of our consumers (apart from instantaneous water heaters) are ACconsumers! See toasters, televisions, irons. In most countries around the world, however, households are now only supplied with alternating current, which would also prevent the disadvantages described here between 3-phase inverters and single-phase consumers from occurring in the first place. Why do so few countries have a three-phase household grid and so many have an alternating current household grid? And what about the difference between 110/115 volts and 220/230 volts? And then 50Hz or 60Hz?

Here is the first "secret": Practically all countries in the world have a three-phase grid on the distribution board (high and medium voltage). The reason is quite simple: costs! When power (current x voltage) is transported, 2 conductors are needed for the connection between the power source (generator, power station) and the consumer (consumer, your house, your washing machine) - regardless of whether it is direct or alternating voltage. Both conductors must have the same cross-section in order to transport the current on the outward and return journey.
What about "earth"? Electricity can flow through the earth back to the power station, then we only need one conductor, one cable! Yes, that's what we learned at school... or at least that's what we understood. That was and has always been largely wrong! As far as I know this technology (earth-return) is only still used in Australia , but it has the same basic problems as everywhere else in the world: the earth is a very, very poor conductor! Therefore, very high voltages must be used here, and not every area (depending on the nature of the earth!!) can be connected to such a network. Earthing conductors have to end up in very damp earth (not water or stones themselves. Damp earth!), which can make very deep constructions necessary.
Therefore, at least 2 conductors: there and back, regardless of whether direct current (which, by the way, would decompose earth conductors, electrolysis) or alternating current.

If you need more power on this line (village, town, not just your washing machine), you will soon need 2 lines next to each other = 4 conductors, because otherwise the cable cross-sections would simply be too rigid or too heavy. And even more power? Then 3 lines = 6 conductors. And so on.

And this is exactly where the "magic" of three-phase current comes into play! With three-phase current, the phases are (naturally) offset from each other so that the current flow (and thus the power transport) always goes from one main phase to the consumer... and goes back on the other two phases = other two lines/cables. Depending on the phase angle, this is somewhat simplified, but is sufficient for understanding or understanding without having to study physics. With three-phase current, the same power that would otherwise require 6 lines can be transported on three lines - this is truly magical, and halves the material costs and simplifies power lines by 50%!

What about 110/220 volts?

The basic voltage on which all power grids were based was 50-55 volts. Why? An arc lamp was once the most powerful light source there was (long before LEDs 🙂 ). And "light" was the only drive for building power grids. Electric motors, and therefore vacuum cleaners, elevators and washing machines, came much later. Arc lamps "burn" with alternating current at around 50 volts. Two arc lamps in series required 100 volts, or 110 volts for slightly more burning/ignition reliability. Current was already a major problem back then, so it was easier to operate two carbon arc lamps in series at 100-110 volts than two lamps in parallel at 50-55 volts.

However, this could not simply be transferred to 4 lamps in series = 220 volts, ignition was the problem.

Later, there was the carbon filament lamp, in which a carbon filament in a vacuum practically created a short circuit and became so hot that it glowed - but did not burn updue to the lack of oxygen. In contrast to arc lamps, which were incredibly dangerous in private hands (the arc has a temperature of approx. 10,000°, see also plasma welding!), electric light could now be used not only outside on the street, but also in the home: A gigantic market was born!
As luck would have it, 100-110 volt mains voltage was also ideally suited to these new light sources and spread very quickly. However, they still used direct current because Thomas Edison thought it would be great. The World's Fair in Chicago then acted as a game changer : Tesla with the Westinghouse was able to supply this World's Fair with energy from a distant hydroelectric plant (hear hear!) much more cheaply because he could bring the power in from afar using high voltage and low current via very inexpensive cables. However, changing the current and voltage via transformers was only possible using alternating current instead of direct current! That's why we have Tesla's alternating current everywhere today, whereas Edison's direct current would have made it much easier to supply energy through solar systems...

This meant that alternating current was now the norm. 50 Hz resulted from the trade-off that direct current was still "slow" enough for the electric motors that were emerging at the time (very simplified), but could already be transformed. Higher frequency = smaller transformers, so 60 Hz grids are cheaper - once again the price. Incidentally, this is also the reason why the railroads in Germany, for example, run at 16 2/3 Hz: Direct current would be cool, but it cannot be transformed. Not ideal from today's perspective, but back then there were only electric motors available. Inverters didn't even exist yet, and we couldn't even dream of the performance they have today.

OK, 110V/50Hz, everyone happy. But 220/230 volts?

Once again, it was the lighting that drove this change! For the first time, the new tungsten lamp was able to provide a much brighter and yet safer light than the widely used carbon fiber lamps in an inexpensive (yes, the price...) way. And: it could be operated at significantly higher voltages! Higher voltage = lower current. Lower current = cheaper cables & transformers (the "alternating current" was already the standard anyway!). And here something quite crazy happened: Many of the new energy producers in the world paid (yes!) their customers to replace their old carbon fiber lamps with the modern tungsten lamps. This meant that in Europe and Germany the new 220 volt technology supplied mainly by Siemens could be used to save costs (and also less power loss = higher efficiency). Why not in the USA? Because the spread of carbon fiber lamps was already far too advanced there! The USA was a victim of its progress, the changeover would have been too expensive.

USA = 220 Volt?

Yes, the distribution grids in the USA are indeed 220 volt grids! Every household in the USA is supplied with 220 volts. However, for historical reasons (see last paragraph), most consumers are connected to one phase against neutral in the fuse boxes (only there!). This results in the usual and familiar 110V. Everywhere in the USA you can also replace the fuse ("breaker") with a Line 2 Line Breaker, which then clamps between the two main conductors, and out comes... 220 Volt/60 Hz alternating current!

Why 50Hz/60Hz?

Almost by chance. The frequency depended on the generators & poles (= winding bundles) of the generators and their rotational speed. Siemens, as the market leader, specified 50 Hz (it could also have been 48 Hz or 56 Hz!), which then became widespread throughout the world. Westinghouse in America preferred 60 Hz. From a transformation point of view (higher frequency = more effective and smaller = cheaper transformers) 60Hz or more would be desirable, from an electric motor point of view of the time, lower frequencies (down to direct current) would be desirable. So 50Hz to 60Hz was the best compromise. Today, this would be assessed differently, but the grids are no longer interchangeable and are too closely interconnected.

Enough with digressions.

Back to the inverter

If the PV system delivers 3 KW, the inverter now feeds 1 KW per phase into the domestic grid. 3 x 1 = 3 KW. However, your washing machine now draws 2 KW from a single phase (while it is heating, normally it requires significantly less power). This means that your PV system feeds 2 KW into the grid on the other two phases, while the third phase consumes 1 KW of power from your own PV and 1 KW from the public grid. (These values are also shown here in simplified form).

This applies to any load with a power consumption of more than 1/3 of the current PV power.

If the PV in the above example were to produce 6 KW (= convert light to electricity), you would feed 4 KW into the public grid as expected, and consume 2 KW yourself without having to purchase/pay for grid electricity.

How can you increase your own use?

A) Less is more

To make better use of self-generated electricity, it is advisable to use appliances with the lowest possible maximum power consumption. Heating appliances are usually the most worthwhile for this, e.g. washing machine, dishwasher, dryer, hairdryer, stove, oven, microwave.

A dishwasher with a maximum power consumption of 1.5 kW (which it only draws when heating up) can be operated 100% without purchased mains electricity if 4.5 kW of energy is generated (regardless of whether it is combined heat and power, wind energy or solar power).

For a tumble dryer that draws up to 2 kW (but practically continuously), 6 kW of power is already required.

It will never pay off to dispose of a working washing machine and purchase a less powerful appliance instead.

But the next time you buy a new one, you can check the camping accessories catalog to see if there are alternatives. Of course, this only makes sense if the appliance in question is also used at times when self-generated power is available.

If the only hairdryer in the house is only switched on by the daughter in the evening just before the disco, i.e. at a time without self-produced photovoltaic energy, the 800 watt hairdryer will not bring any savings compared to the 2000 watt model due to the longer running time.

B) Distribute.

Here we must distinguish between temporal distribution and phase distribution.

Temporal distribution

If the washing machine is used FIRST and THEN the tumble dryer, both appliances can benefit from the self-generated electricity. If they run at the same time, their combined energy consumption will exceed the available power.
Also note this in different rooms, i.e. entertainment technology in the living room oven in the kitchen washing machine in the basement games console in the children's room.

Phase distribution

Make sure that your regular consumers in particular (refrigerator, freezer, heating, waterbed, circulation pump, router...) are distributed as evenly as possible over the 3 phases.

It is therefore very helpful if you

a) label your sockets with the respective phase number (the "real" phase is not important here, it is only a matter of assigning the loads to the 3 individual phases available) and

b) list these consumers in a list, with main operating time and power consumption.

It can be helpful if you operate the computer, which is often used during the day, on a different phase than the monitor, which is operated at the same time. And ideally the printer on the 3rd phase. In this way, a 300 watt computer system could be operated 100% from photovoltaic power even in the early morning or late evening hours.

At the beginning, concentrate on the permanent loads such as heating, (freezer) refrigerators, routers, NAS. If, for example, you evenly distribute the heating with a consumption of approx. 100 watts, the refrigerator with approx. 80 watts, the router with approx. 10 watts, etc., you can even run the permanent load devices in winter when it is cloudy from free solar power. This is much more efficient than the water boiler with 2 KW, which is only switched on twice a week for 5 minutes. With a dual waterbed, for example, it can be helpful to start the first heater at 9 in the morning when the PV system is already supplying 900 watts or more. The 2nd heater then switches on at around 12 noon. By the evening, the bed is then safely back to full operating temperature and both heaters are off at night. Another advantage: you no longer sleep on an alternating magnetic field.