More and more mobile homes are being equipped with solar systems in order to be independent of a power connection for several days. Unfortunately, the systems are often put together incorrectly, so that the hoped-for success is only inadequately achieved. The calculation is often wrong. The article here is intended to provide a remedy. First, all bills for the most commonly used shunt controller (price 20-90 euros) are made. The MPT controllers will only be discussed later (price 110-250 euros).

Consumption profile

The calculation of the requirement in Ah (ampere hours) is quite simple and can be calculated by anyone simply from the power specification of the consumer, divided by 12V and multiplied by the switch-on time. z. B. 2 halogen lamps of 10W over 3 hours consume 20W / 12V * 3h = 5Ah. This is how you do it with every consumer. The water pump 2A and 30 minutes makes 1Ah. At the end you have the daily power consumption of e.g. B. 20Ah.

Solar panel

The often used maximum power Wp (wat peak) of a panel still says too little. The solar cell is actually not a voltage source but a current source, so the adaptation of this source to the battery is the most important thing. In other words, an 80Wp panel, adjusted incorrectly, puts less power into the battery than a 50Wp, adjusted correctly.

How can that be?

Because the panel, as already said, is a power source, ie it produces electricity of e.g. B. 3A for any voltage from 0. 0V to approx.0.42V. Then the current decreases parabolically to zero at a voltage of 0.52V.

If this solar cell works at a voltage of 0.2V, it gives 3A * 0. 2V = 0. 6W
If this solar cell works at a voltage of 0.4V, it gives 3A * 0. 4V = 1. 2W

The power increases because the voltage increases, but at some point the current decreases faster than the voltage increases, the power drops to the point where the cell generates the highest voltage of 0.52V, but it does no current flows, so no line is output. The point where the product of current and voltage, i.e. the power, reaches the highest value is called the MPP most power point. In our example, this is:

Ump = 0. 48V and Imp = 2. 8A so Pmp = 1. 34Wp the highest performance.

The complete table:
If this solar cell works at a voltage of 0.2V, it gives 3A * 0. 2V = 0. 6W
If this solar cell works at a voltage of 0.4V, it gives 3A * 0. 4V = 1. 2W
If this solar cell works at a voltage of 0. 48V it gives 2. 8A * 0. 48V = 1. 34W
If this solar cell works at a voltage of 0.52V, it gives 0A * 0. 52V = 0W

Please note: a cell delivers a fairly constant current that is proportional to solar radiation and we try to set the voltage so high, but not too high, so that the greatest product of voltage and current is achieved will. That means for a solar cell actually the fairly constant current, which is a constant parameter and not the output power, as it is totally dependent on the connected voltage and is not a constant.

For each panel we have to calculate the current first. The single solar cell at 25C and solar radiation of 1000W / m2 produces the maximum output at 0.48V at 25C. Since the manufacturers specify the power at the only, best operating point in Wp, we have to calculate the Imp current from the knowledge of the Ump voltage. If our voltage is lower than the Ump (17V with 36 cells), we will get less and less power, because the current remains almost the same, but the voltage is much lower (e.g. 12v), so the power is also smaller.

Example:

An 80Wp panel with 60 cells produces 60x0. 48 = 28. 8V and 2.78A, a current of approx.2.8A flows into the 12V battery with a common shunt controller.

A 50Wp panel with 36 cells produces 36x0. 48 = 17. 28V and 2. 9A, a current of approx. 3 flows into the 12V battery. 0A with a common shunt controller.

A 60% (80Wp to 50Wp) larger panel brings less power to the battery.

In order not to make it too complicated: a solar panel for operation on a 12V battery should have 36 cells in series. A panel with 72 cells, connected in parallel as two strings of 36 cells, behaves like a 36 cell system. 2 or 3 or 4 panels with 36 cells each connected in parallel behave like a 36 cell system with 2, 3 or 4 times the power.

Actual current

It is wrong to calculate the current of a 50Wp panel as 50Wp / 12V = 4A. As shown before, it is 3. 0A with the panel perpendicular to the sunbeam. Because the sun is on May 21st, 6th, 21st, 8th, 21st 9 at noon

Berlin 58 61 58 49 38 degrees above the horizon
Stuttgart 62 65 62 55 42 degrees above horizon
Marseille 67 70 67 58 47 degrees above horizon
Palermo 72 76 72 64 52 degrees above horizon

stands,. we have to multiply the current of 3A by sin (height of the sun), so that in Stuttgart on August 21 at noon on the sun (13:30) only a current of 2.45A flows into the 12V battery. Two hours later, only 2. 16A are flowing, which means a power at 12. 8V of 27W, not a trace of 50W.

As we can see here, through the practical horizontal installation of the panel and the use of the shunt controller (95% of all controllers), we only get half of 50W.

Yield of the panel

In corresponding books it is assumed that the daily yield in Ah is about 5 times the current.

In our example 5 x 2. 45 = 12. 25Ah at 50Wp
In our example 5 x 3. 9 = 19. 5Ah at 80Wp
In our example 5 x 4. 9 = 24. 5Ah at 100Wp

MPT controller - most power tracker controller.

A shunt controller has been assumed in the calculations up to now. This is a very simple controller that feeds the solar power directly into the battery, like a switch, until the battery is full, then switches the power off, or feeds the power to a second battery, e.g. the starter battery. Is there anything else?

Yes, there is. The solar panel does not work with a shunt controller in the optimal working point Ump (U most power). This is because it is somewhere else, at around 15 to 17V, depending on the radiation and temperature.

The greater the voltage difference between the Ump and battery voltage (12V to 14.3V), the greater the gain of an MPT controller.

The MPT controller takes the greatest solar power at the best operating point, converts the power with low losses to a required voltage of 12V and thus increases the current that flows into the battery t.

MPT sample calculation

50Wp panel delivered 3A into our 12V battery with a shunt controller. The 50Wp result in Ump = 17V and Imp = 2. 95A. The 50W is converted to the new voltage of 12V with 92%, i.e. the current is 92% * 50/12 = 3. 83A, a whopping 25% more than with a shunt controller.

This high value of 25% only occurs with full irradiation and a low temperature of 25C. With every degree warmer, the Ump drops by 0.34%. If the panel is 30 degrees warmer (55C) the voltage drops from 17V to 15.3V. If the battery voltage reaches 13.8V, the voltage difference would only be 1.1V and the gain of the MPT circuit hardly worth mentioning, approx. 5%.

The average gain over different solar conditions is 10-15% in practice.

Is an MPT worthwhile compared to a shunt controller

Realistically, about 15% can be achieved through use. You have to compare the surcharge of a 15% larger panel with the surcharge of the MPT compared to a shunt controller. With small systems up to 100W it is hardly worthwhile, with larger systems like on our roofs, you would never do without an MPT. If you want to increase the yield without looking at the costs, the use of an MPT is mandatory.

Other ways of increasing the yield

It is clear to say that cleaning the solar panel, every couple of days, often brings 20-40%. A layer of dirt, such as on the windshield, blocks sunlight quite efficiently. Therefore apply the panels in such a way that cleaning should be possible.

The time-consuming installation of the panels brings, depending on the height of the sun, from a few percent (midsummer) to 60% in winter.

The complex equatorial tracking of the panel brings additional percentages, but in terms of price on the motorhome it is not competitive with an additional horizontal or inclined panel.

Of course, better equipped shunt and MPT controllers can have a display. The following are displayed: the battery voltage, the battery charge and the charging current, so that you have a good control whether the system is supplying the correct electricity or whether it is worth cleaning the solar panel.

With high-quality MPP controllers, the profit is often also displayed, in the form of solar power that would flow into the battery without the complex technology and the actual larger battery power with the expensive MPT . This display of the ´more´ is, so to speak, the soul massage for the increased price of the MPT.

Graphics:

On the two graphics I show the current (lower curves) and the lines (upper curves) depending on the voltage of the battery (X-axis).

The first graphic shows the ratios at 1000W / m2 and 25C

The second graphic shows the more practical case, lower radiation and higher temperature.

How do I get the right MPT controller?

Since we are not advertising here, everyone has to search for the terms MPT or MPPT or MPP and determine the efficiency (92 or 94 or 96%) of the Compare converters. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
lower radiation and higher temperature.

How do I get the right MPT controller?

Since we are not advertising here, everyone has to search for the terms MPT or MPPT or MPP and the efficiency (92 or 94 or 96%) of the Compare converters. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
lower radiation and higher temperature.

How do I get the right MPT controller?

Since we are not advertising here, everyone has to search for the terms MPT or MPPT or MPP and determine the efficiency (92 or 94 or 96%) of the Compare converters. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
How do I get the right MPT controller?

Since we are not advertising here, everyone has to search for the terms MPT or MPPT or MPP and compare the efficiency (92 or 94 or 96%) of the converter. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
How do I get the right MPT controller?

Since we are not advertising here, everyone has to search for the terms MPT or MPPT or MPP and compare the efficiency (92 or 94 or 96%) of the converter. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
everyone has to search for the terms MPT or MPPT or MPP and compare the efficiency (92 or 94 or 96%) of the converter. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
everyone has to search for the terms MPT or MPPT or MPP and compare the efficiency (92 or 94 or 96%) of the converter. In fairness one should assume the same conditions, e.g. conversion from 17V to 12V (5V difference) is much more difficult (lower efficiency of 92%) than the conversion from 17V to 14V (only 3V difference). the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter
the smaller the difference, the higher the actually measured efficiency of the converter. So be careful when comparing. Since there are many possible circuit concepts, the devices differ greatly in terms of their efficiency.

Greetings Peter


torcman, 2009-03-23