Planning An Off Grid Photovoltaic System: 100% ElectricitySelf-sufficiency

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Achieving complete 100% self-sufficiency with an off grid photovoltaic system is feasible, but requires knowledge of the various adjustment screws. I explain that in detail here.

Grid photovoltaic system

First, let’s look at the Pareto principle. If you already know it, you can skip this step. But it is excellently suited to understanding the difficulty of 100% self-sufficiency.

The Pareto principle

Let’s make it short. 20% power is required for 80% of the result. In order to achieve the remaining 20% of the result, 80% power is required. At least that’s the theory.

The Pareto principle shows that a lot can be achieved with little effort. The many details to achieve everything cost time/effort/performance. Basically, this principle can be applied to all other areas of life. In order to have something perfect, it takes an enormous amount of your capacity.

If we relate this to stand-alone systems, we will probably be able to cover a good part of the electricity requirement over the year with a small storage system and a small photovoltaic system. In the dark season of November, December and January, however, we need many times more storage and, above all, PV modules. There are also other problems: The sun is lower and shines for a shorter time. On top of that, there could probably be snow.

So you can see that with a small solar powered generator we can consume a lot of electricity from it. However, 100% self-sufficiency is associated with many problems that have to be overcome. You will definitely be able to decide for yourself at the end of this article whether you still want it or whether you can do it at all.

Foreword: off grid or feeder

Before we begin, the decision to make an off grid solar system or feed-in system is probably one of the most important criteria. The design of the entire system depends on this. Feed-in systems aim to feed as much as possible into the public power grid and thus earn money. The repurchase of the electricity results in self-sufficiency on the balance sheet. In the case of stand-alone systems, it is not primarily the annual balance that is important, but how much electricity is available at what time. There is no accounting self-sufficiency here, but real self-sufficiency.

Hybrid systems produce a better mix. This can either be a zero feed-in system that does not feed into the grid but uses the existing power grid (must be registered!) or with a battery charger that can only be drawn from the public grid and is to be regarded as a consumer. But let’s start with the most important components and basic ideas of the system.

A flat module provides an excellent amount of electricity over the many hours of sunshine in summer. In winter, on the other hand, the sun is low and there may be snow on it. The largest system is of little use there.

A notice:

  • Statistically, a 1kWp (1000 watt peak) PV system collects around 1000 kWh over the year. Depending on latitude, shading, environmental and natural influences, this varies somewhat. In northern Germany near Hamburg it is between 800 and 850 kWh per kWp. In contrast, in Munich about 1050 kWh per kWp can be collected.
  • These are values over the entire year and summer is the above-average factor here. Especially with the hours of sunshine.
  • For off grid systems there is still the limiting factor of the battery size, which is absolutely necessary to achieve a high level of self-sufficiency. The maximum output power (inverter) is also linked to the battery. A small battery can only deliver a small current.

Orientation, angle and height of the photovoltaic modules

The performance of the photovoltaic solar panel modules decides everything. It is the generator of your self-sufficiency. Through direct power consumption, it mitigates conversion losses through storage. Let’s start with the most difficult topic of all.

Structures like Stonehenge show the equinoxes. These two important annual events were also carved in stone at the Pyramids of Giza. It shows the two days where day and night are of equal length. Exactly between the two days lies the longest day with the shortest night (summer solstice) and the shortest day with the longest night (winter solstice). We should focus on the latter to cover the most difficult period of self-sufficiency. Once that’s done, the rest of the year shouldn’t be a problem.

Module sky alignment

The following table shows you in which direction your system should be aligned if possible.

Since the sun only shines briefly over the southern horizon in winter, any deviation from the ideal is associated with more photovoltaic installations.

Module angle

While the sun is very high in the summer and modules mounted flat bring an enormous amount of yield here, the opposite is the case in winter. However, that is only half true. In winter, a steep module angle in the sun is excellent (resp. facade modules). The diffuse light capability of photovoltaic modules, which has greatly improved in recent years, shows the tendency that orientation and angle are no longer essential to yield. If we take the winter, it depends above all on the hours of sunshine. For the rest of the time, when there are mostly clouds, we need the brightness of the sky as an energy source. A flat system is again an advantage. What is important is the percentage of sun and clouds. In order to find an ideal dimension here, the module angle should not be too steep. Around 50° seems to be a good value. I believe that, like in finance, risk should be spread. That means “as well as” for the angle and alignment of solar modules. So there doesn’t seem to be a silver bullet here.

Module height

In the case of advice and contributions from photovoltaic companies, one factor is often not mentioned. The module height. In other words, at what height the PV module finds its permanent place. Not all PV systems are mounted on a house roof. But they should if it is the best place to collect electricity.

In winter, a low installed module is of no use if there are a lot of shadows from houses and trees.

Anyone can easily check this for themselves at any time of the year: look at the sunrise and sunset. Look at your house or the houses around you and pay attention to the shadow. And this is exactly where no photovoltaic modules should be placed or attached.

Power storage: Battery with sufficient dimensions

In addition to the photovoltaic modules, the second most important point for a possible 100% self-sufficiency is the storage. On the one hand, this compensates for fluctuations in the PV modules. On the other hand, it stores the excess electricity produced so that it can be made available when there is no sun or the output of the modules is not sufficient. Solar power box may be able to help you out of a lot of trouble.

Most storage systems are designed for overnight power consumption. This is the most cost-effective way to use storage in a financially sustainable manner. It is assumed that the storage tank is full by nightfall and can be completely filled again the next day.

This usually works with small memories without any problems.

However, 100% self-sufficiency means that our storage facility must also guarantee 100% security of supply. Even if the sun doesn’t show up for two weeks. Living on the battery alone for two weeks should probably not be the goal and is also not necessary. The photovoltaic system has to do a good job here so that self-consumption can be covered during the day and the storage tank can still be sufficiently charged.

Let’s assume that your modules provide the necessary electricity to charge a large part of the storage even when there are clouds and to cover the electricity requirements of the day. So it needs a memory size with fewer self-sufficiency days. Only the long nights during the winter solstice need to be covered (+/- 2 weeks at least).

There will continue to be days with extremely dark clouds, heavy rain and thunderstorms. So light snow is less of a problem here.

We should now find out how many such days there are in a row. The season is relatively unimportant. A weather record: Thuringian Forest with 242 hours (approx. 10 days) of fog in 1996. But these are rare and isolated events. On the other hand, 7 days in a row rain with dark clouds is more common. So dark that the PV system only manages a few watt hours.

If there is only the possibility to use electricity from the off grid photovoltaic system, creativity is required here. If the memory is limited, the power consumption should be massively throttled. Since neither a petrol/diesel generator nor gas is available in this post, we need a different solution for this case. There is a separate chapter on this later in the post.

Storage technology

We are struggling with conversion losses in all areas. The modules have to deliver enough power to serve direct consumption and store the surplus. One should note here all losses of the system itself! The amount of electricity that comes in is not the same as that that can be extracted or used. The first loss is the photovoltaic module itself. Partial shading and bypass diodes, as well as pollution in short. The current continues via the first cable to the charge controller. Low voltage, high current, long and thin cables contribute to the first significant losses.

  • The charge controller provides the necessary current to the direct consumers (DC/AC, inverter). The efficiency of the charge controller depends on its type, regulation and voltage differences.
  • And inverters also have their own consumption when idling. This electricity cannot be used for anything else.
  • The current continues to flow into the marine battery. Depending on the storage technology, the losses from storage AND power consumption can vary significantly. For lead storage, the Peukert equation and temperature coefficient should be mentioned here.

One should refrain from so-called AC storage. Here, the direct current of the photovoltaic modules is first converted into alternating current in order to then convert it back into direct current for the battery. Luckily, these are not common in off grid solar systems.

And then there’s the big issue of system voltage, which I’ve already covered in a separate post. The system voltage is often decisive and another permanently high loss factor.

Calculation bases of the storage and the PV modules

So far there have been few numbers. Now we want to calculate and make an assumption. Examples are primarily for you to derive.

The following scenario:

daily power requirement of 2.5 kWh

Electricity requirement during the day 70% (6 a.m. to 6 p.m.), at night 30% (6 p.m. to 6 a.m.)

Although we only have 30% power requirement at night, the limiting factor is the brief brightness in winter. If we assume 6 hours of brightness, the memory must be filled and the power requirement covered during this time.

Calculate PV power

2.5 kWh : 6 hours = 417 W PV power

The wrong assumption would now be that there is full sunshine in these 6 hours. If we assume cloudy days, the PV system only produces 10% – 30% of its output. Let’s calculate with the average of 20%, because that is often the norm in winter, but the result is quite different. Because we need 5 times the power

417 Wp x 5 = 2085 Wp

A 2 kWp system for just 2.5 kWh of electricity per day shows the difficulty and questions the sense. In summer with 12 hours of sunshine (12x2kWp) over 24kWh can be generated per day. That is almost 10 times the daily electricity requirement and is not available to anyone due to the closed off-grid system. The performance of the PV modules is therefore only minimally utilized in summer. More on that in the conclusion below.

Calculate memory

If we assume that we have built such an oversized system, the memory can be very small.

24h – 6h = 18h

18 hours must be covered from memory.

2.5kWh : 24h x 18h = 1.875 kWh storage capacity

A storage unit with a net capacity of at least 1.875 kWh is required.

If we take the next usual storage size of 2.4kWh (gross), we should be able to cover most of the electricity requirements all year round, we strongly suggest you can try Renogy solar powered calculators tool to easily get your power needs.

This is only the half truth

As mentioned above, there are really gray days that are often continuous in winter. In order to ensure 100% supply here, the system must be at least three times as large. So around 6kWp.

Conclusion: 100% necessary or wishful thinking?

The daily power requirement of 2.5kWh as in the example does not come from anywhere, but this is my daily consumption. For this I built a 2.5kWp system + 7kWh LiFePo4 battery. My assumption was that I could cover 80-90% of my consumption. That works fine too. On many winter days, however, the system is not sufficient because the storage tank cannot cover several days and is often only slightly recharged. That’s just enough for the fridge and some light. In the extremely bad year 2021, almost nothing worked from the end of November. Only small consumers could be covered. This shows that there can be extreme fluctuations every year.

The difficulty lies in the last percent, which would require a multiple of further expansion. The Pareto principle was precisely why the introduction to this article.

Here one should question the necessity.

Stand-alone systems are a closed system and the large amount of electricity in summer is not available for the energy transition. An off grid system should therefore only be built if the public grid cannot be used. Garage and garden shed should be mentioned here. In my opinion, expanding the entire house with it is very selfish. Mostly out of frustration due to the official requirements, laws, the network operator, landlord or a general reluctance to be self-employed as an electricity seller. Everyone is welcome to decide for themselves and find a healthy balance. The off grid system itself clearly has its limits. And they are often not due to the budget or the limit of the construction area. For the summer, however, you can think about how you can give the electricity away in a sensible way.