Planning aid for a photovoltaic system

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Updated – June 2, 2024

Setting up a photovoltaic system is not rocket science, but it does require a certain amount of orientation, which this planning aid would like to provide here in the context of an exemplary PV system.

For the legal regulations governing the installation and commissioning of PV systems, see here. PV systems that are operated on the public power grid and generate more than 600 W of power must be approved by an approved electrical company. This usually also takes care of the registration procedure.

Wishlist

The system should generate around 2 kW of power, still offer an expansion reserve of 4 kW, consist of as few modules as possible, supply electricity even in the event of a power failure, otherwise minimize the chargeable electricity consumption on sunny days and be mounted on a gable roof facing south.

Once this wish list has been determined, it's time to select the components. A complete parts list is listed at the end of this article.

PV modules

The first thing to take into account here is the official mold: in Germany, modules with a glass cover area of up to 2 square meters are only permitted for roof installation. Larger modules are only approved for open-space installation. And no, it is not the actual “glass” cover area that counts, but rather, how could it be otherwise, the external dimensions of the module, i.e. including the frame...

Can be read in the MVV BT, there under B 3.2.1.25 and B 3.2.1.27.

This means that 450 W modules are usually the upper limit. If you want to stick to the 2 kW power limit, you will install 6 x 330 W and thus achieve 1,980 W. Taking into account the efficiency of 19.7 %, the choice falls on a module from Heckert Solar NEMO 2.0 60M, with dimensions of 1,670 x 1,006 mm at around 400 euros. For 6 pieces, the total is around 2,400 euros.

Parallel / series connection

Test question: why 6 x 330 W = 1,980 W and not 5 x 400 W = exactly 2,000 W?

Well, a look at the specifications of the planned inverter reveals that it requires at least 64 V DC no-load voltage to work reliably, and a maximum of 145 V DC.

The 330 W modules generate an idle voltage of 41.1 V. Three of these connected in series results in 41.1 + 41.1 + 41.1 = 123.3 V DC at 990 W.

If you connect a second series of three modules in parallel, the result is – still – 123.3 V, but now 2 x 990 W = 1,980 W.

Parallel connections should always be carried out with the same voltage and therefore the same number of modules. Here the limit value is the (recommended) maximum nominal power of the inverter of 4,800 W. You could therefore connect a maximum of 14 modules in parallel, but then you would not reach the starting voltage, the minimum no-load voltage of 64 V.
Alternatively, a parallel connection of 7 modules and a series connection of a second parallel connection comprising 7 modules would be conceivable. These then result in 7 x 330 W = 2,310 W x 2 = 4,620 W at 2 x 41.1 V = 82.2 V, which means that all specifications of the inverter are met.

The series connection, on the other hand, has the maximum no-load voltage of the inverter as the upper voltage limit, here 145 V. Since you shouldn't push anything to the limit, but rather stick to the recommended voltage value, which is given here as 115 V, 123.3 V is good in the race.

Mounting material

Given a module width of 1,006 mm, the required mounting rail length is 6 x 1,006 = 6,036 mm, plus 5 x 12 mm module spacing ( = 60 mm) for the middle clamps and 2 x 10 mm ( = 20 mm) for the end clamps, a total of 6,116 mm.

To assemble the 1,670 mm long modules, three rails each 4,400 mm long, one of which should be cut to around 2000 mm, should be used.

10 middle clamps are required and 4 end clamps are required. Screws for fastening 14 pieces. Profile connector 2 pieces.

A selection of the different products from Schletter, one of the leading providers of PV mounting systems.

Material for roof installation

The mounting rails are held on the battens (the ones on which the roof battens are attached, i.e. those that run perpendicular to the eaves) using roof hooks.

Although roof hooks do not necessarily represent the smallest part of the cost, they should be given special attention: the entire investment depends on them, in the truest sense of the word!

Roof hooks are available for a wide variety of roof tile shapes. Therefore, it is important to first determine the pan that will be installed and then select the roof hooks. One roof hook should be provided for each mounting rail for each setting batten. If you have three mounting rails, you need to calculate three times the number of roof hooks.

Each roof hook should be fastened to the batten with at least two screws. This results in twice the number of screws in relation to the number of roof hooks.

Each pan must be carefully provided with a recess for the support hook using a cut-off grinder. There should be some play left and right. Likewise, the hook should not rest directly on the lower pan in order to prevent the (older) pan from breaking due to wind pressure or snow load. This means there should be some play above the hook towards the underside of the pan.

A recess that is too generous should be avoided so as not to effectively provide entry for wasps, other vermin or even gusts of wind.

The installation of the roof hooks is here described in detail.

Inverter

A hybrid inverter is required as an inverter, e.g Steca Solarix PLI 5000-48 at 899 euros. Steca is another trademark of KATEC, which also owns the brand Costal and stands for longevity and reliability.

It has a nominal power of 4,800 W, to be protected with a circuit breaker of 40 A, a nominal battery voltage of 48 V, with a maximum charging current via PV and alternating voltage of 140 A, nominally 80 A. The AC connection is 6 .. 12 mm², DC side at least 35 mm² (better 50 mm²) required.

The wall is attached using three 5 mm screws not included in the delivery are.

When choosing the installation location, permanent accessibility should be ensured, as well as the shortest possible cable path for the AC connection, especially(!) the connection to the batteries, in order to prevent unnecessary losses.

A LAN cable connection to the router or switch for configuration and diagnostic purposes should also be considered.

Laying the three cable routes (AC, DC, LAN) in separate ones installation pipes suitable diameter makes sense. The cables must under no circumstances run in a cable duct.

Batteries

The choice of battery type depends primarily on the willingness to invest: LiFePo4 batteries cost two to three times as much as conventional solar AGM batteries and only weigh about a third, which is, however, insignificant when installed stationary. In a motorhome, however, every kilo saved counts.

Rough difference between LiFePo4 and AGM, apart from the energy source used and weight: LiFePo4 batteries maintain their voltage almost the same throughout the entire discharge period, while AGM batteries have a faster voltage drop.

Differences in quality have a particular impact on the number of guaranteed discharge cycles and the guaranteed capacity still available after they have been reached.

The quality is similar with AGM batteries, but here the differences are rather marginal if you prefer well-known manufacturers. Experience and quality depend on each other and therefore also the price.

By the way: you usually calculate with kW when it comes to power. The battery performance in kWh is calculated from the battery capacity in Ah multiplied by the voltage in V. A 250 Ah battery with 13.6 V therefore delivers 250 x 13.6 = 3,600 Wh, or 3.6 kWh.

This provides a good overview of LiFePo4 batteries website, a recommended AGM battery is e.g this.

When connecting the batteries, always use very high currents (140 A and more are not uncommon) with regard to the cable cross-sections to be used (50 mm² and more) note! The linked table lists AC and DC currents because the cable load is different for AC and DC.

Calculation of cable cross-section

The necessary cross-section of copper cables for DC applications can be calculated here, as can the voltage drop. The yellow fields are editable:

parts list

Electrical installation

• Hybrid inverter Steca Solarix PLI 5000-48
• ? x Batteries 12 V 260 .. 280 Ah (AGM / LiFePo4)

• ? m AC cable 3 x 6 mm²
• 3 x tubular cable lugs 6 mm²
• DC cable (red / black) 35 mm²
• 2 x ring cable lugs 35 mm² M6
• Circuit breaker 40A (e.g. ABB S201B40 S201-B40)
• optional: FI switch 40A (ABB F202A-40/0.03 2-pole type A)
• 3x installation pipes (plus number of pieces depending on the total length required)
• Holder for installation pipes
• ? x Torx screws 4 x 40 mm / dowels for holders of the installation pipes
• 3 x Torx screws 5 x 60 mm / dowels for inverter
• ? m DC cable 35 mm² or larger
• ? x Ring cable lugs / pole connectors for battery poles
• ? x Fuse holder for ANL high-current fuses
• identical number of ANL high-current fuses
• optional: battery disconnect switch (note max. switching capacity)

PV modules and cables

• 6 x 330W NEMO 2.0 60M
• 2 x Y connectors for parallel connection of the 3 modules connected in series
• 1 x required cable length from the plus connection of the first module to the Y connector (e.g. red +)
• 1 x required cable length from the minus connection of the third module to the Y connector (e.g. black -) - the serial connection between plus and minus of modules 1, 2 and 3 are established via the cable connectors located on the modules
• 1 x required cable length from the plus connection of the sixth module to the Y connector (e.g. red +)
• 1 x required cable length from the negative connection of the fourth module to the Y connector (e.g. black -) - the serial connections between plus and minus of modules 4.5 and 6 are made via the cable connectors located on the modules
• 1 x required cable length from the central Y-connector outlet to the inverter (e.g. red +)
• 1 x required cable length from the central Y-connector outlet to the inverter (e.g. black -)

Roof mounting material

• 3 x Schletter mounting rail Eco05 4,400 mm
• ? x Roof hooks – in accordance with existing battens and roof tile design
• 2 x number of roof hooks - screws for fastening in the batten

Tool

• Crimping pliers for tubular cable lugs up to 10 mm²
• Crimping pliers (with long lever or hydraulic) for ring cable lugs up to 50 mm²
• Phillips screwdriver
• Drill / cordless screwdriver
• (Stone) drill bits/screwdriver bits
• Ratchet set / torque wrench 1 .. 3 Nm
• Cutting grinder / cutting disc for concrete
• Cable drum with suitable cable length

Hints

The parts list does not claim to be complete and merely provides guidance regarding the materials and tools required. All components may need to be adapted or supplemented with additional items depending on the actual requirements.

All electrical installation work must be carried out by qualified personnel in compliance with applicable regulations. When working on electrical installations, they must always be kept safely disconnected from the power supply!
To inform uninvolved third parties, appropriate information signs must be attached to the shutdown devices during the work.

ps If you need personal support in the implementation for a fee, you are welcome to Ticketing make!