Off-grid PV system

Container Home ERS SouthBuying of solar panels can be accessed by a description of the choice in purchasing, and by explanation the most important purchase information. Customer has to be introduced in all possibilities of his future solar system. There are two choices in front of him:

1.  to buy solar panels and meet the needs of its total or partial, it is common for off-grid and hybrid systems, or

2.  to obtain a certain amount of electricity produced in a year, or to install PV modules (a common question from customers), or just to cover some surface with solar modules?

(Note: There are demands on major systems to produce green electricity, and as a target value is taken CO2 emissions derived from fossil fuels.)

PVSYST off-grid solar panels example1. PVSYST, the list of devices: their strength and the average time of exploitation

The first, shown above, is the usual customer’s request present in electrification of homes, residential facilities that have no close connection to the grid, garage, solar LED lighting, solar billboards and advertisements, autonomous system monitoring and alerting, independent weather stations, boats and small ships, the tourist attractive floats, trailers and campers, etc. Therefore, the application is a multi-purpose, and can be implemented easily, of course, with expert consultation and / or supervision. Only it is not advised when consumers are of large forces, because there is a rapid degradation of the battery bank.

When buying an autonomous system, off-grid solar energy system, the customer is often required either average and maximum monthly consumption of electricity that would replace a power supply system (given in kWh), or the list of electrical devices, which means their power and mean time work during the day, see Figure 1. Just to emphasize that in  fridges and freezers power daily consumptions are well known, so it’s sometimes enough to say their number in the household. Thus, such a list is necessary to optimize the solar system, and to do that just visit the post Solar Calculator.

(Note: For new devices, class AA or AAA, which consume the least energy, it could only be stated the specifications of their daily consumption in kWh.)

PVSYST exemple resultsFigure 2. PVSYST, the results of the analysis


Another important thing for off-grid solar systems is the autonomy of stored energy in battery bank. It is important to be aware: the batteries don‘t have full 100% efficiency, ie. beside that they accumulate so much power, but battery efficiency range is between 95-85%, and solar panels never give Pm or nominal watt-peak power. Then, when they are fully charged and lately don`t spend some time, the accumulated energy decreases, Figure 3. And most importantly, the batteries mustn’t be emptied up to 50% of their capacitance, unless a permanent damage will occur. That means if average need is 6kWh in household, the minimum capacity of the battery bank must be 12kWh. In addition, every customer must knows that the major electric energy consumers over 1.5kW, do rapid discharge and have side effects in batteries. Figure 2 shows an example of calculation of such a system in PVSYST where autonomy is defined for 4 days. This is very important for cloudy days when is a low insolation.

RITAR RA12 120D 12v120AhFigure 3. RITAR RA12-120D (12V120Ah)

The battery bank can be organized-connected in series (the sum of the battery voltages) or parallel (the sum of the amperage). So, if it is wanted two 12V DC batteries be connected serially, then will be received the system of 24V DC, but the same strength of current (amperage) will stay. In case if  the battery bank is 240Ah labeled then the accumulated “solar energy” is 24V*240Ah=5.76kWh, and only 2.88kWh can be used.

Plasmatronics and Apollo charge controllers

Figure 4. Apollo and Plasmotronics charge controllers

Third, the battery unit is charged and controlled by special electronic devices, named charge controllers. Here it is very important to adjust the power of controllers with the power of installed solar panels and battery power unit!

Then, what is common for off-grid and grid-tie (or grid-connected) systems, are basic elements: solar panels, inverters, cabels, meter units, construction. For this system, it is desirable the PV panels (classification by type of solar cells) be monocrystalline and polycrystalline in terms of higher power. For smaller needs, such as boats, trailers, etc.. can be used and amorphous PV moduls. Why such a proposal? Due to reliability and sustainable characteristics of the solar panels. Mono and poly have far more stable voltage and current characteristics than amorphous, and suitable for cases where they need to be placed on inaccessible places (eg on roofs). Otherwise, amorphous are cheaper!

Selectronic inverter

Figure 5. Selectronic 1700W 24V Inverter

Inverters are different for off-grid and grid-tie solar energy network. Then, each differ in nominal AC power and DC voltage, some specification of such an inverter can be seen in Figure 5. For autonomous systems are developed inverters with integrated charge controllers, such as Apollo Stackable Inverter / Charger. It is therefore very important to know the approximate power of consumers and the time of their exploitation. In addition, there are DC-DC inverters, that are used for example in video surveillance, lighting, …

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One Response to “Off-grid PV system”

  1. Allen Chen says:

    Hi There,

    I am a solar general contractor here is the San Jose, California. I am interested in your article of off grid. I am looking for a solution of
    15KW off grid system. However I can’t find right solution, too many products out there in the market, can you make suggestion ?

    1. Does current inverter provide charge controller function ?
    2. Will those inverter provide the function of trickle charge, over charge or over discharge or battery management system to protect the battery.
    3. As a system provider to end user, we like to have equipment as little as possible. Do we have any thing that is available in the market including inverter itself, charge controller, monitoring LCD,
    power distrinution.
    4. If PV provide enough charge to battery system, the excess electricity goes to where ? Heating up water or what… ?
    5. What is most people required of autonomy days ?
    6. What is the actual cycle time for most lead acid battery ? people said 2000 cycles of 80% DOD (means only 20% juice used ?) If the application is home daily use, charge and discharge from home owner
    how many cycles we counted per day ?
    7. Can you provide me or someone a block diagram or wire diagram show me how does those equipment connected and how to calculate right or needed power for 15KW with 3 days autonomy ?

    Where are you located ?

    Thank you for your time

    Allen Chen

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