Archive for Category Balance of Systems, BOS

1. Inverters and BOS in Roof Top PV Systems

BOS, Balance of Systems and Inverters are integral parts of any Solar Power Generating unit. It is imperative that specified BOS and inverters are used for any given configuration of a solar power system for optimal energy efficiency. BOS includes the wires connectors, AJB’s etc. For Energy Efficiency, it is important to use the wires with correct thickness to avoid heat loss and voltage drop. Ohm’s law ( V=IR ) quantifies the voltage drop as a function of current and resistance. The figure below broadly indicates the wire thickness v/s current function to minimize the voltage drop.

The MPPT Charge Controller

MPPT (Maximum Power Point Tracking) Charge Controller tries keeping the panel at its maximum voltage and simultaneously produces the voltage required by the battery. A basic charge controller simply prevents damage of batteries by over-charging, by effectively cutting off the current from the solar panels (or by reducing it to a pulse) when the battery voltage reaches a certain level. On the other hand, a Maximum Power Point Tracker (MPPT) controller performs an extra function to improve your system efficiency.

What does the MPPT Controller Do ?

Besides performing the function of a basic controller, an MPPT controller also includes a DC to DC voltage converter, converting the voltage of the panels to that required by the batteries, with very little loss of power. In other words, it attempts to keep the panel voltage near its Maximum Power Point, while supplying the varying voltage requirements of the battery. Thus, it essentially decouples the panel and battery voltages so that there can be a 24 volt system on one side of the MPPT charge controller and panels wired in series to produce 48 volts on the other. Thus, offering the ability to provide some charging current even in dull conditions when a simple controller would not help much.

Inverter Efficiency

Solar PV system produces DC current, which is converted to AC using an inverter. Although inverters come with wide ranging efficiencies but typically affordable solar inverters are between 80% to 90% efficient.

Battery Efficiency

Batteries are needed for charge storage. Lead acid batteries are most commonly used.
Since lead acid batteries are usually charged at the float voltage of about 13.5 V and the discharge voltage is about 12 V, the voltage efficiency is about 0.88. In average the coulomb efficiency is about 0.92. Hence, the net energy efficiency is around 0.80 A lead-acid battery has an efficiency of only 75-85% (this includes both the charging loss and the discharging loss). From zero State of Charge (SOC) to 85% SOC the average overall battery charging efficiency is 91%- the balance is losses during discharge. The energy lost appears as heat which warms the battery. It can be minimized by keeping the charge and discharge rates low. It helps keep the battery cool and improves its life.

2. Photo Voltaic Electricity from the Sun

Photo-Voltaic essentially means Volts produced by photons. The panels are made out of silicon wafers, which let out (release) electrons when impinged upon by photons ( light). This is a rare property of some semi-conductors. The intensity of Solar radiation changes during the course of the day, year and weather conditions. To facilitate calculations in planning a system, the total amount of solar radiation energy is expressed as Peak Sun Hours. United States Department of Energy indicates the amount of solar energy that hits the surface of the earth every +/- hour is greater than the total amount of energy that the entire human population requires in a year.

Solar Panels

The output of a solar panel is usually stated in watts (V x A = W)
Since the intensity of sunlight contacting the solar panel varies throughout the day, we use the term "peak sun hours" as a method to average out variations into a daily average.


Deep Cycle batteries are preferably used in Solar Generators for back-up. Lead-acid batteries are the most common in PV systems because their initial cost is low. Lead-acid batteries are available in both wet-cell (requires maintenance) and sealed no-maintenance versions.

Using an Inverter

An inverter is a device which changes DC power stored in a battery to standard 120/240 VAC electricity (also referred to as 110/220). In an inverter, direct current (DC) is switched back and forth to produce alternating current (AC). Then it is transformed, filtered, stepped, etc. to get it to an acceptable output waveform. The more processing, the cleaner and quieter the output, but the lower the efficiency of the conversion. The goal becomes to produce a waveform that is acceptable to all loads without sacrificing too much power into the conversion process.

Efficiency Losses

In all systems there are losses due to such things as voltage losses as the electricity is carried across the wires, batteries and inverters not being 100 percent efficient, and other factors. These efficiency losses vary from component to component, and from system to system and can be as high as 25 percent.