Adding Solar PV to Existing Inverters

How to get real savings in Electricity bills and keep the batteries safe!

Electricity supply in India has improved enormously in the recent past. The Southern region is expected to have an high Peak shortage (Power cuts !). The Eastern region is expected to have an energy surplus in 2015-161. With increasing power availability, consumers are looking to add Solar PV to help in reducing electricity bills. In the absence of grid tied options, adding Solar to existing Inverter based back up makes economic sense. Adding Solar to existing Inverters allows the reuse of the investment already made in Inverters & Batteries.

With this approach, the Solar PV panels & a Charge Controller that controls the charging from both the existing Inverter and Solar PV is retrofitted to the existing Inverter. The combined system is expected to work as a single unit, maximizing the use of Solar PV and providing sufficient power back up in case of power cuts.

Most retrofit charge controllers control the Mains input to the Inverter, switching off the Mains supply, when Solar PV is available so that the loads connected to the Inverter are powered from Solar PV (and the batteries).

A good ‘retrofit’ charge controller MUST control the Charge Rate to the batteries if they aim to really save electricity bills with the addition of Solar PV. To get real savings in electricity bills, the Solar PV addition must generate enough energy to completely service the day time loads connected to the Inverter. For example, in a typical home, the day time energy consumption might be 2-3 Units (kWHr). A kWp of Solar PV panel would generate about 4-4.5 Units per day. A Solar array suitably sized would be capable of generating the required energy. The conflict arises becase of the size of battery back up available. Most existing Inverters would have Lead acid batteries of ~ 150Ah capacity. Lead acid batteries must not be charged at a rate higher than 10 -15% of their battery Ah. When charged at an higher rate, the batteries reach gassing Voltage before they are fully charged.However, a Solar PV array to take care to day time loads might deliver Current that is higher than the optimal Charge rate. Installing larger batteries is expensive & not very useful. Installing smaller PV capacity is counter productive – in the end, the energy deficit would be bridged by the use of Grid supply and will result in increased bills. Not what the user wants either. The retrofit charge controller Must monitor the net current delvered to the batteries and Must control this current to the optimal charge rate – so that Loads get the required power and the batteries are used safely as an effective buffer for the variations in load and generation.

Therefore a good solution to add Solar to existing Inverters to be of real benefit to users must allow sufficient Solar capacity to take care of day time loads and keep the batteries safe at the same time.

1Page i.. Anticipated All India Power Supply Position for the Year 2015-16. Load Generation Balance Report 2015-16. Central Electricity Authority.


System Integrator Question Series – What Panel should I use with my PWM charger?

The other day, in a meeting with a system Integrator from Trivandrum (He has been a great supporter of our products and has given us tons of valuable feedback and some good business too) we were asked if it was OK to use a 60 Cell panel with 24 V battery system and a PWM charger.  We had to tell him that it would be better to use a 72 Cell Panel with a 24V battery system esp with a PWM charger. (Similarly a 36 Cell panel with a 12V battery system).

The reason is simple.The Current – Voltage curves of the 60 Cell Panel seem to indicate that the MaxiEldora250Pmum power point is at 30V. Given that the 24V batteries are going to operate at a maximum of 28V and therefore the Maximum power Voltage is higher than the highest operating battery Voltage, (See the I-V curve on the left – this one is from the Panel data sheet of Vikram Solar  the PWM charger ought to have no difficulty charging the batteries at all the various Battery voltages of a 24V battery system. However – these curves are at STC (Standard Test conditions ) and even at STC,  at lower Solar availability, the maximum power point has shifted to a lower Voltage value. And real life conditions are very different from STC . For one, the ambient temperatures when we get 1000W/m2 of Solar power is likely to be  much higher than 25 deg C that the STC uses. The actual cell temperature is probably close to 65- 70 deg C. Panel data sheets refer to the drop in Voc and the Vmpp with a temperature coefficient. (e.g the same Panel data sheet the Temperature coefficient is mentioned as -0.31% per deg C. Apply that to the STC values and we find that the actual maximum power point is now nearer 24-26V and not the 30V that we see in the standard I-V curves. Now with battery voltage at 24V or more – the PWM charger is forced to operate to the right of the knee instead of the left of the knee.  ( in the curve above , the maximum power point is close to the knee of the curves).

The problem with operating at the right of the knee instead of the left of the knee is obvious once we look at the curves above. On the right of the knee, we see that the curve is dropping sharply and that means that a small shift to the right on Voltage will mean a large drop in available Current – and therefore the Power ( Power: multiply I and V). Whereas if we operate to the left of the knee- the value of Current (I) does not change much and we end up getting more power in the exact same conditions. We may even be operating on the knee and getting the maximum power with the PWM charger if the knee is ‘right’ enough. Take a look at a similar IV curve for a 72 Cell panel below. Even under real life conditions, we are likely to operate at the left of the knee and that is why it is advisable to use a 72 cell panel with 24V battery systems and 36 cell panels with 12V battery systems. Eldora200P








REhub now adds an LCD screen

As REhub started getting sold in different parts of India – one request we kept getting was the need for a display – it does not surprise me at all . I jog regularly and need the iPod Nano telling me the minutes I took to run each kilometer and the calories I presumably burnt to both make the run feel real and to keep me honest.

Having installed Solar PV  - there is the urge to know the Units of electricity produced by the installed PV. That number tells us if the unit is actually delivering power from the Sun and makes us feel good about this little positive contribution to Earth !
So we now have a LCD screen with our newest product REhub PWM. The LCD makes it easy to know what the unit is doing, if Solar power is available, the state of charge of the batteries and if Power is available from the utility Mains. The unit keeps a running cumulative count of the total energy produced from Solar.  We see an added benefit from the LCD. Now remote debugging of issues would be easier and we hope our support would be better with the LCD present .

The next step is to have the data accessible on the net. I can already hear the requests coming in !

Update : This article on Newyorker about how fitbit took over this persons life. Start measuring something and it becomes an obsession!

PWM, MPPT and measuring real output

Yesterday I received a call from a system integrator – who has been installing solar systems using PWM charge controllers and is looking at switching to MPPT charger.  He has an experimental set up with a 200 AH batteries, 48V, 2500 Wp Solar panels : 250 Wp X 10 nos , a PWM charger and a MPPT charger which he had acquired from his recent visit to Bangalore. (Its not from AMBRT though)

He was aghast that contrary to his expectations the back up power from PWM charger was longer than that of the MPPT charger.

Questioning him further – revealed two common issues with comparison. 1) Unlike the PWM charger , the MPPT charger is expected to have a different input Voltage and current compared to its output. If the MPPT charger is working to its full potential- then the Voltage in the input is likely to be the Maximum power point Voltage – Vmpp.  If you would like to compare two chargers – they are ideally done in parallel  connecting to the same battery voltage and are extracting power from two arrays under same conditions of solar incidence.  If this is not possible, then calculate the total energy delivered – on a few days – hopefully with similar solar incidence.

Note : The wiring of the array using PWM charger may not work for the MPPT charger. In the above example, with just two panels in series, the Voltage of the array could be in the lower end of the input Voltage range of the MPPT charger , BUT the current  might be higher than the input current limit. Therefore the array may have to be rewired with 3 Panels in series instead of two in series.

2) Battery Voltage matters. As I found out in this case, the MPPT charger was not pushing the battery voltage to beyond 54V – based on settings in the charger. The PWM charger on the other hand was pushing the battery voltage to much higher – so definitely not an apples to apples comparison. What was probably happening was that the MPPT charger was backing out when the battery voltage reached 54V. The MPPT charger was then regulating to keep the battery voltage nearly constant , allowing the battery to draw as much current it wanted. The MPPT charger was no more operating in the MPPT point.

so what is the best way to compare both the chargers?

If the array cannot be made parallel and each half connected to a different charger – the next best thing is to

a) Connect the charger to the batteries on two different days – note the sunlight conditions and judge if they are similar.

b) Log the Battery Voltage and measure the Current at the output of each of the chargers in regular intervals. – Say every ten minutes – through the day.

c) Make sure a load is connected to the battery so that the battery is not fully charged. For example in the 48V system , adjust the loads so that the Voltage of the batteries are maintained at around 51V.

d) Multiply V and I and sum the values to determine the amount of units ( kWHr) of energy delivered in the same time frame. This should give you an approximate estimate of the performance of the two chargers.

Note : For the MPPT charger – the wiring of the panels may have to be changed due to the different input current ratings.

Does MPPT make sense in Indian Conditions

As we interact with a variety of different solar system installers, we have got this question repeatedly – does MPPT make sense in Indian conditions.

The implied question is – MPPT controllers cost more than the PWM counterparts – is it worth the while to add this cost to the installation.

First : The difference between MPPT and PWM chargers. There are ton’s of literature on this subject on the net- briefly  PWM chargers essentially short the panels to the battery and the power from the panels is extracted at the battery voltage. i.e Power extracted is Vbatt X I – which is the current dictated by the PV panel’s characteristic I-V curve at Vbatt. This Voltage and therefore Current I, when different from its Maximum power point Voltage, the PWM charge controller ends up extracting lower energy from the Panel compared to the maximum possible. The panels are matched to the battery voltage and the Vmpp of the panels chosen is always higher than the highest battery voltage. That is why, for example for 12 Volt battery systems, the so called ‘12V panels’ have Vmpp of 17V.


In Indian conditions, the cells in the PV panels are operating under ambient temperature conditions that are much higher than the ideal name plate Wp conditions i.e 25 deg C. The maximum power point voltage shifts to a lower value – and hence closer to the battery voltage.  The difference between the Vmpp and Vbatt is considerably reduced . Hence the extra power extracted using a MPPT charger is reduced. 

Note that the MPPT charger is going to deliver extra energy – BUT the addition over PWM chargers is reduced because of typical India conditions (at say ambient of 36 deg C and NOCT – 47 deg – the difference is about 15% ) For a 100 Wp system – this is only about 10 Wp in typical conditions and based on today’s Solar PV prices, that allows only ~Rs500 extra for a MPPT charger. However as the Wattage increases – the head room for MPPT is better – for a 500 Wp install the extra cost afforded for a MPPT charger is ~Rs 2700/- and so on. That is to get the same performance of a MPPT charger with a PWM charger – the installer has to spend Rs 2700/- worth of PV panel extra in a 500 Wp installation.

However this calculation completely ignores the other benefits of using MPPT chargers.  With MPPT chargers, the need to match the PV panel Voltage with the Battery Voltage is no more present. The system integrator can choose mass produced PV panels that cost lesser on a per Wp basis.

Further if the MPPT charger is designed well – the input range of acceptable voltages is high and this means that the input can be operating at higher voltage – thus reducing the wiring and connector costs.

In conclusion : MPPT chargers can have the effect of reducing the system costs and deliver more energy even in Indian conditions.

How a good MPPT charge controller (e.g. REhub) reduces system costs

REhub  is based on an high efficiency MPPT charge controller with the ability to accept a wide input MPPT range ( e.g 17 – 65 V for a 12V battery system).

System_integrators can use this to their advantage by

a) Selecting PV panels that are mass produced and therefore of lower cost. E.g to set up a 400Wp installation – using 4 X 100 Wp panels cost more than using 2 X 200 Wp panels. Yes, the 100 Wp panels are selected in installations today because of the need to match the PV panel output voltage to that of the battery system voltage – not needed any more with REhub.

b) Wire the panels in an all series arrangement. Typical Voltage at the Maximum Power point ( Vmpp ) for a 200 Wp , 60 Cell panel is about 30V. Wired in series the combined voltage is at 60Vmpp. REhub converts the input power at Vmpp to Vbatt ( the battery voltage) efficiently. The higher input voltage would mean that wiring costs are lesser compared to having all panels in parallel and operating at 17 V.

c) Lesser civil work and structurals on the roof top. Lesser panels : lesser cost .

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