An argument for Solar Part – II

In my earlier post ( I discussed how in the places where power cuts are common, Solar makes immediate sense and how REhub makes it even more easier to decide on Solar. What about places where Power cuts are not so common but the electricity tariffs are high ?

15TVKSEB_eps_2059993eTake the state of Kerala . Kerala has power tariff slab that not only increases with the number of units consumed per month but also penalizes the customers exceeding a certain number of units consumed. Once the units exceed a limit, the consumers are charged the higher rate for all units consumed and not just for the extra units. Let us say that 334 units were consumed in a given month. On exceeding 250 units consumed, the charge is Rs 5 for all the units I.e for 334 units and not for the extra 84 units beyond 250. In a normal telescopic rate based tariff, the consumer would have paid Rs 1696/- versus as per the non telescopic tariffs he would have to pay Rs. 1904/- . Imagine the benefit if Solar PV helps in reducing the units so that the total falls into a lower slab. 22 + states increased the electricity tariffs last year and we are headed to a situation where the cost of electricity from Solar is already lesser than alternatives – what experts call Grid parity. As more and more states allow net metering or reverse metering, the case for Solar becomes even more stronger. In the case of net metering (or reverse metering) the Grid becomes the storage and the consumer is paid for the amount of units put into the Grid. In this scenario, the constraints are not the storage or the day time load in a house. The constraint is now shifted to the amount of shade free roof top available and the money available for investment. And I will show in a future post how much of a good investment that is with Karnataka and Tamilnadu’s reverse metering policy for small rooftops. For now, let us stick to adding Solar to existing back up and go into the mechanics of the addition in some detail. That’s in the next post of the series.

Image from The Hindu article on power tariff revision in Kerala - link here.

An argument for Solar on your rooftop – Part I

Take a typical home in Hyderabad or Coimbatore or for that matter most cities and towns in India. The uncertainty surrounding power cuts and the need to tide over them has spawned a whole industry around Inverters and Batteries & everyone seems to have one in their home for power back up. Yet the use of Inverters has perversely increased the demand for electricity – leading to more black outs and at an individual level increase in electricity bills. The round trip efficiencies of the Inverter and batteries are so poor that for every two units of energy that is put-in, one gets barely 1 unit of useful energy. Effectively the consumer is paying double for the unit of energy used during blackout hours. Even if one is willing to pay more for the convenience of always-on availability of power, the power cuts are so long that batteries just do not have sufficient time to recharge. A typical lead acid battery takes more than 10 hours to recharge completely under ideal charge rates. Most Inverters in the market are undersized relative to the size of the batteries and would take longer to recharge the batteries.


With REhub – stands for Renewable Energy hub – one can now add Solar to existing Inverters, effectively upgrading to Solar without having to invest in a completely new system again. Amberroot’s Solar chargers efficiently extract power from the Solar photovoltaic panels (Solar PV) and keep the batteries charged.

Care must be taken to size the Solar panels: install too little and Solar becomes nothing more than a token and worse can end up increasing the electricity bills, install too much and extractable energy is wasted because there are not enough loads or storage to utilize the energy that can be generated. Ideally, Solar installation must match the day time energy requirement in the absence of option of grid export. In battery based storage systems an additional factor comes into play. When Solar is added it becomes the second charger to the existing set up and depending on the AH (Ampere hour – the storage capacity of the batteries) the combined charge rate of both the Solar PV and Inverter can exceed the ideal charge rate to the batteries. If charged at a rate (speed of charging – higher current put into the batteries in a shorter time would mean higher charge rate) higher than what is ideal, the batteries end up getting ‘surface charged’ with diminished capacity to hold charge and eventually shortening the expected life of the batteries.

This is where REhub makes a big difference. By virtue of monitoring the currents going in and out of the batteries and dynamically adjusting charge rate (output of REhub to the batteries), REhub is the only device that adds Solar in way to maximize the extraction of Solar without exceeding the recommended charge rate to the batteries. Power cuts do not mean compromises anymore. Switching on the extra fan or light in the daytime or watching a Soap in the afternoon need not lead to an anxiety on the availability of power in case of further power cuts in the late evenings or night time. Adding Solar enhances the convenience that was promised by the addition of the inverter – at a cost that is lower month on month both to the consumer and to the environment.

What about cities and places where blackouts are not common ?

The next post in this series will cover that.

Will I save Money If I Install Solar ?




Yes that is the bottom line isn’t it ? Should be straight forward to answer if – the Ifs and but(s) appear and the issue becomes too confusing.  Let me explain why.

Installed Wp Vs What we actually get on field. 

Wp rating of the Solar Panel is what you pay for. However the Wp rating is based on Standard Test Conditions (STC) – which differ widely from the conditions in your installation. For example STC assumes 25 deg C for the Solar Cell temperature. What you have on your roof top is an ambient temperature of say 35 deg C and the Solar cell temperature operating at closer to 70 deg C ! As the output of the Solar module drops with increased Cell temperature -the maximum that you can get on the roof top is already lesser than what the name plate suggests. A good rule of thumb is about 75% of the rated Wp that you can get as the maximum.

Now that you have calculated the maximum Wp that can be extracted from the Solar Panel- you need to know how many units of electricity the panel would extract per day.

In India – you can assume 5.5 hrs equivalent of Wp – roughly for the calculation of Units generated. The extract-able energy from the Solar PV panels varies through the day and there would be a time close to noon when this is close to the maximum that can be extracted from the Solar Panels. Let us say Sun is available for 10 Hrs. When we say 5.5 hrs of Wp equivalent – what that means is that the amount of energy that can be extracted in Ten hours is equivalent to having 5.5 Hours of maximum Wp extracted.

Based on the thumb rule above and the 5.5 hrs Wp equivalent – an one kWp module will deliver approximately – 5.5 X 1 X 0.75 = 4.1 kWhr Or about 4 units per day .

The energy generated is only one side of the equation. It is useful only if the energy generated can be consumed !

Ex-tractable energy  Vs What is actually consumed

In the absence of Grid tie option – where a solar installer can export excess energy into the local grid and can get paid for the same- offgrid installations can end up wasting extractable energy unless Solar Panels are undersized. There would be periods when the batteries are full and loads not enough to consume all the available energy from the installation. In this case – the charger ends up extracting lesser than the maximum power available – thus reducing the returns from the installation. Its difficult to match the load timing to the Solar power availability. Note however – this author feels that if the objective is to obtain sufficient back up – then that needs to be addressed first. That need would drive the panel sizing and the ‘wasted’ energy is not such a bad thing.

Yes – Installing Solar PV does save money. 

Solar energy when used – saves on the power otherwise drawn from the grid for charging the batteries or powering the loads. However most offgrid systems still have a payback period of > 8 Years and compares well with alternatives like Diesel and not having power at all. In states like Kerala where the rate per Unit of electricity for increased consumption is much higher – Solar PV would deliver a shorter payback.

* Picture of the electricity Meter is from BESCOM site.

How much Solar PV should I install in my home

This is another very commonly asked question.Here are some guidelines to determine the size of install in India.

Money as a constraint : If Rooftop schemes with reverse or net metering is available in your locality ( not unless you are in Gandhinagar for example install as much as you have cash to spare (at today’s prices its about Rs 120 Per Wp , landed cost inclusive of the electronics , structures etc) – till you hit the next constraint which is space.

Shade free space as a constraint : You will need about 100 SqFt per kW installed with area free of shade , south facing.

However -rooftop net metering is going to take some time to become a reality. If you still want to add Solar PV (while you keep following up with the local utility about reverse metering )  - its usually because a) You think you want to do some good to the environment  b) you are in the unfortunate town or village where power cuts are the norm.

A). For someone who is looking to do ‘good’ – and cannot export the power generated to the Grid – you look at the two constraints : Money and Space to determine what is the size of install. You may have to rewire the connections – and add storage because of the nature of Solar PV power production. Accept the fact that not all extractable energy can be used all the time.

B) Solar PV as a back up power supplement : Unless you are prepared to ’ waste’ otherwise extractable Solar energy – the size of the install is much lesser than the total connected loads. Sizing the Solar PV is related to the hours of power cut, the size of the batteries and the average power consumption in your home. Sadly there are no electronics that manage the usage of Solar PV with Grid, loads and back up in a way that is seamless to the end user- yet.  Till such solutions are available – be clear on whether you want assured back up ( even in the face of ’ wasting extractable solar energy) Or Maximal usage of Solar.

We will look at some simple math in a later post for the sizing.

Recently Bridge to India Folks put up a Solar Calculator – Check this out :