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Sizing batteries and inverters for a solar PV system

By on April 22, 2016

In our previous post on: Solar Glossary – photovoltaic, panels, modules, cells, voltage, watt and current we talked about various technical aspects to look for before buying a solar panel. In this post we will talk more about batteries and inverters and their sizing. A larger sized system may cost more whereas a smaller sized system may not fulfill all your requirements and thus sizing a solar PV system before buying is very important. Although we have already talked about sizing of PV panels on the previous posts, we have included the same again in this post for the sake of completeness of information.

Basics of sizing

The most important thing that one needs to know before sizing a PV system is the energy requirements of a setup. (Along with all the electrical values mentioned in article mentioned above) A few things that can help are:

  1. Wattages and counts of all the appliances that need to be run on solar PV.
  2. If you do not have wattages then you can look at the current requirement (in amperes) of the appliances and calculate wattage with this simple formula:

Watts = Ampere x 240 (voltage)

  1. Electricity bills of the setup. Used to check the monthly electricity units used in a setup. Daily units can be obtained by dividing month units by 28/29/30 or 31 (depending on the number of days in the month for which the bill is generated)
  2. Daily usage of each appliance in hours. This is required if you do not have a sample electricity bill. This helps in calculating the number of units of electricity used in a day using the formula below:

Units = (Watts x Hours) ÷1000

Two things are absolutely essential are: Total wattage of appliances (which denotes the instantaneous electricity requirement of a setup) and Total Units (which denotes the total electricity used in a day).

Sizing a PV panel

To size a PV panel, the most essential thing to know is the Total Units consumed in a day by the appliances in a setup (unless it is a direct connected system or a grid connected system for which details are mentioned here: link). The size of PV system should not be less than the one that can generate total units consume in a day. Every PV panels has a peak wattage (Wp) mentioned on them.  A 1 kWp (or peak kilo watt) system would generate 5 to 7 units in a day. Thus the right size of PV system (in kWp) should be estimated by dividing maximum daily usage units divided by 5.

You can buy a bigger system if you are going for a grid connected system where extra electricity produced will be sold back to the electricity provider (or discom). In such cases you can optimize the size of PV system based on the space that you have for installing PV panels (more on it on this link).

Sizing Batteries for PV system

If you are not going for a grid connected system or a direct connected system, you need batteries to store the energy generated using PV panels. Along with sizing of the PV panel, it is important to size the batteries as well. Because if you purchase more batteries then they will not get fully charged, if you buy fewer batteries, you may not be able to get the maximum benefit out of the solar panel.

Most big PV systems use deep cycle (or deep discharge) batteries that are designed to discharge to low energy levels and also to recharge rapidly. These are typically lead acid batteries that may or may not require maintenance. In case of solar mobile phone chargers or other small chargers the batteries may be Lithium Ion, etc.

Batteries have energy storage ratings mentioned in Amp-hour (Ah) or milli-Amp-hour (mAh). They also have a nominal voltage that they generate (typically deep discharge batteries are 12 V batteries, cell phone batteries are 5 V batteries, etc). To calculate the total energy a battery can store you can use following formula:

                        Units = (Volt x Ah) ÷1000 or (Volt x mAh) ÷1000000

We have already talked about how to calculate the total units required in a day and also the sizing of the PV system. Batteries should be sized in such a way that the units of energy generated by the PV system should be equal to the number we have calculated above.

So assuming we have a 1 kWp system and we assume that on an average it generates 6 units a day and if we have to buy 12 V battery for it, the Ah (or storage) of battery required would be:

                        (6 x1000) ÷12 = 500 Ah

Sizing Inverter for a Solar PV system

A power inverter or inverter is a system that converts Direct Current (or DC) to an alternating current (or AC). A solar panel produces DC current, batteries also generate DC current, but most systems we use in our daily lives use AC current. Inverters also have transformers to convert DC output voltage to any AC output voltage. Depending on the type of system (grid or off-grid) various types of inverters are available.

Sizing of inverter depends on the wattage of appliances connected to it. The input rating of inverter should never be lower than the total wattages of the appliances. Also it should have the same nominal input voltage as that of the battery setup. It is always better to have inverter wattage about 20-25% more than that of the appliances connected. This is specifically essential if the appliances connected have compressors or motors (like AC, refrigerator, pumps, etc), which draw high starting current.

Most inverters available in market are rated on kVA/VA or Kilo Volt Ampere/Volt Ampere. In ideal situations (power factor of 1) 1 VA = 1 Watt. But in real power factor varies from 0.85 to 0.99 (more about power factor on: What is Power Factor correction and how MDI (Maximum Demand Indicator) penalty can be avoided). So one can assume 1.18 VA = 1 Watt. So if you have a setup where the total wattage of the system is 1000 Watts, it means your inverter size required is more than 1180 VA or 1.18 kVA (add some extra to be on a safer side).

The higher the VA of an inverter, more is the number of appliances it can support, but more costly it would be. So it is important to size it right while buying. Also for a grid-tied system, as there are no batteries connected, the size or VA of the inverter should match the wattage of PV panel for efficient and safe operation.

Here is a quick video that can guide you on Rooftop Solar PV buying in India:






About the Author:
Abhishek Jain is an Alumnus of IIT Bombay with almost 10 years of experience in corporate before starting Bijli Bachao in 2012. His passion for solving problems moved him towards Energy Sector and he is keen to learn about customer behavior towards Energy and find ways to influence the same towards Sustainability. .