A Solar Mango Guide for Residential Energy Consumers

Solar Mango provides solar solutions for industrial and commercial energy consumers (more on our services here), but we do receive a large number of enquiries from residential consumers on the feasibility of rooftop solar PV for their dwellings. As we are unable to reply to many such enquiries, we prepared this guide to address many of the common concerns surrounding residential rooftop solar power and help residential consumers decide on a solar plant.

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PDF format of the Guide can be downloaded from here

Within this guide you will find

 

Key Takeaways
  • A solar PV plant costs approximately Rs. 1 Lakh per kW with installation without batteries or subsidies, and will last 25 years
    • 1 kW of solar plant generates about 4 kWh of electricity per day on average over a year
    • The inverter is the only major component likely to need replacement
  • If you are considering solar to save on your EB bills, we recommend evaluating solar only if your residential EB tariff is Rs. 5.00/kWh or greater, subject to state specific incentive schemes that can lower the cost of solar
  • As solar plants generate power during the day when most residents are not at home, the plant has to sized carefully for daytime load rather than peak load which is usually experienced only at night
  • Solar PV plants can be installed in apartment buildings provided installing solar panels on the common rooftop is permitted
  • A solar water heater is a better choice for heating water than a solar PV plant

 

Overview of Solar PV
Solar PV converts sunlight (not heat) to generate electricity through a photovoltaic process. A solar plant consists of
pv

  • Solar PanelsThese are mounted on the roof and convert sunlight into DC power
  • Mounting Structures – These secure the solar panels to the roof of your building
    • Most mounting structures require your roof to be penetrated. If you are worried about water seepage, non-penetrating options are also available
    • Mounting structures should be strong enough to hold your solar panels even at high wind speeds, such as during cyclones
  • Inverter – This converts the generated DC power into the AC power that is required by your appliances. Through a charge controller it also regulates battery charging if required
  • Balance of Systems (BoS) – All the other components of the solar plant, such as cables, junction boxes, fuses, etc.

The size of solar plant you require depends on your electrical load and the number of kWh (units) you consume. The size of solar plant you can install is limited by the extent of shade-free rooftop space available.

The expected lifetime of a solar PV plant is 25 years.
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Kinds of Solar Plants
Solar plants can be

  • Grid-tied – The plant works in conjunction with grid power. Such a solar plant will turn off if grid power is not available, and may not suit locations with frequent power failure
  • Hybrid– In conjunction with a diesel generator or battery bank, the plant can deliver solar power even during a power failure
  • Off-grid – The plant only works off-grid. It can sync with batteries or a diesel generator but not grid power. Such plants can be installed in areas where the grid is absent, but not recommended for areas where grid is present, even if grid power is only available intermittently

The only difference between the two kinds of plants is in the type of inverter used.

It should be noted that a solar PV plant must have another source of power (grid/DG/battery) to function – the other source is used to provide a reference voltage as solar power is continuously varying. In the absence of reference power, the solar plant will not generate electricity even in bright sunlight.

  • A grid-tied plant synchronises only with the grid and will shutdown in the absence of grid power
  • A hybrid plant can synchronise with either gird or DG or battery and will shutdown if all three are absent
  • An off-grid plant can synchronise with DG or battery and will shutdown if both are absent

What kind of a roof do I need for a solar plant?

Solar PV plants require a south-facing roof (in the northern hemisphere). The roof should also be free from shadows; portions of the roof that have shadows should be excluded while calculating the roof space available for the solar plant. Shadows falling on solar panels reduce power generation, and may also damage the panels.

The kind of roof determines if solar panels can be installed. Ability of the roof to support the weight of the panels and mounting structure even in strong winds is the critical parameter.

  • Concrete Roofs – Solar PV plants can be easily installed on flat concrete roofs. Inclined roofs may pose a challenge depending on the angle and direction of the inclination
  • Metal Roofs – This will depend on the type of structure and the weight it can withstand
  • Asbestos/Other Sheet Roofs – These are the least likely of the 3 to be able to support solar panels, though it is possible in some cases.

How much of roof space do I require?

Approximately 100 SF is required for 1 kW of solar plant.

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Can a solar plant be installed only in individual homes, or also in residential apartments?

Solar PV plants can be installed on the rooftops of residential apartment complexes. A few issues should be considered when evaluating rooftop plants for apartments

  • Roof ownership – In many apartment buildings the roof is considered common area. Therefore permission from the building society/association may be required before an individual can install the solar plant
  • Distance from roof to apartment – Since apartment complexes typically feature multi-storeyed buildings, the number of floors between the apartment where solar power is to be used and the roof where the power is generated may be an issue as DC power provided by the solar panels suffers a great deal of loss as distance increases
    • Using thicker DC cables is a possible solution to minimise loss
    • Another solution is to place the inverter on the roof as the AC power from the inverter does not suffer much loss over distances compared to DC power. Issues such as permission to install the inverter and location of inverter (it needs a well-ventilated room) need to be addressed
  • Earthing – The solar plant generates both AC and DC power and both need to be earthed. While AC earthing can be combined with existing building earthing, DC will require separate earthing for which permission will be required as well

It is also possible that rather than an individual apartment owner having a personal solar plant, the building association as a whole can install a solar plant to supply common facilities such as common area lighting.

Can I run my entire home on solar PV?
This will depend on the kind of loads you run, but usually running the entire load off solar PV is not recommended

  • Solar power is only generated during daytime. With heavy rain or mist solar power may not be generated even during the day (30-60 days of generation may be lost in a year due to these factors)
  • The solar plant may not run loads with heavy starting current requirements, such as water pumps. Air conditioning also may not be supported
    • It may be possible to run inverter air conditioners off your solar plant

In such situations a solar plant can be used to support part of the load, similar to a home inverter/UPS.

How do I know my electrical load?

The sum of the wattage of individual appliances represents the load i.e., a 60 W fan and 40 W light represent a 100 W load. If these two appliances run continuously for an hour they consume 100 watt-hours. If they run 10 hours they will consume 1,000 watt-hours or 1 kilowatt hour (kWh), also known as one unit.
We can illustrate this calculation with an example:

Appliance Number Wattage Total Wattage Hours used between 10 and 4 Energy (Wh/day)
Lights 4 40 160 5 800
Fans 3 60 180 6 1,080
TV 1 120 120 3 360
Refrigerator 1 300 300 4 1,200
760 3,440

Dividing the Total Wattage and Energy by 1,000 gives us the load in kW and energy consumed in kWh i.e., Total Load = 0.76 kW; Total Energy Consumed = 3.44 kWh.

Your electricity consumption is billed on the basis of number of kWh consumed. Since solar is only available during daytime, it is the load and energy consumption during daytime that will need to be ascertained to calculate the size of the plant. Residents are usually away from home during the day and most energy consumption is at night. It is therefore important to calculate the size of the plant based on daytime use to avoid oversizing the plant.

The above example gives a simple calculation to calculate the load and energy consumption. In real world situations it may be difficult to determine the number of hours that appliances are used during the day. We can instead ascertain the energy consumption by noting the electricity meter reading at 10 AM and 4 PM for a few days. The difference between the readings is the energy consumption (kWh) for that period. If the electricity meter is a digital meter it usually also provides the load in amperes and kW. This can be noted several times each day to determine the average load.

A detailed note on estimating electrical load and energy consumption can be found here – www.solarmango.com/in/faq/8#calculating-the

What should be the solar plant capacity based on my load?
Solar plant capacity is typically calculated as load + 25% safety margin i.e., an 8 kW load will require a 10 kW solar plant.

In the example given previously, we can calculate the solar plant capacity for the 0.76 kW load as

             Load (kW)                  0.76
      -----------------------  =    ------------  = (100-25)%
       % before safety margin         1.0133 kW

This load can be supported (without batteries) by a 1 kW solar PV plant.
Please note that the capacity of the solar plant is determined by the capacity of the solar panels and inverter i.e., both the panel and inverter capacity will have to be 10 kW for the plant to be a 10 kW plant.

A detailed note on sizing the solar plant can be found here –

www.solarmango.com/in/faq/8#Sizing

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What would happen if my solar plant does not match the load?

If the plant is smaller than required

  • Grid-tie system
    • When grid power is present – The deficit will be compensated by electricity from the grid
    • During power failure – The solar plant will not provide power (irrespective of size of plant)
  • Hybrid system
    • When grid power is present – The deficit will be compensated by electricity from the grid
    • During power failure – The inverter may register an overload and be shutdown/damaged, unless another source of power (DG or batteries) can compensate
  • Off-grid system
    • As this is an off-grid system, there is no question of grid power being present. The inverter may register an overload and be shutdown/damaged, unless another source of power (DG or batteries) can compensate
  • Grid-tie system
    • When grid power is present – Excess solar generation is fed into the grid
    • During power failure – The solar plant will not provide power (irrespective of size of plant)
  • Hybrid system
    • When grid power is present – Excess solar generation is fed into the grid
    • During power failure – The solar plant will generate only as much energy as required by the load i.e., the excess plant capacity is wasted
  • Off-grid system
    • As this is an off-grid system, there is no question of power being exported to the grid. The solar plant will generate only as much energy as required by the load i.e., the excess plant capacity is wasted

If the plant is larger than required

Please note that if you do not have a net meter, feeding excess solar generation into the grid will cause your electricity meter to turn in the forward direction for energy supplied i.e., the power sent to the grid will be added to your electricity bill.

What is the cost of a solar PV plant?

The approximate cost of a rooftop solar PV plant is Rs. 1 Lakh per kW, before considering incentives. This includes installation and all components other than batteries. Batteries can add 30% or more to the cost of the plant, depending on the extent of backup required.

The cost of a rooftop solar PV plant is discussed in detail here – www.solarmango.com/in/faq/2.
1 kW of solar plant will occupy 100 SF and generate 4 kWh of power per day on average over a year.

Are any subsidies provided?

The MNRE previously provided a 30% subsidy on capital cost of a rooftop solar plant, but that subsidy is currently not available for residential plants.
Individual states may have special schemes for residential rooftop solar. Please check with the Energy Development Agency of your state.

Can I benefit from Accelerated Depreciation?
Depreciation can be claimed only on business assets therefore residential solar plants cannot benefit from accelerated depreciation.

What are the operating/running costs?

Other than washing of solar panels once in every few weeks, solar plants require little by way of maintenance as there are no moving parts; operating and running costs are minimal.
The inverter is the only major component that may need to be replaced during the 25-year lifetime of the plant.

What is net metering?
metering

Net metering is a method by which solar PV plant owners can be compensated for any excess solar power that they supply to the grid. The conventional uni-directional electricity meter is replaced with a bi-directional meter that spins in reverse for energy that is supplied to the grid. The consumer is billed by the utility only on the net energy consumption indicated by the meter.
In the absence of net metering any energy supplied to the grid will be added to the consumer’s electricity bill.

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Will using solar power be cheaper than using grid power?

This depends on your current EB tariff.
A 1 kW solar plant (without batteries) will last for 25 years and give you about 4 kWh (units) of power per day. Assuming the power output degrades by 0.5% a year, we will get about 34,000 kWh over 25 years. As this is a grid-tied (no batteries) plant, we can consider 30,000 kWh as total usable energy generation after adjusting for losses during power failure.

At Rs. 1 lakh per kW, the per kWh cost is

                   Rs. 1,00,000
                  ---------------  =   Rs. 3.33/kWh
                    30,000 kWh

Please note that this does not include any cost other than direct upfront cost for the plant. Other costs such as inverter replacement and AMC can push the cost/kWh higher.

Adding some margin for unforeseen costs, Solar Mango recommends considering solar power for your residence only if your current residential tariff is Rs. 5.00/kWh or greater. We urge you to conduct a thorough due diligence on costs by speaking to vendors with experience installing in and around your location before deciding on your solar plant.

It should be noted that cost of power from your solar plant is fixed for the next 25 years while the EB power tariff keeps rising.

Can I use batteries to obtain power at night?
With the appropriate hybrid inverter, a battery backed system will charge batteries during the day and the energy can be used at night. When evaluating such a configuration, the consumer should be aware that

  • Batteries are expensive, and need to be replaced every few years
  • Repeatedly draining the batteries will shorten battery life even further
  • Some amount of energy is lost both when charging and discharging a battery; batteries deliver only about 75% of the energy that is fed to them

What certifications/warranties should I look for in my solar PV plant?

  • Solar Panels
    • Warranty – At least 5-year manufacturing defect warranty and a 25-year output warranty (typically 90% of power output at year 10 and 80% of power output at year 25)
    • Certifications – IEC 61215/IS 14286, IEC 61730, and IEC 61701/IS 61701 if coastal location
  • Inverter
    • Warranty – 1 year, extendable to 5 years
    • Certifications – IEC 61683/IS 61683, IEC 60068-2
  • Other components – 1 year warranty

A detailed examination of warranties and certifications, including failures not covered by warranties, can be found here – www.solarmango.com/in/faq/7

Should I use a solar PV plant or a solar water heater to heat water?
Using a solar water heater is a more cost-effective option than using solar PV to generate electricity which is then used to heat water.
Solar water heaters are a different kind of solar technology where the heat from the sun, rather than the light, is collected to heat water.
cells

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