Constraints in Roof Top Solar

The need to substitute diesel with solar is unique to India and a few other developing nations whose hunger for energy is not matched by growth in conventional power supply, necessitating large diesel systems (and now large solar systems). This is an emerging area where many solar system providers are still evolving solutions with varying degrees of effectiveness. Here we take a look at the features, constraints, and innovative solutions in substituting diesel with solar.

Some of the limits facing rooftop solar PV are

  • Solar power is generated only during the daytime on non-rainy days
  • Solar power may not handle the heavy starting currents required by heavy machinery
  • The output of the solar plant varies as the sun moves across the sky, and as clouds move across the sun
  • Limited roof area constrains the capacity of solar PV plant that can be installed

Due to these reasons, rooftop solar ideally acts in conjunction with diesel generators, taking up part of the load and saving, but not eliminating, diesel expenditure.

Integrating rooftop solar with a diesel generator

The question that now arises is how well can rooftop solar power integrate with diesel power if both are to be used simultaneously. There are several issues to be considered here

  • Frequency & Power Quality: As the loading factor[1] of the diesel generator varies (as a result of introducing an intermittent solar power source into the mix), the frequency of the output AC power can vary beyond acceptable levels reducing the quality of power. Some modern devices require high quality power to operate
  • Reverse Current Flow: When the output of the solar power exceeds the load demand, some reverse current will flow into the diesel generator. There are acceptable levels to which this can happen beyond which the diesel generator trips, terminating the reference voltage for the solar system causing it to shut down
  • Efficiency and Minimum Loading: Diesel generators operate in various modes. Running a generator at loads below the minimum load factor[2] for prolonged periods affects the efficiency of operations which in turn has a bearing on fuel consumption, maintenance costs, and useful life. Operating below the minimum load is possible if a sizeable solar system is integrated into the supply and results in decreased lifecycle savings of the solar-diesel generator system.
    • This should be specially considered by those who have only one diesel generator, or use only one diesel generator at a time, as the generator’s load will reduce once solar is introduced. In a multi-generator system, it is possible to balance the load by turning off some generators completely, and running the other generators at full load and optimum efficiency

Economics of substituting Diesel with Rooftop Solar PV

The financial returns from substituting diesel with rooftop solar have to be calculated carefully because they have very different cost structures

  • Diesel generator – The initial cost of the system is low but running cost (both fuel and maintenance) is high
  • Rooftop solar PV –Initial investment is high but running cost is very low


In order to compare the costs involved with both options, we model the expenses involved in both cases in a cash flow statement over the life of the PV plant (25 years). Any savings in expenditure over diesel power is treated as ‘revenue’ for the solar plant. The resulting cash flow is then used to calculate financial metrics such as IRR and payback period.

One of the factors to be considered when estimating the economics of rooftop solar as a substitute for diesel is the timing of load-shedding

  • If load-shedding occurs during the entire time when the solar system is generating power, then the entire output of the solar system will help in abating diesel consumption. Here there is a very clear gain as solar power is cheaper than diesel power, but this scenario isn’t very realistic
  • If load shedding occurs during part of the time when the solar system is generating power, then part of the output of the solar plant abates diesel (during load shedding time) and part of the output of the plant abates grid power (during non load-shedding time). This is a more realistic scenario but the gains from using solar will have to be carefully calculated if grid power for the consumer is cheaper than solar power.

Returns under different scenarios

Considering these factors, we have estimated the returns from partial substitution of diesel with rooftop solar for a 100 kWp system under varying proportions of diesel substitution (10% diesel substitution means 10% of solar power will substitute diesel, and the remaining 90% will substitute grid power), price, and diesel cost escalation. The results from our analysis are given below.


Capital cost(Rs. Lakhs)

Diesel price escalation (%)

Diesel substitution (%)

Project IRR

Payback period (Years)



















Both the likely and optimistic scenario offer attractive returns; the pessimistic scenario offers a fairly good IRR but has a rather long payback period.

Please note that these calculations are only indicative in nature and are based on a number of assumptions. We urge you to calculate the returns for your plant based on the constraints and features specific to your installation.

Solutions to support different loads

Due to rooftop space constraints or issues with integrating diesel and solar power, you might be faced with limits on the kind of load or extent of load that can be supported. Many innovative solutions are being implemented, and they can be broadly classified into two approaches

Light loads

In this solution the rooftop solar system is used to support non-critical loads that don’t require heavy starting current, such as lighting. Such a system requires the light points to be wired through a separate circuit that can be powered only through solar.

This solution is favoured by those who are severely restricted in the size of rooftop plants they can install.

Critical loads

Here the critical loads that must run continuously, even during a grid failure, are identified (such as a portion of the plant, or a critical machine) and their electrical circuit is isolated from the rest of the plant’s circuitry, sometimes through a separate feeder. The rooftop solar plant has a battery backup and is the primary source of power for the critical load, keeping it running even during grid failure. Any excess power generated by the panels is first used to charge the batteries, and then supplied to the rest of the plant. Any shortfall in power (perhaps due to an overcast day) is made up through a diesel generator.

This system is essentially a solar powered industrial UPS and is favoured by those running critical loads that cannot wait for a diesel generator to start up.

Can rooftop solar power replace all of my diesel consumption?

Typically, no; it might be possible in some cases based on factors such as the nature of the load, timing of load shedding, etc. but for many commercial/industrial units 100% replacement of diesel power with rooftop solar is unlikely, while partial replacement of diesel consumption is possible.

  • Even with pessimistic assumptions, a rooftop solar plant can generate 16.7% IRR in returns from savings over diesel
  • Due to issues with integrating diesel and solar power we recommend limiting the rooftop solar PV plant size to 25% of the diesel genset capacity, subject to some vendor specific solutions
  • The extent of diesel abatement that is possible also depends on the timing of load-shedding. If grid power is available when the solar plant is generating power, then it is grid power, not diesel power, that is being abated
  • Rooftop Solar PV is not likely to entirely replace diesel power in your organisation. It can abate diesel consumption by taking up part of the load

To know more on whether rooftop solar would replace diesel entirely, see here
1Load factor refers to the demand load/capacity. Therefore, for a load of 400 kW and a DG capacity of 1000 kVa (800 kW) the load factor becomes 50%.
2 Generator suppliers usually specify a minimum load factor below which the life of the lubricant, generator efficiency fall and the engine starts to slobber (emit black smoke).