At Solar Mango we repeatedly remind our clients and our readers about the importance of sizing a rooftop PV plant correctly when integrating with a diesel generator, as the solar plant can affect the efficiency of the DG. A poorly sized PV plant can cause greater diesel consumption than before, despite supporting part of the load. We recently performed an analysis on PV sizing for a client that could serve as an interesting case study on Solar PV plant sizing when integrating with a Diesel Generator.

Why integrate Solar PV with Diesel?

Solar PV plants need to be integrated with a diesel generator because the solar plant may not be able to support the entire load all of the time. This may be due to

  • Infirm power – The power output from a solar PV plant is dependent on sunlight and is therefore not constant
  • Restricted roof space – The capacity of solar plant that can be installed on the available roof may not be sufficient to support the entire load
Understanding Diesel Generator Efficiency

The efficiency of a DG is not constant; it varies with the load imposed. The generator is most efficient when the load it supports is near its rated capacity, and drops as the load decreases. The fuel economy of the DG varies with its efficiency; the kWh/litre decreases as the efficiency decreases.

This is an important consideration when integrating a PV plant with a diesel generator as the solar plant supports part of the load, causing a decrease in load supported by the diesel generator accompanied by a decrease in fuel efficiency. This decrease in efficiency need not be linear. Fuel economy may decrease only slightly as the load starts decreasing, and then plunge beyond a certain point.

Each DG has its own efficiency curve, known as specific fuel consumption. This information should be provided by the DG manufacturer for the model that you have installed.

Sample DG Capacity and Efficiency

Let us analyse the efficiency of a sample 100 kVa (80 kW at power factor = 0.8) diesel generator

Load Efficiency
Full load 4.00 kWh/ltr
70% load 3.80 kWh/ltr
60% load 3.20 kWh/ltr
50% load 2.20 kWh/ltr
25% load 0.90 kWh/ltr

Table 1: Sample DG Load and Fuel Consumption

We can see that in this case the fuel consumption only decreases slightly until about 60% load, and then reduces significantly.

With this data we can create varying load scenarios to assess the impact of the solar plant on the genset’s fuel consumption.

Solar Plant Capacity 0 kW 25 kW 40 kW 50 kW 75 kW
Effective solar capacity (80%)* 0 kW 20 kW 32 kW 40 kW 60 kW
kWh contributed by solar in one hour 0 20 32 40 60
kWh contributed by DG after introduction of solar 80 60 48 40 20
Load on DG after introduction of solar 100% 75% 60% 50% 25%
DG fuel consumption after introduction of solar (litres) 20.00 15.79 15.00 18.18 22.22

Table 2: DG fuel consumption (one hour) under different scenarios

* The capacity of the solar PV panels is rated under Standard Test Conditions. As these conditions may not hold true at the implementation site, 80% of the panel capacity is considered to be the effective plant capacity

The table illustrates that in these conditions, a 40 kW PV plant returns the least diesel consumption (15 litres) while a larger, 75 kW solar PV plant would cause diesel consumption to increase to 22.22 litres which is more than the diesel consumption before introduction of solar!

Please note that the above information is illustrative. Actual load/diesel consumption of DG at your site should be used to verify DG’s efficiency.

PV plant sizing should also consider other factors, such as ability of DG to handle reverse current flows. Solar PV plants are typically restricted to about a third of DG set capacity.

Case Study on PV plant Sizing for a Software Company

We recently performed an analysis on the impact of 80 kW solar plant on the fuel consumption of a 380 kVa diesel generator for a client. The data we gathered included

DG Details   Solar PV Details  
DG Capacity (kVa) 380 Plant Capacity (kW) 80
DG Capacity (kW, 0.8 power factor) 304 Effective Plant Capacity (kW) 64
% of load on DG before solar 98.36% Solar kWh/kW/day 4
No. of working days per month 22 Estimated solar kWh substituting diesel per day 58.54
Average diesel consumption per month (litres) 2,000
Average diesel consumption per month during solar hours (litres) 1,900
Average diesel fuel efficiency during solar hours (kWh/litre) 3.8
Average hours of load shedding per day during solar hours 1.10
Litres consumed per day during solar hours 86.36


In this scenario we estimated diesel savings of 12.61%. A summary of our analysis is provided below.

  Before Solar After Solar
% of DG Load during solar hours 98.36% 77.30%
DG fuel efficiency during solar hours (kWh/ltr) 3.8 3.6
kWh generated per day during solar hours
         DG 328.18 269.64
         Solar 58.54
Diesel consumed per day during solar hours (ltrs) 86.36 74.90
Diesel consumed per month during solar hours (ltrs) 1,900.00 1,647.82
Diesel consumed per month during non solar hours (ltrs) 100.00 100.00
Estimated diesel savings per month after introducing solar   12.61%


As the client has a 380 kVa generator, a larger solar plant can be installed without causing reverse current issues. A 100 kW solar plant (keeping within the one-third limit) will increase the diesel savings to 17%, but due to rooftop space restrictions the solar plant capacity had to be limited to 80 kW.

The sizing of rooftop solar PV plants can become more complex if the client has more than one DG. Depending on the load, it may be possible to increase the proportion of solar PV beyond one-third of the combined DG capacity if one DG can be turned off completely and the other DG can run near its optimum efficiency level.


In this particular case, savings of 12.61% on monthly diesel consumption is possible. We were able to perform this analysis because the client maintained detailed records on fuel consumption and load.

Each installation is different and solar solutions have to be tailored to the conditions at each site. We always recommend that clients seeking diesel saving solar solutions should analyse their consumption and requirements carefully to avoid consuming even more diesel than they did before installing the solar plant.