Updated: Dec 8, 2020
The fuel for a solar power plant required to generate electricity in sunlight. And generators like diesel generators, petrol generators or biomass generate required their own specific fuel to generate electricity. Also, there are solar generators which are popular in some countries, which are basically portable off-grid power plants with battery backup.
Can Solar power plants have generator backup? And which type of solar inverters support generator backup?
What is the type of solar power plant, is determined by;
type of inverter used in it (micro-inverter, string inverter, off-grid inverter or hybrid inverter),
connected with grid or not (on-grid type or off-grid type),
have battery backup or not
So, what type of solar power plant can have generator back up and which type of solar power plants required generator backup.
Both on-grid type and off-grid type solar power plant can have generator backup, but the requirement of a generator back for both is completely different.
On-grid [grid tie] solar power plant with generator backup.
On-grid solar power plant requires a reference power source for the generation of electrical power in synchronization with the reference power. Gird power or Generators can be used as a reference power source.
Any solar inverter which can be controlled by an export limiter (also known as zero export device or solar export device) are suitable for generator backup. With the available technology, only On-grid type inverter (also known as String Inverter) and hybrid inverters are commutable with an export limiter.
Whereas micro inverters and off-grid type inverters both aren’t compatible with an export limiter.
Places where the repetition of power failure is high (like above 5-10 hours per day) or where grid power is not available. Are the most suitable place for the on-grid type solar power plant with generator backup.
Also, if the load is domestic (like lights, Fan and AC) and continuous load is not much higher than 3 kW. Then going on-grid solar with the generator isn’t advisable. In this case, an Off-grid solar power plant with a suitable rating of battery backup is more suitable.
But both grid power and batteries are out of the equation, a gird-tie solar power plant with generator backup is a suitable option.
If you already have a generator and you what to sync it with On-grid solar power plant. Then export limiter (also known as Zero Export Device) is requiring for the same.
Export limiter limits the power generations of solar inverter, so that surplus power (when power generation > connected load) can’t be generated from the solar power plant.
Otherwise, surplus power will trip the breaker of the generator. And, if the breaker isn’t available it (surplus power) will burn down the armature coil of the generator. Learn more about how to operate the generator with the grid-tie solar power plant.
Backup generator size for On-grid [grid tie] solar power plant
Generator size should be around 1.25 time of the continuous load. This is because the generator needs to serve load, while sunlight isn’t available or solar inverter/plant is in maintenance.
Solar Panels and Inverter rating
If the frequency of power failure isn’t much high and you want to sync your generator with the on-grid plant, then the plant size doesn’t matter. You can have any size of the plant.
But if the rate of failure of grid power is very high or gird power isn’t available. Then the solar plant size should not higher than 1.2 times of the continuous load.
If the plant size much higher than the continuous load, you won’t get full utilization of the plant and lots of power generations will lose in the process of export limitations.
Which make of on-grid [grid tie] inverters are compatible with export limiters?
All major grid-tie solar inverters like ABB, SMA, Hitachi, Consul Neowatt, Huawei, Solar Edge, Kaco, Detal, Solis, Kirloskar, polycab, Sungrow, Growatt, Fronius, REFU Sol, Schneider, Zever solar and many more are compatible with Export limiter [a.k.a Zero Export Device].
Can I use a large backup generator for on-grid solar power plant?
Generator size should not be higher than 3.3 times of the continuous load. This is because continuous running generator at load lesser than 30% of its rated capacity effect it negatively.
Learn more, about the effect of continuous running of generator at load less than 30% of its rated capacity.
Can I use a smaller size of backup generator for On-grid solar power plant?
Generator should be capable of taking load when the sunlight is not available (night and heavy raining days). Generators consumes more fuel at overload, ideal load on generator should not higher than 80% of its rated capacity.
Advantages of On-grid solar power plant with generator backup
Independent of grid power availability. But can also work with Grid too, learn more.
Reduces the fuel consumption of the generator, up-to 75%. Generators consume less fuel per hour at low load.
Increase the life span of the generator. Operating Generator at low load (not less than 30%) increase its life span and reduce its maintenance cost.
No requirement of battery backup.
Highly profitable for factory load, where having battery backup is not possible due to high inrush current. And generator is the only option available during power cuts.
Disadvantage of On-grid solar power plant with generator backup
Not, economical for the lower size of plant with domestic loads.
Maintenance cost of the generator increases the O&M cost of Plant.
If the new generator is brought for the plant, it will increase the ROI period of the investment on plant + generator.
Off-Gird solar power plant with generator backup.
Off-grid solar power plant means, gird isn’t connected with the plant. Off-grid type of solar inverters are used in this type of solar power plant.
Off-grid solar inverter store the power generated from solar panels into battery (or bank of battery) via off-grid solar inverter and load (appliances using energy) use this stored power in battery via again off-grid solar inverter.
Or in simple words, off-solar inverters charge batteries from solar panels and once the battery is charged at a certain percentage (like 20% to 80%) as per settings done in Inverter, it starts generating AC power for the load from charged battery meanwhile battery keep charging as long as sunlight is available.
If the sunlight isn’t available and batteries can’t get charged. But you can charge batteries by the mean of an external power source. Both gird power or generator can be use as external source of power. Most of the off-gird solar power plants have facility to charge batteries with gird power or generator.
Don’t forget to check the same purchasing off-grid solar inverters for your solar power plant.
While charging batteries, external power source will also serve power to the connected loads.
As long as external power source is connected with off-grid solar inverter, batteries will keep charging and no power can be drawn from them.
Once the external power source is disconnected, batteries will start powering the connected loads.
Whole working process of off-grid solar power plant in absence of sunlight is like, the working process of a normal home inverters.
Backup generator size for Off-grid solar power plant
Generator size should be compatible for powering continuous load as well as charging of the battery.
In this case, there is no such ideal size of the generator. If the generator size of small, but enough to provide minimum power required to charge battery as well as powering the continuous load. Then it will take some time to charge battery but can take care of the load as long as fuel is available.
And if you have a large size of generator, then it will take less time to charge batteries.
Battery required minimum 10% current to get charged. But since there are power losses involved in between generator, solar invert and batteries. So, the minimum current required for the charging of the battery is 12 to 13%
Minimum size of generator to backup off-grid solar power plant is equal to continuous load + 12% of battery load (in kW or W).
For example, if the continuous load is 800 Watt (or 0.8kW) and the connected battery is 24V, 80Ah. The Minimum continuous power required to powered load as well as charge battery, will be equal to 800 + 12% of (24 x 80) = 1030.4 Watt.
Don’t forget to considered efficiency of the off-grid solar inverter.
Minimum size of generator > (continuous load + Battery Load)/Inverter Efficiency
Considering minimum or small size of generator will keep generator loaded 100% or above all the time and this may heat up.
Suitable size of the generator is ≥ (continuous load + Battery Load)/Inverter Efficiency.
Can I Use a Larger backup Generator for off-grid solar power plant?
A larger generator can be beneficial if you have heavy loads like an air conditioner or water pump. As I already mentioned, it recommended to run generator at minimum load of 30% of generators rated capacity.
(continuous load + Battery Load)/Inverter Efficiency ≥ 30% of the Generator rating.
Advantages of Off-grid solar power plant with generator backup
Increase the life span of battery, by keeping them charged during high load demands and less sunlight availability.
Generator backup reduces the chance of complete blackout.
Generator backup, provide power source during servicing of battery or plant.
Disadvantage of Off-grid solar power plant with generator backup
Generator backup increase the cost of the system.
Required, storage of fuel for the generator.
Required, time to time maintenance of the generator.
Here is my guru mantra to choose the right system, don’t choose the system but choose the disadvantages you can live with.
If you have any question, please leave a comment below.