Hello PV Experts!We all know the calculation for PV system sizing. We always talk about ENERGY (kWh). Maybe this is due to the idea of net metering. Even when we talk about Off Grid systems... we start with the Needed Energy.

I wonder why don't we just look to the needed POWER. Why should we focus on the consumed energy during a time interval if the instantaneous power cannot be generated to take of or keep the devices running.

Thanks for all your feedback!

Hello Firas. The answer to your question actually lies in your last statement. We must look at the instantaneous power requirements for off grid systems especially. However, that factor is usually reserved for the battery inverter that is providing power when the grid goes out or in a stand alone system.

Solar PV inverters generally do not provide the amount of instantaneous power required for loads for a sustained amount of time. We only have so much sun and when you look at the power curve for a typical day 95% of the system power is produced between the hours of 10am and 3pm. Most house load profiles do not match this curve as the loads are usually highest in the morning when people are getting ready for work/school and then at the end of the day when everyone gets home. Both of these time periods are outside the solar window so the PV probably won't be able to sustain the power required by the loads.

We use kw/hr because it is a much more predictable measurement by which to size systems and unless the solar system is extremely large (probably not very practical for small homes with small roofs) the solar cannot sustain the loads.

Dear Greg,
Thank you very much for your detailed answer.
You are totally right about the mismatch between the Max solar irradiation and the the energy consumption peak hours. But here is a different case: in hot countries, the middle east for example, the max irradiation will give you maximum power to run fans or small AC's. This will be needed at home, at school, at office..
The irradiation might be somehow more stable during a day comparing to Europe or America.
On the other hand, batteries, although indispensable, are something you need to replace very frequently. Even for the deep cycle batteries. Here I am also talking about long hours of electricity cut outs, particularly during the day. And the batteries become the most expensive block in the system and people get more reluctant to use PV system eventually.
In fact, I am trying to build a PV system including an inverter/charge controller which combines charging the batteries from available source (AC mains if not the PV when possible) and delivering AC output to the selected load in case of mains cut out.

You comments are highly welcomed!

Firas,
Energy is a more general and comprehensive parameter related to both power and the time this power is in use. Either power or its usage time can affect the final consumed energy and this is what a PV system shall be designed for, with uncertain and variable power over a variable time. For guaranteed instant enough power in an off-grid system you shall have batteries and in on-grid system there is no guarantee as it depends on irradiation level.
By the way, the system you are going to build actually is an off-grid inverter so what you are looking for?

What is a typical array size for a 60,000 sq.ft. roof? Panels must lay nearly flat with about an 8 degree tilt facing magnetic South, and I need to estimate walk-around space. I intend to use 245Wp p-c Si modules of 16% efficiency.

Dear Babak,
When you have plenty of sunshine with almost no variation, and people fed-up with batteries problems they don't want to hear about the Energy philosophy. This is the problem!

Firas,
Sunshine is powerful but intermittent and weather dependent while we used to get power from a reliable and stable source like a power cable fed by a distribution company. don't forget that most of solar power being generated now around the world is fed to an already established base load stable system using fossil fuel and without them a stable power network cannot be established with just renewable energies

Dear Sir,
Thanks for your comments and suggestions. Please help us for following;

We want to operate of Single phase welding machine and Drilling machine at our site.
Where there is no Grid power is available

a. Single Phase wielding machine - 1 no.
Maximum current 10 to 12 amps
b. Single Phase Hand drill machine - 800w
We will use any one unit at a time.

This systems has to work 9am to 4pm of the day.

Please suggest the system with protections.
If possible, kindly send us one line diagrame

Thank you sir

Regards
Cyril Samuel
+91 8897 321 718

Firas,

Solar production is not constant through out the day even it is a very bright sunny day.This happens because solar depends on insolation(sun light) , temperauture, losses like mismatch, wiring, dust etc., If you run your loads with instantaneous production from solar panel, the life of motor/ lamps/ other loads fluctuates as per production and reduces the life of loads. So you need to have atleast requirement nominal battery/grid connection to support constant input to your day loads.

I want to make it clear, Energy is time integral of power.So we always mention production/ consumption in time as energy(KWH/Units).

Dear Cyril,

Where is your work located? What about using a diesel Generator? Are you working in a protected area, I mean you cannot use any generator with harmful emissions? Or would there be a problem with the diesel supply? If not, maybe you should consider this solution.
If the only solution is an RE one, then give us your location so we could check the weather and irradiation data. We are all here for help.

Dear Praveena,
first, you have a nice name PV ;)
Thank you for your contribution. If you read my first reply, you would see that the system will in fact contain batteries. But, concerning the fluctuations due to solar radiation, soiling and what so ever.. I think that we can set up a margin and oversize the modules to compensate for any drops. Batteries are indispensable for night or winter. We are talking about a kind of APS. Now, if the batteries are to compensate for low production of PV modules, then the inverter should be able to "mix" the power coming from the modules with that produced from the batteries, right?