Small Solar Systems

nilufery Registered Users Posts: 1


I am new graduated electrical and electronics engineer and working a company which Works on solar and wind energy systems.  My boss wants me to design some small solar systems on roof. (like 1 kW, 2kW, 5 kW, 10 kW and 25 kW). These systems will be off-grid or grid-tied according to what client wants.

 For an off-grid system my equipments will be, pv panels, inverter, voltage regulator and accumulator. How should i choose these equipments. What voltage values should my equipments(panel,inverter,accumulator) be? I mean, they should be compatible with the others. Can you explain me how to decide that?

 My second question is about grid-tied systems. If the house needs more power than the PV system can supply, then the extra is drawn from the grid as usual. Does inverter do that job? What kind of inverter should i choose? Do i need any other equipment?

 If you can help me, i would be very appreciate. Because i am new at this job and i want to learn more and improve myself.


Thank you from now


  • BB.
    BB. Super Moderators, Administrators Posts: 33,406 admin
    Welcome to the forum Nilufery,

    Some of these design issues are specific to your location--I think you are in/around Turkey?

    So--Studying local building and electrical codes is important. I know when I got out of engineering, the school never even mentioned codes, standards for cabling/wiring, standard practices, etc. In North America, the National Electric Code is used pretty widely (there are others).,aps,318

    You want to design systems that are safe and can be installed by local electricians--Knowing what will be in common usage (parts, wiring, breaker panels, etc.) will help you a lot. If you can go on a job and watch (or even be a "helper")--Practical experience.

    Regarding Grid Tied systems... Those are solar panels=>Grid Tied/Utility Interactive Inverter(s)=>AC branch circuit=>main breaker panel (typically). A GT inverter is (typically) a "current mode source" that follows the AC voltage wave form--And for all the world looks like an "AC Generator" that is "Charging" a giant "AC Battery". The GT inverter "injects" current into the AC mains--Some of the energy may be used locally, and the rest of the energy goes "backwards" through the utility meter--And turns it backwards (simple setup). There can be other options (depends on country/utility) where you may install two meters--One for the house and a second for just the solar system (depends on size of system, the utility billing/tariff plans, etc.). But the thing to understand is that the GT inverter does not "support" the local loads--It just "charges" the utility grid--The Utility grid sets the local line voltage and line frequency and the GT inverter just "follows" the grid (the utility grid is a "voltage source" which effectively powers the loads--The GT inverter does not set voltage/frequency).

    Note for "typical" grid tied systems, if the utility power fails, the GT inverters turn off (for safety) and the customer has no power.

    If you want power during an outage, you need a local battery bank/accumulator + an Off Grid AC inverter + AC transfer switch to run some local AC loads.

    There are variations available. There is one company (SMA out of Germany) that makes a GT inverter than supply ~1.5 kWatts or so when there is full sun on the array (a little backup AC power). And there are "hybrid" inverters (plus battery bank) that acts like a GT inverter when the utility is up (excess solar power is sent to the utility) and like an off grid inverter when the utility has failed (supply local AC loads from battery bank+inverter).

    For all the world, the GT inverter + Utility Grid looks almost exactly like a giant automotive electrical systems. You have the Battery (accumulator) that sets the "12 volts" for the car electrical system, runs the ignition, computers, lights, radio. And the Alternator in the car "recharges" the battery when the motor is running. If the motor is running slowly, and you have lots of loads (lights, heater, etc.), the battery and alternator will "share" the loads (battery is discharged slowly as the alternator, a current source, shares some of the current).

    I am in the United States, so the brand/model of equipment I know about will be different for your location--We do have folks from other parts of the world here--And they may be able to help.

    When designing a system (whether grid tied or off grid)--Understanding the customer's needs (grid tied, off grid, hybrid inverter system), their loads (AC and DC, by season).

    Grid Tied systems--You need to figure out how many kWatt*Hours per month they use, and size the system to (on average) give them a (rough) "net zero" billing at the end of the billing cycle. These calculations can be greatly affected by utility rules. Some utilities do not allow GT connections. Others may have Time of Day and Seasonal Billing differences (I get $0.12 in the summer mornings, and $0.30 per kWH summer afternoons/evening). GT power can be cheaper than utility power--But the whole set of calculations are (in the US and Canada) very much based in local politics.

    For off grid systems--Very roughly, Off Grid power is ~10x the cost of utility power--So if a customer has reliable utility power, it almost never makes economic sense to install an off grid power system (cost of batteries, conversion losses (AC->DC->AC, etc.), batteries that have ~5-7 year life, charge controller+inverters that have a ~10+ year life, backup genset for bad weather, etc. cause Off Grid Systems to be very expensive). However, if the customer has unreliable power or lives miles from utility power, then an Off Grid or Hybrid system can make sense.

    Because off grid power is so expensive, customers need to understand their loads and power needs. It almost always costs less to do conservation (insulation, LED lighting, most efficient appliances they can find, delay some loads when there is bad weather, guse propane/oil/etc. for cooking and hot water (or solar hot water), etc.).

    We have some links to books, and various other sources that can help (plus some common answers to questions):

    I suggest that you start with a specific design and go through the process... It will help you understand the basics. It is very difficult to try to learn "everything" at once (in English, we talk about "drinking from a fire hose").

    Pick something (Grid Tied or Off Grid), and ask the questions here. I would suggest that you start with your own home/apartment. Get a Watt*Hour meter and a copy of a couple electric bills (perhaps a summer/winter bill if they are different).

    Another helpful option is a Current Clamp DMM (digital multi meter)--There are AC Clamp Meter versions (cheap, accurate) that are handy if you only work AC power systems. If you are going to do DC systems (with batteries/accumulators), then an AC/DC Current Clamp DMM is very nice for diagnosing problems & understanding how a DC power system works. DC current clamp meters are more expensive and a bit "different" in how to use them (there is a "zero meter" function for DC current, and there is a "direction" to current flow that you don't worry about for AC systems). Typically a current clamp meter will have 40 and 400 Amp ranges (good enough for our needs). (Fluke meters are very good, but very expensive, you can find other DC clamp meters that are "good enough" for your needs for much less)

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset