ELEC5206 Chap.12 Designing a Grid-Tied Home PV System
Designing a Grid-Tied Home PV System
The design lecture applies the whole unit to an Australian home. A seven-step procedure runs from the specification, through the inverter and its output circuit and the PV source circuit, to the inverter input and a final review whose deliverable is a single-line diagram. A rule of thumb of about 170 W per square metre sizes the array from unshaded roof area, so a 30 m² roof in Sydney supports a 5 kW system.
String length is then checked against the inverter’s voltage window: the coldest morning raises module voltage toward the 600 V limit and summer heat lowers the MPP voltage toward the tracking floor. A second scheme replaces the string inverter with microinverters certified for Australia, trading cost for module-level tracking.
A second worked design fits a single string on a smaller roof and checks both ends of the voltage window, including an approximate hot-afternoon check that the lecture leaves qualitative.
What this chapter covers
- 01
The seven-step design procedure and the single-line diagram
- 02
Sizing the array from unshaded roof area
- 03
Specification: grid, temperatures and the 6 kW versus 5 kW discrepancy
- 04
Choosing the inverter and its output circuit
- 05
Cold-temperature string voltage against the inverter limit
- 06
Nominal MPP voltage against the tracking floor
- 07
Final review: cables, grounding, ratings and markings
- 08
The microinverter alternative and its compatibility checks
Worked example · free
Fitting one string on a 26 m² roof
- 1Area rule: 170 × 26 = 4.42 kW, so 13 modules of 330 W (4.29 kW).
- 1Cold correction: KT = 1 + (−0.0029)(3 − 25) = 1.0638, so each module can reach 41.5 × 1.0638 = 44.15 V.
- 1Maximum modules in series: 600/44.15 = 13.6, so at most 13.
- 1One string of 13: Vmax = 13 × 44.15 = 574 V < 600 V, with only 26 V of margin.
Key terms
- Single-line diagram
- The design deliverable that shows every string, disconnect, inverter input and AC connection on one line.
- Temperature correction factor
- The multiplier KT = 1 + βT(Tmin − 25) applied to open-circuit voltage at the coldest temperature.
- DC disconnect
- A switch that isolates each PV string from the inverter input, usually supplied by the inverter maker.
- Combiner box
- An enclosure that joins several strings in parallel, unnecessary when the inverter accepts each string directly.
- Area rule of thumb
- The lecture's estimate of about 170 W of PV per square metre of unshaded roof.
- MPPT voltage floor
- The lowest PV voltage at which an inverter can still track, 125 V in the lecture's design.
Designing a Grid-Tied Home PV System FAQ
Why does the coldest temperature decide the string length?
Module voltage rises as temperature falls, so the open-circuit voltage on the coldest morning is the highest the inverter will ever see. The string must stay below the inverter or code limit at that voltage, which the lecture checks by multiplying the string's open-circuit voltage by a correction factor of one plus the voltage coefficient times the minimum temperature minus 25 °C.
Why does the lecture's specification say 6 kW when the system is 5 kW?
The specification table prints 6 kW, but the array actually designed is twenty 250 W modules, the 5 kW that a 30 m² roof supports at about 170 W/m². Work from the area-based figure and point out the discrepancy rather than mixing the two.
When would microinverters be the better scheme?
When shading or module mismatch would cost a string system significant power, because each module then gets its own tracker. The design is modular and straightforward, but needs one unit per two modules in the lecture's example, with the maker's dedicated cabling, which raises cost.
Why check the MPP voltage against the inverter's lowest tracking voltage?
Heat lowers the MPP voltage, so on a hot afternoon a short string may fall below the inverter's tracking floor and stop harvesting. The lecture's string of ten modules sits at 309 V nominal against a 125 V floor, a margin wide enough to survive summer temperatures.
Exam move
Memorise the seven steps as a checklist and practise turning a roof area into a module count. Drill the two voltage checks with your own module data until both are automatic: cold maximum against the limit, nominal MPP voltage against the tracking floor. Then rehearse a short comparison of string inverter and microinverter schemes for a partly shaded roof, ending with a recommendation and the code compliance caveat.
Practise drawing the single-line diagram for both schemes from memory.
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