Lý thuyết điện & an toànElectrical theory & safety · BàiLesson 1/6
Chapter 01 - 01 Electrical Foundations for Solar PV Systems
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Solar Knowledge HubNền tảng điện cho hệ điện mặt trờiElectrical foundations for solar
Gần như mọi quyết định trong một hệ, từ chọn tiết diện dây tới lý do nâng điện áp chuỗi pin, đều quy về hai công thức điện học cơ bản.Nearly every decision in a solar system, from wire sizing to raising string voltage, comes back to two basic formulas.
Cả U và V đều chỉ điện áp, cùng đơn vị volt, chỉ khác thói quen tài liệu. Chuẩn IEC (tài liệu kỹ thuật điện, tủ điện) hay dùng U; còn datasheet biến tần, pin, tấm pin quen dùng V. Gặp ký hiệu nào dùng theo ký hiệu đó, bản chất không đổi.Both U and V mean voltage, same volt unit: only the documentation habit differs. IEC electrical docs use U; device datasheets use V. Use whichever symbol you meet: the math is identical.
Tổn hao nhiệt tỉ lệ bình phương dòng (P = I²R). Dòng nhỏ hơn 8 lần → tổn hao trên cùng sợi dây nhỏ hơn khoảng 64 lần. Đó là lý do hệ hiện đại nâng áp chuỗi pin lên hàng trăm–nghìn volt.Heat loss scales with the square of current (P = I²R). 8× less current → about 64× less loss on the same wire: why modern systems push string voltage to hundreds–thousands of volts.
| Thành phầnComponent | Liên hệ V-I-R-PV-I-R-P link | Rủi ro khi R tăng / I quá lớnRisk if R rises / I too high |
|---|---|---|
| Tấm pinPV panel | Tạo ra điện áp & dòng một chiềuProduces DC voltage & current | Dòng quá lớn → tổn hao, sụt ápExcess current → loss, voltage drop |
| Dây một chiềuDC cable | Có điện trở gây sụt áp = I × RResistance causes drop = I × R | Sụt áp → hao điện, giảm hiệu suấtDrop → wasted energy, lower efficiency |
| Đầu nối MC4MC4 connector | Tiếp xúc kém làm điện trở tăngPoor contact raises resistance | Nóng giắc, cháy đầu nối, mất công suấtHot, burnt connector, lost power |
| Biến tầnInverter | Điều khiển điện áp, dòng, công suấtControls voltage, current, power | Quá dòng / quá áp → lỗi, hỏngOver-current / -voltage → fault |
| Pin lưu trữBattery | Sạc xả theo điện áp & dòngCharges/discharges by V & I | Quá dòng → nóng pin, giảm tuổi thọOver-current → heat, shorter life |
| Cầu chì & aptomatFuse & breaker | Bảo vệ theo ngưỡng dòngProtect by current threshold | Quá dòng không cắt kịp → cháyUncleared over-current → fire |
Thuộc một phép tính nhanh: I = P ÷ V. Tải 5kW trên hệ pin 51.2V kéo dòng ≈ 98A: con số này quyết định cỡ dây và cỡ aptomat.Memorize one quick sum: I = P ÷ V. A 5kW load on a 51.2V battery draws ≈ 98A: that number sets wire and breaker size.
Nearly every decision in a solar PV system (from choosing cable cross-section to the reason for raising string voltage) comes down to two basic electrical formulas. This document presents those two formulas, clarifies a common notation confusion, and shows why the industry consistently pushes for higher voltage and lower current.
For: newcomers who need a solid grounding, technicians who use it for quick calculations, and sales staff who need to explain the principles to customers.
Quick Summary
Ohm’s law states that voltage equals current multiplied by resistance, written concisely as V = I × R. Power equals voltage multiplied by current, written as P = V × I. The four quantities (voltage, current, resistance, and power) are linked through formulas derived from these two.
The symbols U and V both refer to voltage and are completely equivalent; the difference is purely a matter of documentation convention. European-standard electrical engineering documents tend to use U, while electronic device datasheets and international documents tend to use V.
Heat loss in a cable is proportional to the square of the current. Therefore, at the same power level, raising the voltage to lower the current reduces losses very rapidly, keeping cables cooler, the system more efficient, and operation safer.
The Two Foundational Formulas
Ohm’s law describes the relationship between three quantities in an electrical circuit. The voltage applied across a resistor equals the current flowing through it multiplied by the resistance value, that is, V = I × R. From this we derive: resistance equals voltage divided by current, and current equals voltage divided by resistance.
Power is the rate of energy consumption or transfer, equal to voltage multiplied by current: P = V × I. Combined with Ohm’s law, two additional useful forms emerge. Power equals the square of the current multiplied by resistance: this form explains resistive heat loss in cables. And current equals power divided by voltage: this form is used constantly when calculating the current drawn by a load or by a PV string.
The four quantities (voltage, current, resistance, and power, abbreviated V-I-R-P) form the framework around which all calculations in a solar PV system revolve.
Customers don’t need to memorise the formulas, but a consultant should know one quick calculation by heart. Current equals power divided by voltage. A 5 kW load on a 51.2 V battery system draws approximately 98 A, a figure that determines cable sizing and circuit breaker rating, and is exactly why high-load systems typically opt for a higher battery voltage.
U or V: They Are the Same Thing
A frequently asked question is when to use P = U × I versus P = V × I. The short answer is that both formulas are identical, because U and V are both symbols for voltage, measured in the same unit: volts.
The difference is purely a matter of presentation convention. The international IEC symbol standard uses U for the voltage quantity, so electrical engineering documents, panel wiring diagrams, and industrial electrical work in Vietnam and Europe commonly show the letter U. Electronic device datasheets, inverters, batteries, and solar modules, on the other hand, conventionally use V, following English-language documentation. Whichever symbol you encounter, use it as written: the underlying calculation is unchanged.
Why the Solar Industry Raises Voltage and Lowers Current
This is the most important design principle derived from the two formulas above. The resistive heat loss in a cable segment equals the square of the current multiplied by the cable resistance. Because current appears as a square term, halving the current reduces losses to one quarter.
Take the same 1,000 W of power as an example. On a 48 V system, the current is 1,000 ÷ 48 ≈ 20.8 A. On a 400 V system, the current is only 1,000 ÷ 400 = 2.5 A. A current eight times smaller means resistive heat loss in the same cable is approximately sixty-four times lower. The cable runs cooler, voltage drop is reduced, the risk of arc faults is lower, and equipment lasts longer.
This is why modern solar PV systems raise string voltage to hundreds (and in large installations, over a thousand) volts on the DC side, and why battery storage is available in both low-voltage and high-voltage variants depending on load scale, a topic covered in document 01-05.
Ohm’s Law in Each Component of the System
Every part of a solar PV system is a direct application of V-I-R-P. The table below shows what happens when resistance rises or current becomes excessive.
| Component | V-I-R-P relationship | Risk when R increases or I is too high |
|---|---|---|
| Solar module | Generates DC voltage and current | Excessive current causes losses and voltage drop |
| DC cable | Has resistance causing voltage drop equal to I × R | Voltage drop wastes energy and reduces efficiency |
| MC4 connector | Poor contact increases resistance | Hot connector, burned terminal, power loss |
| Inverter | Controls voltage, current, and power | Overcurrent or overvoltage causes faults and damage |
| Battery storage | Charges and discharges according to voltage and current | Overcurrent heats cells and shortens service life |
| Fuse and circuit breaker | Protects based on current threshold | Overcurrent that is not interrupted quickly causes fire |
A loose MC4 connector leading to a fire is a clear chain of cause and effect, fully explained by the formula. Poor contact raises the resistance at the joint; heat dissipated at that point equals current squared times resistance, so it surges; the heat causes heavy oxidation that increases the resistance further, creating a runaway feedback loop that ends in a burned connector. This is why correct torqueing and the use of matched, approved connectors is a small step with life-critical consequences.
Ohm’s law and the power formula per All About Circuits and Electronics Tutorials. The U symbol per IEC 60027, adopted in Vietnam through TCVN 7447-1:2010 (equivalent to IEC 60364), while V follows IEEE and manufacturer datasheets (Victron, Jinko Solar). The relationship of resistive loss scaling with the square of current is Joule’s law. The calculated values of 20.8 A and 2.5 A have been arithmetically verified.