Essential Electrical Equations and Conversions — PV System Design

A set of vocabulary flashcards summarizing equations, unit conversions, PV string sizing, and protective-device guidelines from Sean White’s Solar lecture.

Power (P)

Electrical rate of doing work; calculated as Voltage × Current (P = V × I) and measured in Watts.

Voltage (V)

Electrical potential difference; calculated as Power ÷ Current (V = P ÷ I) or Current × Resistance (V = I × R), measured in Volts.

Current (I)

Flow of electric charge; calculated as Power ÷ Voltage (I = P ÷ V) or Voltage ÷ Resistance (I = V ÷ R), measured in Amperes.

Resistance (R)

Opposition to current flow; calculated as Voltage ÷ Current (R = V ÷ I), measured in Ohms (Ω).

Ohm’s Law

Fundamental relationship stating that Voltage equals Current times Resistance (V = I × R).

Energy (E)

Electrical work done over time; calculated as Power × Time (E = P × t) and measured in Watt-hours (Wh).

Watt (W)

Unit of power equal to one Joule per second; derived from Volts × Amps.

Watt-hour (Wh)

Unit of energy representing one Watt of power sustained for one hour.

Kilowatt-hour (kWh)

1000 Watt-hours; common billing unit for electrical energy (kW × h).

Amp-hour (Ah)

Battery capacity unit representing one Ampere of current supplied for one hour (A × h).

Energy storage calculation

Total stored battery energy found by Volts × Amp-hours (Wh = V × Ah).

Standard Test Conditions (STC)

PV module rating conditions of 25 °C cell temperature and 1000 W/m² irradiance.

Voc (Open-circuit voltage)

Maximum module voltage under no load; basis for cold-temperature string sizing.

Vmp (Maximum-power voltage)

Module voltage at peak power output; basis for hot-temperature string sizing.

Delta T (ΔT)

Temperature difference between actual operating temperature and STC; used with voltage coefficient to adjust module voltage.

Voltage temperature coefficient

Manufacturer-specified %/°C value indicating how module voltage changes per degree of temperature change.

Longest string calculation

Uses cold-temperature Voc to ensure series string voltage stays below inverter maximum DC voltage.

Shortest string calculation

Uses hot-temperature Vmp to ensure series string voltage stays above inverter minimum (low) operating voltage.

Inverter maximum voltage

Highest DC voltage an inverter can accept; limits modules in series for cold conditions.

Inverter minimum (low) voltage

Lowest DC voltage required for inverter operation; determines minimum modules in series for hot conditions.

DC PV fuse sizing

Select fuse by multiplying PV short-circuit current (Isc) by 1.56 and rounding up.

General fuse/equipment minimum sizing

For non-PV circuits, multiply rated current by 1.25 and round up to choose fuse and conductor ampacity.

Kilo (k)

Metric prefix meaning 1000 (10³).

Mega (M)

Metric prefix meaning one million or 1000 k (10⁶).

Giga (G)

Metric prefix meaning one billion or 1000 M (10⁹).

Milli (m)

Metric prefix meaning one-thousandth (1/1000 or 10⁻³).

Percent-to-decimal conversion

Convert a percentage to decimal by dividing by 100 (e.g., 50 % → 0.5).

Wire sizing resource

Detailed conductor sizing guidelines available at solarSEAN.com/wiresizing.