Off-Grid Solar Glossary of Terms

Definitions as they are used in off-grid system design, not marketing. Where a term is commonly misused, the note explains the practical difference.

A-C

Absorption charge: The constant-voltage stage at the end of bulk charging, held until current falls to a set fraction of capacity. Too short and the battery stays undercharged; too long and it ages faster.
Albedo: The fraction of sunlight reflected from the ground. Snow raises it, dark soil and water lower it. Relevant to bifacial modules and to panels mounted over reflective surfaces.
Ampere-hour (Ah): Charge capacity. Meaningless without a voltage — 100 Ah at 12 V is 1,280 Wh, not a fixed amount of energy.
Autonomy: The number of consecutive days a system can run its load with no meaningful solar input. Set by battery capacity and usable depth of discharge, not by array size.
BMS (battery management system): Electronics that protect a lithium pack from over-charge, over-discharge, over-current and short circuit, and that balance cells. A BMS does not replace correct controller settings or fusing.
Bulk charge: The first charging stage, delivering full available current until the battery reaches its absorption voltage.

Derating: Reduction of nameplate output for real conditions — temperature, soiling, wiring and ageing. A 550 W module rarely delivers 550 W in the field.
Depth of discharge (DoD): The fraction of a battery’s capacity that has been used. Design DoD is the limit set to preserve cycle life: about 80% for LiFePO4, 50% for gel.
Duty cycle: The proportion of time a device is active. A water level sensor measuring for 5 seconds every 5 minutes has a duty cycle of about 1.7%, which changes system sizing completely compared with a continuous load.
Float charge: A lower voltage held after charging completes, to counter self-discharge without overcharging.

Irradiance: Instantaneous solar power per unit area, in W/m². Standard test condition is 1,000 W/m². An overcast winter day may be 80-150 W/m².
Insolation / peak sun hours (PSH): Daily solar energy expressed as the number of hours at 1,000 W/m². A day with 4 PSH delivers 4 kWh/m². Always size on the worst-month value.
Low-voltage disconnect (LVD): A controller function that disconnects the load before the battery is damaged by deep discharge. Set it above the lowest voltage your equipment tolerates.
MPPT (maximum power point tracking): A controller that continuously adjusts its input to harvest the array’s maximum power. Typically gains 10-30% over PWM, more when array voltage greatly exceeds battery voltage.
Mismatch loss: Energy lost when panels or cells with different characteristics are connected in series or parallel. The weakest element limits the string.

Peak sun hours: See insolation.
PERC: Passivated Emitter and Rear Contact — a cell architecture with a rear passivation layer that improves efficiency by reducing recombination.
PWM (pulse width modulation): A simple controller that clamps array voltage to battery voltage. Cheap and reliable, but wastes the difference between Vmp and battery voltage.
Round-trip efficiency: Energy out of a battery divided by energy put in. Roughly 90-95% for LiFePO4, 80-85% for lead-acid.
Soiling: Dust, sand, bird droppings or salt on the glass. In dry dusty regions, losses accumulate at roughly 1% per day without rain or cleaning.
Standby / quiescent draw: Current a device or controller consumes while idle. A controller drawing 20 mA continuously uses 5.8 Wh/day — significant on a small system.
STRING: Panels connected in series. String voltage adds; string current is limited by the lowest-current panel.

Temperature coefficient: How output changes with cell temperature. Power coefficients are typically −0.30 to −0.45% per °C; voltage coefficients are strongly negative and matter for controller voltage limits.
Tilt angle: Panel angle from horizontal. Optimum for annual yield is roughly local latitude, but 10-15° is often better in dusty regions because it sheds dirt while limiting wind load.
Usable capacity: Nominal capacity × design depth of discharge. The only capacity number that matters when sizing autonomy.
Vmp / Voc / Isc / Imp: Voltage and current at maximum power point, open-circuit voltage, short-circuit current and current at maximum power. Voc is what you check against a controller’s voltage limit; Isc is what you check against its current limit.
Voltage drop: Loss along a conductor, proportional to length, current and resistivity, and inversely proportional to cross-section. Keep array circuits under 3% and load circuits under 2%.
Watt-peak (Wp): Nameplate array or module rating at standard test conditions. A starting point for design, not a prediction of field output.

For worked examples using these terms, see the sizing reference or run the calculator.