Off-grid electrical glossary

Definitions

Depth of Discharge (DoD)

Depth of discharge (DoD) is the share of a battery’s nameplate amp-hours you actually use before recharging. Usable watt-hours are Ah × V × DoD. LiFePO4 house batteries are commonly used to about 95% DoD. Flooded or AGM lead-acid is often limited to about 50% for cycle life (sometimes up to about 80% in deep-cycle or emergency use). Lower DoD means fewer usable watt-hours from the same Ah label.

C-rate

C-rate is current relative to rated capacity. 1C on a 100 Ah battery is 100 A; 0.5C is 50 A. Maximum continuous current is C-rate × Ah. Load C-rate is load amps ÷ capacity (load amps themselves come from I = P ÷ V). High inverter, trolling-motor or surge loads can exceed a cheap BMS even when the watt-hour math looks fine. Always use the continuous rating from the BMS or cell datasheet, not a peak burst figure.

Voltage Drop

Voltage drop is the volts lost to resistance in the cable. For copper DC at 75 °C (NEC Chapter 9 Table 8), VD = (2 × 12.9 × I × L) / CM, where L is one-way feet and the 2 covers the round-trip conductor. Percent drop = VD ÷ system volts. A 3% budget is a performance target so inverters and LED drivers do not brown out. Ampacity (how hot the insulation gets) is a separate limit — a cable can meet 3% drop and still be too thin to carry the current safely.

Amp-hours vs Watt-hours

Amp-hours (Ah) are charge. Watt-hours (Wh) are energy. They convert with Wh = Ah × V at the nominal battery voltage. A 100 Ah 12 V pack is 1,200 Wh at 100% DoD; the same 100 Ah at 24 V is 2,400 Wh. Runtime and array math should use watt-hours (or Ah together with voltage). Comparing Ah across different voltages is meaningless.

MPPT vs PWM

A PWM controller connects the panel almost directly to the battery, so harvest is limited by battery voltage. An MPPT controller tracks the panel’s maximum-power voltage and converts extra volts into extra charge current (P = V × I, minus conversion loss). MPPT is worth it when module Vmp sits well above battery voltage (for example a 60-cell grid panel on a 12 V bank). PWM can be adequate when Vmp is already close to the battery. Controller output current is about array watts ÷ battery volts, not module Isc.

Peak Sun Hours

Peak sun hours (PSH) are equivalent hours of 1,000 W/m² irradiance. A site with 5 PSH delivers about the same energy as five hours at full STC sun, even if the sun is up for 12 clock hours. Daily harvest is roughly array watts × PSH × a derate (wiring, temperature, soiling, angle — often about 0.75). Winter PSH can be less than half of summer PSH at mid latitudes.

Ampacity

Ampacity is how much current a cable can carry without overheating the insulation. It depends on conductor size, insulation temperature rating, bundling and ambient heat. Voltage-drop percent is a separate performance budget. Size copper for both: the drop formula for volts lost, and a conservative ampacity table so the insulation is not the fuse. Then protect the cable with a fuse or breaker at 125% of continuous current, not above that ampacity.

Short-Circuit Current (Isc)

Isc is the short-circuit current of a PV module at Standard Test Conditions, printed on the datasheet. String current for overcurrent math follows Isc, not Imp. NEC 690.8 sizes PV conductor and fuse current at 1.56 × Isc (125% × 125%). Combiner fuses are typically required when three or more strings share a bus. MPPT output current at the battery is still about array watts ÷ battery volts, which is not the same number as module Isc.

Surge Watts

Surge (peak) watts are the short burst an inverter must supply to start an induction motor, compressor or pump. Running watts cover the continuous load (P = V × I). Many well pumps and RV air conditioners surge at about 5–7× running watts (LRA / FLA) unless a soft starter cuts that toward 2–3×. Size the inverter’s surge rating for the start, and the continuous rating for the run. Surge does not add extra daily watt-hours.