Glossary of Key Terms

AC (Alternating Current): Electric current that reverses direction periodically (in a sine wave). This is the form of electricity provided by the grid and used by standard home appliances. Off-grid inverters output AC power (typically 120V or 240V AC) to run household loads.

Ampere (Amp): The unit of electric current, symbol “A”. It measures the flow of electric charge. In off-grid systems, wire sizes, fuses, etc., are often chosen based on how many amps will flow through them. For example, a 1000 W load at 12V draws ~83 amps (1000/12), whereas at 48V it draws ~21 amps (1000/48).

Amp-Hour (Ah): A unit of charge commonly used to describe battery capacity. For instance, a battery rated 200 Ah can supply a current of 200 A for 1 hour (or 20 A for 10 hours, etc.) before it’s depleted (though actual usable capacity depends on battery type and discharge rate). To convert Ah to Watt-hours, multiply by the battery voltage. E.g., 200 Ah at 12V = 2400 Wh.

Battery Bank: A group of batteries connected together to act as a single large battery, providing the required voltage and capacity. In off-grid systems, this stores the energy generated by solar (or other sources) for later use when solar isn’t available. Banks can be wired in series (to increase voltage) and parallel (to increase capacity) combinations.

Charge Controller: Also known as a solar charge regulator, this device controls the charging of batteries from the solar panels. It ensures batteries are charged at the proper voltage, prevents overcharging, and often prevents reverse current flow at night. MPPT (Maximum Power Point Tracking) charge controllers optimize the power extracted from solar panels, especially when battery voltage is substantially lower than the panel’s optimal voltage.

Depth of Discharge (DOD): The percentage of a battery’s capacity that has been used. For example, using 50% of the battery and leaving 50% remaining is 50% DOD. The inverse is State of Charge (SOC) – in that case 50% SOC remains. Battery lifespan is heavily affected by DOD; many batteries are rated for a certain number of cycles at a specified DOD (e.g. 500 cycles at 80% DOD). Commonly, off-grid systems try to keep daily DOD around 50% or less for lead-acid, and 80% or less for lithium, to extend battery life.

Direct Current (DC): Electric current that flows in one direction only. The type of power generated by solar panels and stored in batteries. Off-grid systems are built around DC components (panels, batteries, charge controller) and then use an inverter to convert DC to AC for standard appliance use.

Hybrid System: A solar power system that is connected to the grid but also has battery storage (i.e., a grid-tied system with backup). It combines features of off-grid and grid-tied – can provide some backup power during outages, but can also use the grid as needed.

Inverter: A device that converts DC electricity (from batteries/solar) into AC electricity. Off-grid inverters also typically manage the AC output distribution and may include charging capabilities (inverter/chargers can use AC from a generator or grid to charge batteries as well). Pure sine wave inverters produce AC power that is as clean (or cleaner) than grid power, suitable for sensitive electronics. Modified sine wave inverters are cheaper but produce a choppier waveform; these are less commonly used in modern systems due to compatibility issues with some devices.

Kilowatt (kW): 1000 watts. We often rate solar arrays in kW (e.g., a 5 kW solar array). Also, generators and inverters are often rated in kW (or kVA). A kilowatt is a unit of power – the rate of energy generation or use.

Kilowatt-hour (kWh): A unit of energy equal to 1000 watts used (or produced) for one hour. This is the common unit for measuring electricity consumption (e.g., a house might use 30 kWh per day). In off-grid terms, your battery capacity and daily usage are often discussed in kWh. For example, a 5 kW load running for 2 hours uses 10 kWh. If your solar produces 1 kW for 5 hours, that’s 5 kWh of energy generated.

MPPT (Maximum Power Point Tracker): A feature of modern charge controllers that continually adjusts the electrical load on the solar panels to find the “sweet spot” (maximum power point) where the panels deliver the most power. Because solar panel output varies with voltage and current, an MPPT controller can significantly increase energy harvest (especially in cold weather or when battery voltage is low) compared to a simpler PWM controller. Essentially, MPPT ensures you get the most watts possible from the PV array at any time.

Off-Grid System: A power system not connected to the public electricity grid, providing standalone power to a home or facility. It requires energy generation (like PV panels) and storage (batteries) to supply loads 24/7. Off-grid systems must be sized to meet all demands independently, and often include backups due to the lack of grid support.

Peak Sun Hours: A way to quantify solar energy available per day. 1 peak sun hour = an hour of sunlight at 1000 W/m² (full sun). If an area gets 5 peak sun hours, that is roughly equivalent to 5 hours of strong noontime sun energy (which might occur across a 10-hour day of weaker morning/evening sun). This metric is used to estimate solar production – you multiply the array size (kW) by the peak sun hours to get kWh/day (before losses). For example, 4 kW of panels × 5 sun hours ≈ 20 kWh/day (ideal). Peak sun hours vary by location and season.

Photovoltaic (PV): Pertaining to the direct conversion of light into electricity (photo = light, voltaic = electric). Solar panels are PV devices. In off-grid discussions “PV” is often shorthand for the solar panels/array.

Solar Array: A collection of multiple solar panels wired together to act as a single large source. Panels can be wired in series (to increase voltage) and in parallel (to increase current) depending on system needs. An array might be, for instance, 10 panels of 300W each, totaling a 3 kW array. Often used interchangeably with “solar panels,” but technically refers to the group of panels in the system.

State of Charge (SOC): The percentage of charge left in a battery relative to its capacity. 100% SOC means full battery, 0% means empty (though in practice 0% SOC would be at the cutoff voltage, not absolutely zero charge). It’s the inverse of Depth of Discharge (i.e., 30% DOD = 70% SOC remaining). Many battery monitors will display SOC to help users know when to conserve or charge.

Surge (Peak Load): A short-term high power draw that exceeds the normal running power of a device. Many inductive loads (motors in pumps, fridge compressors, AC units) and some electronics have startup surges. Off-grid inverters are rated with a continuous wattage and a surge wattage (for example, 3000 W continuous, 6000 W surge for 5 seconds). The system must accommodate surges to avoid overload. Designing with surge in mind is important for inverter and generator sizing.

Watt (W): The unit of power – the rate of energy consumption or production. 1 watt = 1 joule per second. In practical terms, a 60 W lightbulb uses 60 watts continuously when on. Solar panels are rated in watts (e.g., a 250 W panel under standard test conditions produces 250 W of power in full sun). When talking about electricity, volts × amps = watts (for DC or AC resistive loads). For instance, a device drawing 5 amps at 12 volts is using 60 watts.

Watt-hour (Wh): A unit of energy equal to one watt flowing for one hour. If that 60 W lightbulb is on for 2 hours, it has used 120 Wh of energy. Watt-hours accumulate over time and determine battery capacity and energy usage. 1000 Wh = 1 kWh. This unit is crucial for off-grid system design because we calculate how many Wh are needed per day and how many Wh of storage we have in batteries.