Solar and battery words explained

Power and energy

• Watt (W) — the rate at which power is used or made right now. A 100 W panel in full sun makes about 100 W; a 60 W laptop uses 60 W while running.
• Kilowatt (kW) — 1,000 watts.
• Watt-hour (Wh) — energy: one watt for one hour. A 60 W laptop for 4 hours uses 240 Wh. Batteries store watt-hours; panels make them; loads use them.
• Kilowatt-hour (kWh) — 1,000 Wh, the unit on your electric bill. The average US home used about 28.4 kWh a day in 2024 (EIA).
• Volt (V) — electrical pressure. Battery systems run at 12, 24 or 48 V; US household outlets at 120 V.
• Amp (A) — current, the flow. Watts = volts × amps, so the same power at 12 V is ten times the current at 120 V — see how many amps is 1000 watts at 12 volts.
• AC and DC — household power alternates (AC); batteries and panels are direct current (DC). An inverter turns DC into AC.

Batteries

• Amp-hour (Ah) — battery charge: amps for hours. Multiply by voltage for watt-hours — a 12.8 V 100 Ah battery holds 1,280 Wh. See how many watt-hours in a 100Ah battery.
• Nominal voltage — the battery’s labelled voltage: 12 V for lead-acid, 12.8 V for LiFePO4 (four 3.2 V cells), 51.2 V for a “48 V” LiFePO4 — see a 48V 100Ah battery in kWh.
• Depth of discharge (DoD) — how much of the rated capacity you use. Victron rates its LiFePO4 cycle life at 80% DoD; Trojan recommends 20–50% for lead-acid.
• State of charge (SoC) — how full the battery is. The opposite of depth of discharge.
• C-rate — current relative to capacity: 1C for a 100 Ah battery is 100 A. Lead-acid ratings quote a time instead — the 20-hour rate is the capacity when drained over 20 hours.
• Cycle life — how many charge-discharge cycles a battery lasts before its capacity falls to a set level, usually 80% of new.
• BMS (battery management system) — the electronics in or beside a lithium battery that disconnect it on over- or under-voltage, over-current and temperature.
• LiFePO4 — lithium iron phosphate, the lithium chemistry used in most solar and RV batteries. Compared with lead-acid in LiFePO4 vs lead-acid.
• AGM, gel, flooded — types of lead-acid battery. Flooded needs watering; AGM and gel are sealed.
• Group size (24, 27, 31) — a case size, not a capacity; see the battery group size chart.
• Days of autonomy — how many days the bank can run your loads with no charging.

Solar panels

• Rated watts (STC) — what a panel makes at standard test conditions: 1,000 W/m² of sunlight and a 25 °C cell.
• Peak sun hours — the day’s sunlight expressed as hours of full 1,000 W/m² sun; the same number as kWh/m²/day. See peak sun hours by city and, for aiming panels, the best angle for solar panels in winter.
• Voc (open-circuit voltage) — a panel’s voltage with nothing connected. It rises in cold weather, which matters for controller limits.
• Vmp and Imp — voltage and current at the maximum power point, where the panel actually works.
• Isc (short-circuit current) — the current with the leads shorted; used to size PWM controllers and fuses.
• Temperature coefficient — how much a panel’s power or voltage changes per °C away from 25 °C, usually a negative percentage.
• Series / parallel — series adds voltage, parallel adds current; see two 100W solar panels in series.
• String — panels wired in series.
• System losses (derate) — the share of output lost to heat, dirt, wiring and conversion; PVWatts uses 14% by default.
• Tilt and azimuth — the panel’s angle from horizontal and the compass direction it faces.

Charge controllers and inverters

• Charge controller — sits between panels and battery and stops overcharging.
• PWM — a simple controller that connects panels straight to the battery, pulling them down to battery voltage.
• MPPT (maximum power point tracking) — a controller that runs panels at their best voltage and converts the power to battery current; Victron’s example turns 100 W into 7.7 A at 13 V.
• Inverter — turns battery DC into household AC. Pure sine inverters make clean AC suitable for motors and electronics; modified sine is cheaper and rougher.
• Continuous rating — what an inverter or generator can supply indefinitely.
• Surge (peak) rating — a short burst for starting motors.
• Idle (no-load) draw — what an inverter uses just being on.
• Inverter-charger — an inverter that also charges the batteries from a generator or shore power.

Generators and fuel

• Running watts — what a generator (or appliance) supplies (or uses) continuously.
• Starting watts — the brief extra a motor needs to start; see the appliance wattage chart.
• Inverter generator — a generator whose electronics make clean power and let the engine slow down at light load.
• Dual fuel — runs on gasoline or propane.
• Btu — British thermal unit, a unit of heat. The EIA puts gasoline at 120,214 Btu per gallon and propane at 91,452.
• Transfer switch — a switch, installed by an electrician, that disconnects your house from the grid before a generator powers it. The alternative, backfeeding through an outlet, is dangerous — see generator safety.
• TT-30 / 14-50 — the 30 amp and 50 amp RV plug types. TT-30 is a three-prong 120 V plug; 14-50 is a four-prong 120/240 V plug; see 30 amp vs 50 amp RV.

RV words

• Rig — RV slang for the vehicle or trailer itself.
• Shore power — plugging the RV into a campground pedestal or a house outlet, borrowed from boating.
• Converter — the RV’s built-in battery charger, which turns 120 V AC into 12 V DC. Turn it off when an inverter is feeding the RV’s outlets.
• Pedestal — the campground post with the 30 A and 50 A outlets.
• Dogbone — a short adapter between 30 A and 50 A plugs. It changes the plug, not the power: you still get the smaller service.
• Boondocking — camping with no hookups, running on batteries, solar or a generator.
• Absorption fridge — the propane/electric RV fridge; run it on propane when off-grid.

Grid-tied solar

• Grid-tied — panels connected to the utility grid, usually without batteries.
• Net metering — a billing arrangement that credits power you export against power you import. Rules differ by utility.
• kWh per kW — the yearly energy one kilowatt of panels makes at a location; used to size home systems on how many solar panels for 1000 kWh per month.
• Payback — the years until savings equal the system’s cost; see the solar payback calculator.