Solar terms explained: a plain-language glossary

Solar advice is full of short words and letters: W, Wh, Voc, MPPT, DoD. This glossary explains each one in plain words, with an everyday comparison and a source for every number. If a term stops you in any of our articles, come back here.

The short version

• Watts are how fast energy flows; watt-hours are how much you used or stored. Watts = volts × amps.
• A panel's watt rating comes from a lab test at 1,000 W/m² of sunlight and 25 °C (77 °F). Real days are usually weaker.
• Peak sun hours turn a day of changing sunlight into hours of full lab sunshine.
• Panel voltage rises in the cold, so it must stay under the controller's limit on the coldest morning.
• Lithium iron phosphate (LiFePO4) batteries must not be charged below freezing without a heater, and fuses on the battery side must be rated for DC.

Power and energy

Watt (W)

A watt measures power: how fast energy is being used or produced at one moment. Officially one watt is one joule of energy per second (BIPM SI Brochure). Think of water from a tap: watts are how fast the water is running, not how much is in the bucket. A panel "rated at 100 W" gives 100 watts only in lab conditions (see "Irradiance" below).

Watt-hour (Wh)

A watt-hour is an amount of energy: one watt flowing for one hour. This is the water in the bucket. A 50 W lamp left on for 4 hours uses 200 Wh (a simple example: 50 × 4). Battery sizes and daily energy needs are both counted in watt-hours, which is why our quiz asks how long you use each appliance, not only how powerful it is.

Kilowatt-hour (kWh)

A kilowatt-hour is 1,000 watt-hours, the "unit" on your electricity bill in both the US and the UK. The European Commission's PVGIS tool estimates that 1 kW of panels facing south at a 30° tilt gives about 1,010 kWh in an average year in London and about 1,628 kWh in Denver, Colorado (PVGIS, checked 2026-10-20; worked through in how much power a panel produces).

Electricity basics

Volt (V)

Voltage is the electrical "push", like water pressure in a pipe. UK mains is declared at 230 V (UK supply regulations, regulation 27), and most US sockets run at about 120 V (WorldStandards). Off-grid solar systems usually run their batteries at much lower voltages, 12 V, 24 V or 48 V, which is safer to handle but needs thicker cables for the same power.

Amp (A)

An amp (ampere) measures current: how much electricity flows, like the amount of water passing through a pipe each second. The rule that links them: watts = volts × amps. So 120 W on a 12 V battery means 10 A, while the same 120 W at 230 V mains is only about 0.5 A (our calculation). That is why low-voltage systems need thick cables.

AC and DC

DC (direct current) flows one way only. Panels and batteries produce DC. AC (alternating current) changes direction many times a second: 50 times a second in the UK (UK supply regulations) and 60 times in the US (WorldStandards). Household sockets and most appliances use AC. A device called an inverter (below) turns DC into AC.

Sunlight

Irradiance (W/m²)

Irradiance is the strength of sunlight landing on a surface right now, in watts per square metre (about 10.8 square feet). Panels are tested at 1,000 W/m², a module temperature of 25 °C (77 °F) and a standard light spectrum; together these are called Standard Test Conditions, or STC (PVGIS FAQ and user manual). Bright midday sun in summer can come close to that; a cloudy winter day is far below it.

Peak sun hours

Peak sun hours squeeze a day of changing sunlight into an equivalent number of hours at full lab strength (1,000 W/m²): 3.4 kWh of sunlight per square metre in a day is 3.4 peak sun hours. PVGIS gives a south-facing surface at a 30° tilt about 3.4 a day over the year in London but only 1.3 in December; Denver gets about 5.7 and 3.9 (PVGIS, checked 2026-10-20). They are not the same as daylight hours.

Panels

Open-circuit voltage (Voc)

Voc is the voltage a panel shows when nothing is connected: the highest voltage it produces, printed on its label. Controllers and power stations have a maximum input voltage, and going over it can destroy them.

Voc rises in the cold: about 2.2 thousandths of a volt per silicon cell for every 1 °C drop (PVEducation). For a 36-cell panel on a −10 °C (14 °F) morning, 35 °C below the test temperature, that is about 2.8 V above the label (our calculation: 36 × 0.0022 × 35). Our compatibility check uses the cold of your location's weather record for this reason.

Short-circuit current (Isc)

Isc is the current a panel gives when its two wires are joined directly. It is the most current the panel can push, so it is used to size cables and fuses. It changes only a little with temperature, about 0.06 % per °C for silicon (PVEducation).

Temperature coefficient

How much a panel's power drops as it gets hotter, in % per °C. For crystalline silicon it is usually about −0.3 % to −0.5 % per °C (Sandia National Laboratories). At −0.4 % per °C, cells at 50 °C (122 °F), 25 °C above the test temperature, give about 10 % less than the label (our calculation). Panels in sun run much hotter than the air.

String (series and parallel)

A string is a group of panels wired together. In series (plus of one to minus of the next, like batteries in a torch or flashlight) the voltages add up. In parallel (pluses together, minuses together) the currents add up. Two panels with a Voc of 22 V give 44 V in series, before the cold-weather rise: the usual way people exceed a controller's voltage limit.

Controllers and inverters

Charge controller

A charge controller sits between the panels and the battery and stops charging when the battery is full, like a valve that stops a tank overflowing.

MPPT

An MPPT (maximum power point tracking) controller keeps adjusting the panel's voltage to collect the most power, then converts it to what the battery needs. Victron, a controller maker, reports gains of 10 % to 40 % over the simpler PWM type (below) when panels are cold, very hot or in weak light. MPPT also lets panels run in series above battery voltage, with thinner cables.

PWM

A PWM (pulse width modulation) controller is essentially a switch between the panels and the battery, so the panel is dragged down to battery voltage and some of its power is lost (Victron Energy). It is a low-cost choice for small systems. Our engine counts that loss when you choose PWM. Our article on MPPT vs PWM explains when PWM is fine.

Inverter and pure sine wave

An inverter turns the battery's DC into the AC that household appliances use (energy.gov.au). A pure sine wave inverter produces smooth AC like the mains; cheaper "modified sine wave" types make a stepped imitation that some motors and chargers do not like.

Surge power

Appliances with motors, such as fridges, pumps and power tools, draw extra power for a moment when they start. An inverter's datasheet gives a continuous rating and a higher short-term surge rating; if the start-up surge is above it, the inverter shuts off. Our quiz checks this.

Microinverter

A microinverter is a small inverter for one or two panels, usually fixed behind them (energy.gov.au). Each panel then makes AC on its own, so shade or a failure on one does not stop the rest. Plug-in (balcony) kits use a microinverter to feed AC into a normal socket.

Batteries

Battery capacity: Ah and Wh

Batteries are often labelled in amp-hours (Ah): how many amps they can supply for one hour. Amp-hours alone do not tell you the energy, because voltage matters too. Multiply them: a 12.8 V battery of 100 Ah holds about 1,280 Wh, while a 25.6 V battery of 100 Ah holds twice that (our calculation; Victron lists 1,280 Wh for its 12.8 V 100 Ah model). Always compare batteries in Wh.

Usable capacity and depth of discharge (DoD)

Depth of discharge is how far you empty a battery, in percent; deeper emptying wears it faster. Victron's lithium iron phosphate datasheet gives 2,500 cycles (one cycle is one emptying and refilling) at 80 % depth of discharge, 3,000 at 70 % and 5,000 at 50 %. Usable capacity is the part of the label you can actually use. Our engine assumes about 90 % for lithium iron phosphate and about half for lead-acid, which suffers more from deep discharge (our assumptions, on the methodology page).

LiFePO4

LiFePO4 (lithium iron phosphate, also LFP) is the common chemistry in modern off-grid systems: it tolerates deep discharge and many cycles. Its key limit: it must not be charged below freezing, 0 °C (32 °F), without a built-in heater. Some makers set it higher; Victron's datasheet allows charging only from +5 °C (41 °F), though discharging is allowed down to −20 °C (−4 °F). See LiFePO4 in cold weather; our engine stops charging on days that are too cold.

BMS

The BMS (battery management system) is the battery's guard: it watches cell voltages and temperature and cuts off charging or discharging when something is out of range. Victron's BMS, for example, stops charging above 3.75 V per cell or when it is too hot or cold, and disconnects the load below 2.8 V per cell. Some batteries have the BMS inside; others need a separate one.

Protection and the grid

Fuse

A fuse is a deliberately weak link that melts and breaks the circuit when too much current flows, before a cable overheats. DC is harder to switch off than AC: AC passes through zero many times a second and DC never does, so a spark (arc) can keep burning across the gap (ETI, a fuse manufacturer). Fuses in solar and battery circuits must be rated for DC at the system's voltage, and rated below what the cable can carry. See cable size and fuses.

DC breaker

A DC circuit breaker does a fuse's job but can be reset after it trips and doubles as an on/off switch. A breaker made for household AC is not automatically safe on DC: check for a DC voltage rating on its label.

Consumer unit (UK) and breaker panel (US)

The box where your home's mains is split into circuits, each with its own breaker: the consumer unit (or fuse box) in the UK, the breaker panel or service panel in the US. Anything that feeds power into it, such as a backup system or a rooftop array, is work for a qualified electrician.

Grid connection

A grid-connected system works alongside the public electricity network; an off-grid one is not connected at all. In Great Britain small generators use a "connect and notify" route called G98: you tell the local network operator within 28 days of switching on (National Grid Electricity Distribution). In the US the rules come from your utility and state.

Plug-in (balcony) solar

A small kit, usually one or two panels with a microinverter, that plugs into an ordinary socket and helps power your home. In Great Britain it is legal since 27 August 2026 for devices up to 800 watts of rated AC output that meet a product specification (SI 2026/848). In the US it depends on the state: Utah's 2025 law allows devices up to 1,200 W (Utah HB 340), while many states have no such law. These rules are new and changing: see the UK rules and the US state rules.

What to check before you buy

1. Count your daily energy in watt-hours and compare batteries by usable watt-hours.
2. Check the panels' Voc (added up for panels in series) against the controller's maximum input voltage, with a cold-weather margin.
3. Check the inverter's continuous and surge ratings against your largest appliance.
4. For a cold location, check the battery's minimum charging temperature and whether it has a heater.
5. Check that every fuse and breaker on the panel and battery side is rated for DC.

What can go wrong

• Broken controller on a frosty morning: panels in series went over its voltage limit.
• Battery that never charges in winter: a LiFePO4 battery in an unheated shed on freezing days. The BMS is doing its job.
• Fridge that will not start: the inverter's surge rating is too low.
• Too little energy: the system was sized from summer sun, not from your darkest month.

Questions people ask

Will a 100 W panel give me 100 W? Only in lab-strength sunshine. Our article on how much power a panel produces shows real numbers for a US and a UK city.

Do I need to understand all this to use the quiz? No. The quiz asks plain questions and runs these checks for you.

Next step

When you are ready, size my system: tell us where and how you will use solar, and you get a checked parts list where every number is explained. Our methodology page shows how the checks above are done.

Our results and guides are estimates and do not replace advice from a qualified electrician.