The words, in plain English

Power and batteries

AGM absorbed glass mat, lead-acid, lead acid, flooded lead-acid

A sealed lead-acid battery; cheaper up front, half the usable capacity, shorter life.

Absorbed glass mat. No watering and no spill, but only about 50 percent usable per cycle and a few hundred to a thousand cycles. Fine for a weekend cabin; for full-time living the lithium math usually wins within three years. See it in use →

amp-hour amp-hours, Ah

Battery capacity as current over time. Only meaningful alongside the battery's voltage.

A 100 Ah 12 V battery holds 1,280 Wh (100 x 12.8); a 100 Ah 48 V battery holds 5,120 Wh, four times as much. Compare batteries in watt-hours, not amp-hours, or a 12 V and a 48 V bank will look the same on paper. See it in use →

ANL fuse ANL

A slow-blow, bolt-down fuse common on 12 V inverter feeds; not suitable as a lithium main fuse.

Inexpensive and widely used between a 12 V battery and an inverter. Its interrupt rating is low, so on a lithium bank it can fail to clear a dead short; use it downstream only, and put a Class T at the bank. See it in use →

autonomy days of autonomy

How many days the battery can carry the house with no sun before the generator has to run.

Days of autonomy. Two is a common design point, three is cautious, one is lean. More autonomy means a bigger bank; in a cloudy northern winter, a generator is usually cheaper than the fourth day of battery. See it in use →

AWG American wire gauge, wire gauge

The American wire gauge; smaller numbers are thicker wire.

10 AWG is a typical PV run; 4/0 AWG (four-aught) is a welding-cable-sized battery lead. Size by amps and by distance: too thin and the wire heats and loses voltage. The wire calculator does the table lookup. See it in use →

BMS battery management system

The small computer inside a lithium battery that protects the cells.

Battery management system. It balances the cells, stops charging when they are full or too cold, stops discharging when they are empty or too hot, and opens on a short circuit. When a lithium battery suddenly goes dead it is usually the BMS doing its job; check temperature, voltage, and a latched short before calling it broken. See it in use →

bus bar busbar, bus bars

A heavy copper bar that lets many cables meet at one point instead of stacking on a battery terminal.

One positive and one negative bar, with the shunt on the negative side and the main fuse on the positive. It keeps the battery terminals to a single cable each and makes adding a circuit a ten-minute job. See it in use →

Class T fuse Class T

The fast, high-interrupt fuse that belongs on the main positive of a lithium battery bank.

A lithium bank can dump thousands of amps into a short. Ordinary ANL or MIDI fuses may not clear it and can arc. Class T fuses (Littelfuse JLLN, Blue Sea 5119 series) are rated to interrupt 20,000 A or more, which is why every competent lithium install has one within a few inches of the battery positive. See it in use →

depth of discharge DoD

How much of the battery you use before recharging, as a percentage.

DoD. Lead-acid batteries live long only if you stop around 50 percent; lithium iron phosphate is rated for 80 to 100 percent. That is why a lithium bank can be half the nameplate size of a lead bank and deliver the same usable energy. See it in use →

inverter inverter/charger, hybrid inverter

The box that turns battery DC into the household AC your outlets and appliances use.

Rated in continuous watts (what it can deliver all day) and surge watts (what it can deliver for a few seconds to start a motor). An inverter/charger also charges the battery from a generator or the grid; a hybrid all-in-one adds the solar charge controller too. Idle draw, how much it burns doing nothing, matters on a small system. See it in use →

inverter generator

A quieter, cleaner-power generator that varies engine speed with load; the type most inverter/chargers prefer.

Makes DC first and inverts it to clean AC, so it runs slower at light load (quieter, less fuel) and produces a stable waveform that sensitive chargers accept. Open-frame contractor generators run at 3,600 rpm constantly and produce rougher power; some hybrid inverters refuse to sync with them. See it in use →

Isc short-circuit current, short circuit current

The maximum current a panel can push into a dead short; used to size wire and fuses.

Short-circuit current, printed on the label (about 5 to 6 A for a 100 to 200 W 12 V-class panel, 10 to 14 A for a residential panel). Panels in parallel add their Isc. Wire and fuses on the PV side are sized from Isc times 1.25, and sometimes times 1.25 again for continuous duty. See it in use →

kW kilowatt, kilowatts

A thousand watts. Arrays and inverters are often described in kilowatts.

A 3 kW array is fifteen 200 W panels or about seven 400 W panels. Kilowatts (power) are not kilowatt-hours (energy): a 3 kW array in a 3.5-sun-hour location makes about 8 kWh a day after losses. See it in use →

LiFePO4 lithium iron phosphate, LFP, lithium battery

Lithium iron phosphate, a battery chemistry widely used in newer off-grid systems: safe, long-lived, and usable close to empty.

Also written LFP. Three to six thousand cycles, 80 to 100 percent usable capacity, flat voltage, no maintenance, and a built-in BMS. Its two habits to respect: it will not accept a charge below about freezing (heated models exist), and its short-circuit current is enormous, which is why the main fuse must be a Class T. See it in use →

MC4 MC4 connector, MC4 connectors

The weatherproof click-together connector on the end of every modern solar panel lead.

Male and female halves that lock; rated around 30 A and 1,000 V. The two rules: match the brand on your panels (mixed brands are the leading cause of field fires from arcing), and crimp with the real tool, not pliers. Keep spare pairs and the crimper on the shelf. See it in use →

micro-hydro microhydro, micro hydro

Generating electricity from a small stream with enough drop; the best off-grid source where it exists because it runs all night.

Needs head (vertical drop) and flow (gallons per minute). A few hundred watts around the clock is worth thousands of watts of solar in winter. Permits and water rights apply almost everywhere. See it in use →

MPPT maximum power point tracking, MPPT controller

A charge controller that continuously adjusts to pull the most power the panels can give at that moment.

Maximum power point tracking. The controller sits between the solar array and the battery, converts the array's higher voltage down to battery voltage, and keeps the array at the voltage where it produces the most watts. Compared with a cheaper PWM controller it can harvest 20 to 30 percent more, especially in cold weather and with higher-voltage strings. See it in use →

MRBF marine rated battery fuse

A compact fuse that bolts directly onto a battery terminal, good for 12 to 48 V systems up to 300 A.

Marine rated battery fuse. Sits on the battery post so the protection is as close to the source as it can be. Rated to 58 V, so it works on 48 V banks where MIDI and ANL fuses (32 V) do not. Common for the charge-controller-to-battery run. See it in use →

net metering

Selling surplus solar back to the grid for a credit. Irrelevant off-grid, but it shapes the quotes you will see.

A grid-tied system relies on it; an off-grid system has no grid to sell to, so every watt must be used, stored, or wasted. When an installer quotes you a grid-tied design for an off-grid property, this is the first sign. See it in use →

parallel in parallel, parallel strings

Panels or batteries wired side by side: current adds, voltage stays the same.

Connecting all positives together and all negatives together. Two 5 A panels in parallel deliver 10 A at the same voltage. Parallel strings need a fuse on each string once there are three or more, because a faulted string can be back-fed by the others. See it in use →

PWM pulse width modulation

The simpler, cheaper type of solar charge controller; fine for small 12 V systems, wasteful for anything bigger.

Pulse width modulation. A PWM controller connects the panel almost directly to the battery and switches it on and off to regulate; it cannot use the extra voltage a panel produces, so a 100 W panel may deliver 70 W. Use it for a trickle charger or a tiny cabin; use MPPT for anything you rely on. See it in use →

series in series, series string

Panels or batteries wired end to end: voltages add, current stays the same.

Connecting positive to negative down a line. Three 24 V panels in series make a 72 V string at the same amps as one panel. Fewer amps means thinner wire and less loss, which is why bigger systems run higher-voltage strings, limited by the controller's maximum input voltage. See it in use →

shunt battery shunt, battery monitor

A precision resistor the battery monitor measures across to count every amp in and out.

Installed on the main negative so nothing bypasses it. Without a shunt, a lithium battery's state of charge is a guess, because its voltage is flat across most of the range. The Victron SmartShunt and BMV are the common ones. See it in use →

split-phase 120/240, 240 V

The 120/240 V system a North American house uses; a well pump or dryer needs the 240 V side.

Two 120 V legs that are 240 V between them. A single 120 V inverter cannot run a 240 V well pump; you need a split-phase inverter, two stacked units, or a transformer. Check this before you buy the pump or the inverter. See it in use →

state of charge SoC

How full the battery is right now, as a percentage.

SoC. On lithium, voltage barely moves between 20 and 90 percent, so you cannot read it from a voltmeter; a shunt-based battery monitor counts amp-hours in and out instead. Most lithium BMSs disconnect around 10 percent and refuse to charge below freezing. See it in use →

sun hours peak sun hours, sun-hours

The equivalent hours of full, direct sun per day a location gets, averaged over a month. The number that sizes a solar array.

Peak sun hours. Not daylight hours: a place with 10 hours of daylight may deliver 2.7 sun hours in December and 5.5 in June. Design for the worst month you need to live through. The site's ZIP lookup pulls the real monthly figures from 30 years of records. See it in use →

surge start surge, starting surge, locked-rotor

The brief burst of power a motor needs to start, often three to five times its running draw.

Also called starting or locked-rotor current. A well pump that runs at 1,200 W may need 3,600 W for a second. The inverter's surge rating, the generator size, and the battery's maximum discharge current all have to cover the largest surge in the house, which is why one pump can decide the whole electrical design. See it in use →

two-wire start two-wire auto-start, auto-start, autostart

A generator that an inverter/charger can start and stop on its own by closing two terminals.

Two-wire auto-start. Lets the battery system start the generator at a low state of charge and shut it off when charged, unattended, at three in the morning in January. Most consumer generators have a key fob or button, which does not count; look for the terminals or an automatic-start kit. See it in use →

UL 1741 UL1741, UL listed

The safety standard an inverter is tested to for connecting to the grid and for many insurance and permit purposes.

Listings like UL 1741 (and 1741-SA for grid support) are what an inspector looks for. Off-grid-only inverters are sometimes UL 458 (mobile) or unlisted; if a permit or insurance is involved, ask before you buy. See it in use →

UL 9540 UL9540, UL 9540A, UL9540A

The safety listing for a complete battery energy storage system; increasingly required by code for indoor installs.

Tests the battery and inverter as a matched system (UL 9540A is the cell-level fire test). Many jurisdictions now require a 9540-listed system for anything installed in a dwelling. DIY server-rack banks often are not, which can matter for permits and insurance. See it in use →

Vmp voltage at maximum power

The voltage a panel runs at when it is actually producing its rated power.

Voltage at maximum power. Lower than Voc (about 20.4 V on a 12 V-class panel, about 41 V on a 400 W panel). An MPPT controller needs the string's Vmp to sit comfortably above the battery voltage, which is why a 48 V bank wants at least two or three panels in series. See it in use →

Voc open-circuit voltage, open circuit voltage

The highest voltage a solar panel produces, with nothing connected. It rises as the panel gets colder.

Open-circuit voltage. Printed on the panel label (for example 24.3 V on a 200 W panel, 49 V on a 400 W panel). Add the Voc of every panel in a series string and that sum must stay under the charge controller's maximum input voltage even on the coldest morning, which can push Voc 10 to 20 percent above the label. Exceeding it destroys the controller. See it in use →

watt watts, W

The rate something uses or makes power, right now. Volts times amps.

A 100 W bulb uses 100 W while it is on. Panels are rated in watts under standard test conditions; inverters and generators in continuous watts. Watts tell you about the size of the wire, the inverter, and the moment; watt-hours tell you about the day. See it in use →

watt-hour watt-hours, Wh, kilowatt-hour, kWh, kilowatt-hours

Energy over time: watts multiplied by hours. The unit your battery is really measured in.

A 100 W bulb for 5 hours uses 500 watt-hours (Wh); 1,000 Wh is a kilowatt-hour (kWh), the unit on a utility bill. Daily load in Wh is the first number in any off-grid design, and battery capacity in Wh (or kWh) is the second. See it in use →

Heat

BTU BTUs, BTU/hr, BTU per hour

The unit of heat. A stove, furnace, or fuel is rated in BTUs; a house loses BTUs per hour.

British thermal unit, the heat to raise a pound of water one degree F. A gallon of propane holds about 91,500 BTU; a cord of seasoned hardwood about 20 million; a small cabin may lose 30,000 BTU per hour on the coldest night. Match the stove to the loss, not to the catalog. See it in use →

COP coefficient of performance

How many units of heat a heat pump delivers per unit of electricity it uses. Above 1 is free heat.

Coefficient of performance. A good cold-climate mini-split runs a COP of 2 to 3 at 5 F and drops toward 1 near minus 15 F. Resistance heat (baseboard, space heater) is always exactly 1, which is why it does not work on solar. See it in use →

cord cords, face cord, full cord

The standard firewood unit: a stack 4 by 4 by 8 feet, 128 cubic feet, about 20 million BTU if it is seasoned hardwood.

A face cord or rick is one third of that (16-inch pieces, 4 by 8) and is what many sellers mean. Softwood has half the heat per cord. Wood needs a year split and stacked to get below 20 percent moisture; green wood wastes a third of its heat boiling off water and fouls the chimney. See it in use →

creosote

The tarry deposit from wood smoke that builds in a chimney and can catch fire.

Worse with wet wood, smoldering fires, and cool flue pipe. Brush the chimney at least yearly and check mid-season if you burn a lot; a stove-pipe thermometer keeps you out of the creosote zone. A chimney fire sounds like a freight train and it is how cabins burn down. See it in use →

design temperature design low

The coldest temperature your heating system is sized for; not the record low, the temperature beaten only a few hours a year.

Usually the 99 percent value: 99 percent of winter hours are warmer. A stove sized for the record low would be oversized and smolder the rest of the season. The heat-loss calculator uses it to find the BTU per hour you actually need. See it in use →

direct-vent heater direct vent, wall furnace

A propane or gas heater that draws combustion air from outside and exhausts outside through a sealed pipe, so it does not use room air or need electricity.

The standard unvented-vs-vented answer for cabins: direct-vent wall furnaces (Rinnai, Empire, Williams) are safe in tight spaces and run without power. Unvented (blue-flame, radiant) heaters put moisture and combustion products in the room and are banned in bedrooms in many places. See it in use →

heating degree days HDD, degree days

A running total of how cold a place is over a season; the number that predicts your fuel use.

HDD. For each day, take 65 F minus the average temperature (if positive) and add it up for the year. Southern Vermont is about 7,000; Florida under 1,000. Double the degree days, double the firewood. The site's ZIP lookup uses 30-year normals. See it in use →

mini-split minisplit, heat pump, ductless heat pump

A ductless electric heat pump: an outdoor unit and one or more indoor heads. The only electric heat that makes sense off-grid, and only with a big array.

Heats at a COP of 2 to 3 in cold weather and cools in summer. Adds a substantial daily load in winter, exactly when the sun is weakest, so it is usually a shoulder-season and backup heat source, with wood or propane carrying January. See it in use →

pellet stove

A stove that burns compressed sawdust pellets fed by an electric auger; clean and convenient, but it needs electricity to run.

About 100 W for the auger and fans, so 1 to 2 kWh a day in winter, and it stops when the power does. Pellets are bought in 40 lb bags, roughly a ton to 3 tons a winter. A good fit with a solar system that has winter margin, a poor fit for a lean one. See it in use →

R-value R value

How well insulation resists heat flow; higher is better.

A 2x6 wall with fiberglass is about R-19; a good attic R-49 or more. Insulation is the cheapest heat you will ever buy: a tight, well-insulated 800 square foot cabin can heat on half the wood of a drafty one. The heat-loss calculator's insulation setting stands in for this. See it in use →

resistance heat electric baseboard, baseboard heat

Electric heat that works by running current through a wire: baseboards, space heaters, electric water heaters. One watt in, one watt out.

The load that breaks solar budgets. A 1,500 W heater for 8 hours is 12 kWh a day, roughly four times what a lean cabin uses for everything else. Use it for a bathroom on a sunny afternoon, never as primary heat. See it in use →

Water and waste

blackwater

Toilet waste. The stream that must go to a septic system, a composting toilet, or a holding tank.

Keeping it separate from greywater is the basis of most low-cost off-grid sanitation. A composting toilet removes blackwater from the plumbing entirely, which in some counties is the difference between needing a septic permit and not. See it in use →

cistern water storage tank, storage tank

A large water storage tank, buried or above ground, filled by a well, rain, a spring, or a truck.

Sized in days of use: a week is a minimum, a month if you rely on hauling or rain. Buried tanks stay cool and do not freeze; above-ground ones need shade and, in the north, heat or draining. Poly tanks are the usual choice. See it in use →

composting toilet composting toilets

A toilet that collects waste dry and lets it break down, instead of flushing it to a septic system.

Self-contained units (Nature's Head, Separett) separate liquids and vent with a small fan; bucket systems use sawdust cover and an outdoor compost pile. Legal status depends on the state and county, which the laws pages cover by jurisdiction. See it in use →

first-flush diverter first flush

A simple device that dumps the first, dirtiest water off a roof before the rest goes into the rain tank.

The first few gallons carry bird droppings, pollen, and dust. A diverter is a vertical pipe that fills with that first water and then lets the rest bypass. Along with gutter screens and a settling tank, it is most of what keeps a rain system clean. See it in use →

gpm gallons per minute, GPM

Gallons per minute: how fast a pump or spring delivers water.

A household wants a pump that can do 5 to 10 gpm for showers and fixtures; a spring only needs to sustain the day's total (30 gal a day is 0.02 gpm) if there is a tank. Measure a spring in late summer, its low point. See it in use →

gravity filter Berkey

A countertop water filter that drips through ceramic or carbon elements with no pump or power; Berkey is the best-known.

Handles bacteria, protozoa, and many chemicals from a decent source, slowly. Needs priming, periodic scrubbing, and element replacement by gallons. Not a substitute for a water test or for UV on a surface source. See it in use →

greywater gray water, grey water

Used water from sinks, showers, and laundry, but not toilets. Often reusable for irrigation where the law allows.

Separating it from blackwater (toilet waste) shrinks the septic system and waters the garden. Rules vary wildly by state; many allow simple laundry-to-landscape systems, some require permits for anything. See it in use →

head total dynamic head, TDH

The vertical height a pump must lift water, plus the friction in the pipe, expressed in feet.

Total dynamic head. A well with water 200 ft down feeding a tank 30 ft uphill through a long pipe might be 260 ft of head. Pump curves trade flow for head: the same pump gives fewer gpm at more head. Pick the pump from the curve, not the horsepower. See it in use →

jet pump

A surface pump that sucks water up from a shallow well; limited to about 25 feet of lift.

Sits in the pump house rather than the well. Simple and cheap, but it must prime, it is louder, and it only works where the water table is close. Most drilled wells need a submersible. See it in use →

leach field drain field, drainfield, septic field

The buried perforated pipes in gravel where septic tank effluent soaks into the soil; the part of a septic system that fails and that the perc test sizes.

Also drain field or absorption field. Its size depends on how fast your soil drains (the perc test) and the number of bedrooms. Rocky, clay, or wet land may not perc at all, which is why the septic question comes before the land purchase. See it in use →

perc test percolation test

A soil test that measures how fast water drains, used to size (or deny) a septic leach field.

Percolation test. Holes are dug and filled; the rate of drop decides the field size and sometimes whether a conventional system is allowed at all. Get one, or find the recorded one, before you close on land you plan to live on. See it in use →

pressure tank accumulator, pressure switch

The tank with an air bladder that stores a few gallons under pressure so the pump does not start every time a tap opens.

Also called an accumulator on small 12 V systems. Pre-charge it to 2 psi below the pump's cut-in pressure. Too small and the pump short-cycles, which is what kills pressure switches and pumps. See it in use →

static water level static level, drawdown

How far down the water sits in a well when the pump is off; the number the pump depth and head start from.

Distinct from well depth. A 400 ft well may have water at 80 ft. The drawdown (how far the level falls while pumping) is the other number the driller gives you; together they set the pump's setting depth. See it in use →

submersible pump well pump, submersible

A well pump that sits down in the water, pushing up; the standard for drilled wells deeper than about 25 feet.

Typically 1/2 to 1-1/2 hp, 230 V AC, with a start surge of three times running, which is what sizes an off-grid inverter. Solar DC submersibles (Grundfos SQFlex, RPS, Shurflo 9300) avoid the inverter by pumping slowly to a tank in daylight. See it in use →

UV purifier UV filter, ultraviolet

A water treatment that kills bacteria and viruses with ultraviolet light as the water passes a lamp; needs electricity and clear water.

Common on surface-water and shallow-well systems, after a sediment filter. Draws 20 to 60 W continuously, which is a real load off-grid. The lamp is replaced yearly whether it looks fine or not. See it in use →

Propane

POL POL fitting

The older, large left-hand-thread propane cylinder connection; still on 100 lb cylinders and bulk tanks.

Prest-O-Lite fitting. Modern 20 lb cylinders have the QCC1 (Type 1) hand-tight connector on the outside and a POL thread on the inside. Adapters exist; know which your regulator and hoses need. See it in use →

QCC1 Type 1 connector, ACME

The modern hand-tightened, right-hand propane cylinder connector on 20 lb tanks; also called Type 1 or ACME.

Has a built-in thermal shutoff and excess-flow device. The big green or black plastic nut on a grill hose is a QCC1. Pigtails for an auto-changeover regulator use it on the cylinder end. See it in use →

requalification recertification, requalified

The periodic re-inspection a propane cylinder needs before anyone will refill it: 12 years from manufacture, then every 5.

The date is stamped on the collar; the requalifier adds a new date and an E for a visual inspection. Exchange cages and refill stations refuse expired cylinders. Rotate your cylinders so none lapse. See it in use →

two-stage regulator regulator, auto-changeover regulator, changeover regulator

A propane regulator that drops cylinder pressure in two steps, giving steady appliance pressure as the tank empties and the weather changes.

First stage to about 10 psi, second stage to 11 inches water column. Single-stage regulators are for grills. An auto-changeover version switches from an empty cylinder to a full one on its own and shows a red flag, so dinner does not stop. See it in use →

water column inches WC, 11 inches

The tiny pressure propane appliances run at, measured in inches of water; 11 inches WC is standard.

About 0.4 psi. Low-pressure appliances (range, furnace, fridge) need it exactly; a grill regulator or a high-pressure torch regulator is wrong for them. Yellow, lazy flames often mean pressure, a tired regulator, or a blocked vent. See it in use →

Land and permits

certificate of occupancy occupancy permit

The county's sign-off that a dwelling meets code and may be lived in; some places require it before you can legally live on the land.

A CO. Counties that require one usually also require an approved water source and sanitation, which is where off-grid plans collide with code. The laws pages note where this is a sticking point. See it in use →

easement right of way, deeded access

A legal right for someone else to use part of your land, or for you to use theirs: most often the road in.

Landlocked parcels (no deeded access) are a classic off-grid trap. Get the access easement in writing and recorded, and know about utility, logging, and neighbor easements crossing your land before you close. See it in use →

owner financing seller financing

Buying land with payments to the seller instead of a bank loan. Common for rural parcels banks will not touch.

Usually a down payment and monthly payments over 5 to 15 years, with the seller holding the deed or a note. Read the contract for balloon payments, interest, and what happens if you miss a payment; raw land rarely qualifies for conventional mortgages. See it in use →

setback setbacks

The minimum distance a building, well, or septic system must be from a property line, road, or water.

Typical: 25 to 50 ft from lines for structures, 100 ft between a well and a septic field, 50 to 100 ft from a stream. Setbacks can make a small or oddly shaped lot unbuildable; sketch them before you buy. See it in use →

zoning

Local rules about what you may do on a piece of land: how small a lot, what structures, whether you may live there year-round.

The first thing to check on an off-grid parcel, and the thing listings most often gloss over. Agricultural and rural-residential zones are usually friendly; some counties prohibit living without utility hookups, a certificate of occupancy, or a minimum square footage. See it in use →

Other

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Plain definitions, real numbers, and marked-up data (schema) let an assistant pull a correct answer and cite the source. For a reader the payoff is pages that answer the question in the first paragraph. See it in use →

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Invisible to readers. It is why a calculator page can show up as a tool and a laws page as an answer. Every tool and reference on this site carries it. See it in use →

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