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Before you buy anything, you need to understand the language of electricity. This section covers the core units, the formulas you'll use every day, and the physical components that make up a wiring system.
Electrical Units & Basics#
Every off-grid system is built around five core measurements. Once you understand these, everything else clicks into place.
Voltage
VoltsThink of voltage as water pressure in a pipe. It's the force pushing electricity through the circuit.
A car battery is 12V. Most van and RV solar systems run at 12V or 24V.
Current
AmpsCurrent is how much electricity is actually flowing — like the volume of water moving through the pipe.
A 12V fridge might draw 4A. A 12V inverter running a kettle might draw 80A.
Power
WattsPower is voltage × current combined. It tells you the rate at which energy is being used right now.
12V × 4A = 48W. That's the fridge's power draw at any given moment.
Energy
Watt-hoursEnergy is power over time. A 48W fridge running for 24 hours uses 1,152Wh of energy.
Your daily energy budget is measured in Wh. This is the number everything else is built around.
Amp-hours
Amp-hoursBattery capacity is measured in Ah. A 100Ah battery can deliver 10A for 10 hours — or 1A for 100 hours.
To convert: Wh ÷ Voltage = Ah. A 1200Wh battery at 12V = 100Ah.
The key insight: Voltage and current together give you power (Watts). Power over time gives you energy (Watt-hours). Your whole system design is about matching energy supply to energy demand.
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Essential Formulas#
You only need a handful of formulas to design a complete off-grid system. Here they are, with practical examples for each.
Ohm's Law — The Foundation
Relates voltage, current, and resistance.
Find Voltage
V = I × R
Find Current
I = V ÷ R
Find Resistance
R = V ÷ I
Power Formula — Used Constantly
Convert between watts, volts, and amps.
Daily Energy Budget
Add up all your loads to find how much energy you need each day.
// For each appliance:
Daily Wh = Watts × Hours per day
Fridge 50W × 24h = 1,200Wh
Lights 20W × 5h = 100Wh
Laptop 60W × 4h = 240Wh
Total Daily Load = 1,540Wh/day
Solar Panel Sizing
Work backwards from your daily energy need.
Panel Watts = (Daily Wh ÷ Peak Sun Hours) × 1.25
// 1.25 accounts for real-world losses
// Example: 1,540Wh ÷ 5 hours × 1.25 =
Minimum Solar = 385W
Apply these formulas to your circuits
Enter your load wattage, system voltage, and cable run length — the Wire Calculator does all the maths.
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Four main components make up every off-grid system. Understanding what each one does — and how to choose the right one — is the foundation of a good build.
Solar Panels#
Solar panels convert sunlight into DC electricity. The wattage rating on the label is measured under perfect lab conditions (STC) — in the real world, expect 75–85% of that figure on a good day.
Monocrystalline
Best EfficiencyMade from a single silicon crystal. Highest efficiency (18–22%), best performance in low light, and the most compact. The right choice for most builds where roof space is limited.
Polycrystalline
Budget OptionMade from multiple silicon fragments. Slightly lower efficiency (15–17%) and larger for the same wattage. Cheaper, but the cost difference has narrowed.
Flexible / Thin Film
Specialist UseCan be glued directly to curved surfaces. Lower efficiency and shorter lifespan than rigid panels. Only worth considering when a rigid panel physically won't fit.
Peak Sun Hours: This is the number of hours per day your location receives full-strength sunlight — not daylight hours. Australia averages 4–6 hours. Southern Europe averages 4–5 hours. The UK averages 2–3 hours in winter. USA Southwest averages 5–7 hours.
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Batteries#
Your battery bank stores the energy your panels generate. Choosing the right battery chemistry is one of the most important decisions in your build.
LiFePO4 (Lithium Iron Phosphate)
RecommendedAGM (Sealed Lead-Acid)
Budget OptionDepth of Discharge matters: A 100Ah LiFePO4 battery at 80% DoD gives you 80Ah of usable power. A 100Ah AGM at 50% DoD gives you only 50Ah. You need twice the AGM capacity to match the same usable storage.
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Charge Controllers#
The charge controller sits between your solar panels and your battery. Its job is to regulate the charging process — preventing overcharging.
MPPT
RecommendedMaximum Power Point Tracking
Continuously adjusts its input to extract the maximum possible power from the panels. Typically 15–30% more efficient than PWM. The right choice for almost every build.
PWM
Pulse Width Modulation
Forces the panel to operate at battery voltage, which wastes the potential power difference. Simpler and cheaper, but significantly less efficient.
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Inverters#
An inverter converts DC to AC so you can run normal appliances from your battery bank. Not every build needs one — if you only run 12V devices, you can skip it entirely.
Pure Sine Wave
RecommendedProduces clean AC power identical to the grid. Required for sensitive electronics (laptops, TVs, medical equipment), motors, and most modern appliances. Always choose this type.
Modified Sine Wave
Produces a stepped waveform that approximates AC. Can damage sensitive electronics, cause motors to run hot, and make transformers buzz. Avoid unless you have a very specific, simple use case.
Inverter-Charger (Combi)
Combines a pure sine wave inverter with a multi-stage battery charger and shore power passthrough. The best option for full-time van or boat builds.
Inverter cables carry very high current. A 1000W inverter at 12V draws over 80A. Use the correct cable size and fuse the cable within 500mm of the battery. Undersized inverter cables are a leading cause of electrical fires in van builds.
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Cable Sizing — How to Pick the Right Wire#
The most common cause of electrical fires in off-grid builds is undersized cable. Ampacity — the maximum current a cable can safely carry — depends on the cable's cross-section, insulation rating, and how it's installed. There is no single universal answer: the same cable carries different current depending on where and how it's run.
Rule 1 — Size for Current
Calculate the current first: Amps = Watts ÷ Volts. A 1000W inverter on a 12V system draws 83A. Your cable must handle that current continuously without overheating.
Rule 2 — Size for Length
Longer cables have more resistance, which means more voltage drop and heat. Always factor in the full run length — both positive and negative combined.
Rule 3 — Derate for Installation
A cable in free air carries more current than the same cable bundled with others, in conduit, or in a hot engine bay. Always apply derating factors for your actual installation.
Installation Method Derating — The Factor Most Builders Ignore
Cable ampacity ratings assume ideal conditions — free air, 30°C ambient. Real installations are rarely ideal. Here's how much capacity you lose in common scenarios:
Free Air
×1.0Full rated ampacity. Cable hanging freely with airflow on all sides.
In Conduit
×0.8020% reduction. Heat can't escape as easily. Multiple cables in the same conduit require further derating.
Bundled Together
×0.7030% reduction. The most common van/RV installation. Multiple cables zip-tied together heat each other.
Engine Bay
×0.6040% reduction. Ambient temperatures regularly exceed 60°C. Use XLPE or silicone cable — PVC softens.
In Insulation
×0.5050% reduction. Worst case. Avoid routing high-current cables through spray foam or insulation panels.
Example: A 10mm² PVC cable is rated 38A in free air. In a bundled loom (×0.70), it can only safely carry 26A. If that same loom runs through an engine bay (×0.60), it drops to 22A. Size for your actual installation — not the datasheet maximum.
Cable Insulation Types — They're Not All the Same
The insulation type determines the maximum operating temperature, which directly affects how much current the cable can carry. Higher temperature rating = more ampacity for the same conductor size.
PVC (70°C)
Budget / BasicStandard PVC insulation. Affordable and widely available. Rated to 70°C conductor temperature. Not suitable for engine bays (softens above 70°C) or marine environments (degrades in UV and salt). This is the baseline used in most cable sizing tables.
Avoid in engine bays and marine environments.
XLPE / TFX (90°C)
RecommendedCross-linked polyethylene. Rated to 90°C — the higher temperature rating means approximately 20% more ampacity than PVC for the same conductor size. More flexible, more durable, and better UV resistance. The standard for quality DC wiring in vehicles and cabins.
LSZH (90°C)
Enclosed SpacesLow Smoke Zero Halogen. Same temperature rating as XLPE but emits far less toxic smoke if it burns. Required in some marine and commercial applications. Slightly stiffer than XLPE.
Tinned Marine (90°C)
Marine / CoastalCopper strands individually tinned before insulation. Resists corrosion at terminations — essential in salt water environments. Usually XLPE or LSZH insulated. More expensive but non-negotiable for boats and coastal vehicles.
Bare copper corrodes rapidly in salt air — tinned copper is not optional for marine use.
Silicone (180°C+)
Engine BayRated to 180°C or higher. Used in engine bays and high-heat environments where PVC and XLPE would degrade. Very flexible. Significantly more expensive. Only necessary for cables that run through or very close to heat sources.
CCA (Copper Clad Aluminium) cable is widely sold on eBay, Amazon, and AliExpress as a cheap alternative to pure copper. It looks identical but has an aluminium core with only a thin copper coating. Aluminium has roughly 60% of copper's conductivity — meaning a CCA cable runs significantly hotter and drops more voltage than a genuine copper cable of the same size. The amp ratings printed on CCA cable are based on copper conductivity and are therefore misleading. Always buy from a reputable electrical supplier. If the price seems too good to be true, it's probably CCA.
Welding cables and jumper leads are often advertised with very high amp ratings (e.g. 400A, 600A). These ratings are for short-duration, intermittent use only — not continuous DC loads. A welding cable rated 400A might only safely carry 80A continuously. Using them for permanent DC wiring is a fire risk. Always use purpose-made DC power cable (XLPE or PVC rated for continuous use).
Cable Standards — Why the Numbers Differ by Country
Different countries use different standards for cable sizing. The ampacity numbers in these standards are not identical — a 6mm² cable is rated differently under AS/NZS vs ABYC. Always use the standard relevant to your country and application.
AS/NZS 3008.1
Australia / New Zealand
The primary standard for cable sizing in Australian and NZ electrical installations. Table-based ampacity with derating factors for installation method and ambient temperature.
IEC 60228 / IEC 60364-5-52
International / Europe
International standard for conductor cross-sections and ampacity. Widely adopted in Europe, Asia, and as the basis for AS/NZS. Ampacity tables in IEC 60364-5-52.
ABYC E-11
Marine (USA)
American Boat and Yacht Council standard. More conservative than AS/NZS — requires tinned copper, has stricter derating, and lower ampacity limits for the same cable size.
NEC 310
USA (land)
National Electrical Code. AWG-based. Different derating tables from IEC — the reason AWG and mm² don't map perfectly to the same ampacity.
Use the Cable & Fuse Checker
Select your installation method and cable type — the calculator applies the correct derating automatically.
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Fuse Types — Which One Goes Where#
Not all fuses are the same. Each type has a different current range, blow speed, and job in the system.
Blade Fuse (ATO / ATC / Mini)
5A – 40ABranch circuits — lights, fans, USB chargers, water pumps
The small coloured fuses you see in car fuse boxes. Cheap, widely available, and easy to replace. Use these for any low-current branch circuit in your system.
Always size to the cable, not the load. A 10A cable gets a 10A fuse — even if the device only draws 3A.
Never use blade fuses on high-current runs (inverter, battery main).
MIDI Fuse
30A – 200AMid-range runs — MPPT controller feed, DC-DC charger, distribution panel feed
A step up from blade fuses. MIDI fuses handle higher currents and are commonly used for charge controller cables and DC-DC charger feeds.
MIDI fuses are a good choice for your MPPT controller's battery cable — typically 40A–80A.
ANL Fuse
60A – 750AMain battery fuse for lead-acid (AGM/flooded) systems
A large, flat fuse that sits in a bolt-down ANL fuse holder. Semi-slow blow — designed to handle the surge currents of lead-acid batteries.
ANL fuses are fine for AGM systems. They're the most common main fuse in van builds using lead-acid batteries.
Class T Fuse
60A – 400AMain battery fuse — mandatory for ALL LiFePO4 systems
A fast-blow fuse specifically designed for high-fault-current DC systems. Clears in microseconds — fast enough to interrupt the enormous fault currents a LiFePO4 battery can deliver.
Littelfuse KLKD series is the most common Class T fuse used in van and RV builds. Always install within 150mm of the battery positive terminal.
The rule: Use a Class T fuse for any LiFePO4 battery. Use an ANL fuse for AGM. Size the fuse to the cable's ampacity, not the device's draw.
Fuse Quality — Why Cheap Fuses Are Dangerous
Not all fuses are equal. A fuse that looks identical to a genuine Littelfuse Class T may have a fraction of the interrupt rating. If a cheap fuse fails to clear a fault, the element can arc and weld closed — turning the fuse into a conductor at the worst possible moment.
Interrupt Rating
The maximum fault current a fuse can safely clear. A genuine Class T fuse clears 20,000A+ in microseconds. A cheap counterfeit may only handle 2,000A — and a LiFePO4 battery can deliver 10,000A+ into a dead short.
Blow Speed
Class T fuses are fast-blow by design — they clear in microseconds. Slow-blow fuses (like some ANL copies) may not clear fast enough to protect lithium battery cables from the enormous fault currents involved.
Fuse Holder Quality
The holder matters as much as the fuse. Cheap Mega fuse holders have thin contact plates that arc and overheat at high current. The holder can melt or catch fire even if the fuse itself is genuine.
Counterfeit Fuses
Class T fuses from unknown brands on eBay or AliExpress may look identical to genuine Littelfuse KLKD fuses. If you can't find a datasheet with an interrupt rating, don't buy it.
Trusted Fuse Brands
Buy fuses from manufacturers who publish datasheets with interrupt ratings and are independently tested:
Fuse Standards — What the Certifications Mean
Fuses are tested to specific standards. A fuse that meets the standard has been independently verified to interrupt fault current safely at its rated voltage and current.
UL 248
USA / North America
Underwriters Laboratories fuse testing standard. UL 248-15 covers Class T fuses specifically. Look for the UL mark on the fuse body or packaging.
IEC 60269
International / Europe
International Electrotechnical Commission standard for low-voltage fuses. Widely adopted in Europe, Asia, and Australia. Look for IEC 60269 compliance on the datasheet.
AS/NZS 60269
Australia / New Zealand
Australian adoption of IEC 60269. Required for electrical installations in Australia and New Zealand. Littelfuse and Bussmann products typically meet this standard.
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Now that you understand the components, it's time to put them together. This section walks you through sizing, safety, vehicle integration, and a practical build guide.
Sizing Your System#
System sizing is a step-by-step process. Work through these steps in order — each one feeds into the next.
Calculate Your Daily Energy Load
List every electrical device you'll use. For each one, write down its wattage and how many hours per day you'll run it. Multiply to get Wh/day. Add them all up.
Find Your Peak Sun Hours
Look up the average peak sun hours for your location and the season you'll be using the system most. Use a conservative (lower) figure to avoid undersizing.
Size Your Solar Array
Divide your daily Wh by your peak sun hours, then multiply by 1.25 for real-world losses.
Size Your Battery Bank
Decide how many days of autonomy you need. Divide your daily Wh by your battery's usable DoD.
Size Your Charge Controller
Use the formula: (Total Panel Watts ÷ Battery Voltage) × 1.25. Round up to the next standard controller size.
Size Your Cables & Fuses
Every cable run needs to be sized for the maximum current it will carry. Use our Wire Calculator — don't guess.
Use My Systems to do this for you
Our guided wizard walks through every sizing step and does all the maths automatically.
Still confused about System Sizing?
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Monitoring & Safety#
A well-monitored system is a safe system. These are the instruments that keep your build running reliably.
Battery Monitor (Shunt-based)
Tracks state of charge, voltage, current, and time-to-go. Essential for LiFePO4 — voltage alone is useless for SoC. Victron BMV-712 or SmartShunt are the most popular choices.
BMS (Battery Management System)
Built into most LiFePO4 batteries. Protects cells from overcharge, over-discharge, over-current, and temperature extremes. Never bypass the BMS.
Smoke & CO Detector
Install a smoke detector near your battery bank. A CO detector is essential if you have a gas appliance or generator in an enclosed space.
A shunt-based battery monitor is the only reliable way to track State of Charge on a LiFePO4 battery. The voltage curve is so flat that voltage alone tells you almost nothing between 20% and 80% SoC.
Vehicle Integration#
If your off-grid system is in a vehicle, you have an additional charging source: the alternator.
DC-DC Charger (Recommended)
A DC-DC charger takes the alternator's output and converts it to the correct charging voltage for your house battery. It's isolated, so it won't drain your starter battery.
VSR / Split Charge Relay (Older Vehicles Only)
A Voltage Sensitive Relay connects the starter and house batteries when the alternator voltage rises above a threshold. Simple and cheap, but not suitable for LiFePO4 or modern smart alternators.
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Step-by-Step Build Guide#
Here's the practical build sequence for a typical 12V or 24V off-grid system. Follow this order — it's designed to keep you safe and make testing easier at each stage.
Complete your load calculation and system sizing before buying anything
Draw a simple wiring diagram showing every component and cable run
List every cable run with its length, current, and required gauge
Mount the battery bank in a ventilated, accessible location
Mount the charge controller near the battery (short cable run)
Mount the inverter close to the battery (high current = short cable)
Run all cables before connecting anything
Use conduit wherever cables pass through panels or run near heat sources
Label every cable at both ends before connecting
Connect the charge controller to the battery first (no panels yet)
Connect the solar panels to the charge controller
Connect the DC loads via the fuse box
Connect the inverter last — test with a small load before a large one
Verify the battery monitor is reading correctly (zero current at rest)
Check the charge controller is showing the correct battery voltage
Run each load individually and check the current draw matches expectations
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Common Beginner Mistakes
These are the mistakes that show up again and again in off-grid builds — some are just expensive, some are genuinely dangerous.
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Beginner FAQs
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What's Next?
Electrical Deep Dive
Go deeper on wiring standards, BMS configuration, and advanced system design.
Read Deep DiveCable & Fuse Checker
Get the correct cable size, fuse rating, and voltage drop for every circuit in your build.
Open CheckerSomething missing from this module?
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10 more issues on the troubleshooting page
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LiFePO4 vs AGM vs flooded, true cost of ownership, C-rate
Deep DiveMPPT vs PWM — Full Comparison
Efficiency, cold Voc, cable cost, winter performance
Deep DiveAdvanced Common Mistakes
No Class T fuse, cold Voc error, equalisation on LiFePO4
Deep DiveMPPT Underperforming
Less output than expected — shading, cable sizing, temperature
Fix