12 V solar system for a camper, van or cabin: sizing, parts and wiring

How a 12 V system fits together

Every off-grid 12 V system has the same few parts. The panels and the alternator charge a battery through their own chargers; everything you use draws from the battery, directly at 12 V or through an inverter; and each cable leaving the battery has a fuse.

The parts of the worked example; each section below explains how it was sized.

Start from the energy budget

Everything is sized from one number: the watt-hours (Wh) you use in a day, each load’s power times the hours it runs. A compressor fridge doesn’t run all the time, so use its average draw over a day, not the figure on its label; it rises a lot in hot weather. A DC power meter, or the battery monitor described below, tells you what your devices really use.

The fridge, pump and phone figures are the off-grid calculator’s presets; replace every line with your own.

The laptop runs on a 12 V USB-C charger rather than through an inverter: as the inverter section shows, that alone saves more energy than the lights and the pump use together.

The off-grid calculator with these six loads, 1.5 days of autonomy, 4 peak sun hours and 200 W panels: the numbers in this guide.Open the example in the calculator

Sizing the battery

The battery has to cover the days without sun you want to ride out, using only the part of its capacity it can deliver without harm. For a van that also charges while driving, a day or two is enough; a cabin used all year usually needs two or three. With 633 Wh a day, 1.5 days and LiFePO4 at 80% usable, the calculator asks for 1.2 kWh, or 99 Ah at 12 V: a 100 Ah LiFePO4 battery.

LiFePO4 or AGM?

• LiFePO4 delivers 80% or more of its capacity for thousands of cycles and weighs far less. The same example on AGM, at 50% usable and 85% charge efficiency, needs about 158 Ah. Our LiFePO4 vs AGM comparison has the details.
• AGM costs less up front and can be charged below freezing; it lasts much longer if it rarely goes below half charge.
• The cold. Lithium cells must not be charged below freezing: Victron’s controller manual says lithium batteries are prone to damage when charged below 5 °C, and its own Lithium NG battery accepts charge only from +5 to +50 °C. Discharging in the cold is fine. If your battery will see frost, choose one with low-temperature charge protection or self-heating.

From each maker’s product page or datasheet. Victron’s Lithium Smart and Lithium NG batteries need an external BMS: don’t buy one without it. On any battery, check the BMS current against your largest load (an inverter can need 100 A), the charge voltages, and low-temperature protection.

To read the charge from the voltage, use our 12 V LiFePO4 voltage chart, but expect little from it: at rest, a 12 V LiFePO4 battery reads between about 12.8 and 13.4 V all the way from 20% to 90%. That is why a battery monitor is worth having.

Panels and peak sun hours

Peak sun hours are the day’s solar energy on the panel, in kWh/m², expressed as hours of full 1000 W/m² sun. They depend on the season, the latitude and the panel’s tilt, and they matter more than any other input. As our off-grid calculator notes, in winter they can fall below 2 hours in central and northern Europe, while in summer they are often 5 to 6.

• Size for the worst month you want to cover. For a van used from May to September, that is the weakest of those months; for a cabin used all year, December (June or July in the southern hemisphere).
• Flat or tilted. Panels lying flat on a van roof collect much less in winter than panels tilted towards the sun at a cabin. In the cold months, a steep tilt (around your latitude or more) also sheds snow.
• Look up your place. In PVGIS, open the monthly data, pick your location and tick the horizontal irradiation for a flat roof, or the irradiation at your tilt angle for a cabin; divide each month’s kWh/m² by its days. For the US, PVWatts gives the same in kWh/m² per day.

For our van we design for 4 peak sun hours: check that against the months you travel. The calculator then asks for at least 274 W, after 20% losses, the controller’s 95% and a 25% margin for recharging after cloudy days. Two 200 W panels give 400 W, which collects about 1.2 kWh on such a day and refills the battery about 1.8 days after a day and a half without sun. With 1.5 peak sun hours, a flat roof in winter, the same loads would need 800 W (see it): more than a van roof holds, which is why vans also charge from the alternator.

Series or parallel, and shade

• In series the voltages add and the current stays that of one panel: a thinner cable down from the roof, and the voltage MPPT controllers like. But shade on one panel pulls down the whole string, softened only by the panels’ bypass diodes.
• In parallel the currents add: each panel works on its own, which suits roofs where a hatch, a rack or a tree shades one panel. The cable carries twice the current.
• Fuses in parallel. Each panel’s datasheet gives a maximum series fuse rating: 20 A for the Renogy RSP200D, whose short-circuit current is 11.05 A. With two in parallel, a faulty panel can only receive the other panel’s current, less than 20 A; with three, it can receive 22.1 A, so each panel then needs its own fuse.

From the makers’ datasheets. On any panel, check Voc, Isc, the temperature coefficient of Voc and the maximum series fuse, and measure your roof before you buy.

MPPT or PWM, and sizing the controller

A PWM controller connects the panel straight to the battery, so a panel built for about 18 V works near the battery’s 13 V and wastes up to about a quarter of its power. An MPPT controller converts the higher voltage down and keeps nearly all of it. PWM is fine for one small panel; for anything like our example, MPPT pays for itself.

Current

The calculator divides the panels’ 400 W by 12 V, 33.3 A, and rounds up to a 40 A controller. In practice the battery charges at about 13 V or more, so 400 W gives at most about 31 A: a controller rated for at least that current, and for 400 W at 12 V, will do. Victron’s datasheets say that if more PV power is connected, the controller limits its input power, so a slightly smaller controller only clips the brightest hours.

Voltage: the cold-weather check

A panel’s open-circuit voltage (Voc) rises as it gets colder, and it is highest at a frosty sunrise, before the load pulls it down. Two RSP200D in series give 46 V at 25 °C; with −0.28% per °C, at −10 °C that becomes 50.5 V. Victron’s manual adds a 10% margin in cold climates: 55.6 V, well within a 75 V or 100 V controller. A third panel in series would reach 75.8 V at −10 °C (69 V at 25 °C), over a 75 V controller’s limit, and Victron says damage from too high a PV voltage is not covered by its warranty. Use the coldest temperature you expect; in the mountains, −10 °C is not cold enough.

Voc(cold) = Voc × number in series × (1 + coefficient × (25 °C − coldest temperature))
2 × 23.0 V × (1 + 0.0028 × 35) = 50.5 V

The panels must also start the controller: Victron’s MPPTs start when the PV voltage exceeds the battery voltage by 5 V, and keep going down to 1 V above it. Two 12 V panels in series are far above that; a single one is enough in good light.

From the makers’ datasheets. For one 200 W panel, Victron’s 100/20 (20 A, 290 W at 12 V) is enough; the 75/15 is rated for 15 A and 220 W. On any controller, check: rated current ≥ panel watts ÷ 13 V, maximum PV voltage ≥ cold Voc plus 10%, a LiFePO4 profile with adjustable voltages, and a way to stop charging below freezing (a temperature sensor or a link to the battery’s BMS).

Cables, fuses and isolators

Thin cables waste energy as heat and can confuse the controller, which measures the battery through them. Victron’s Wiring Unlimited suggests keeping the drop below 2.5%; our wire calculator defaults to 3% and also checks the cable against its fuse. For the controller’s rated 40 A over 4.9 ft (1.5 m), it gives 6 AWG and a 50 A fuse; with metric sizes and their own ratings, 10 mm² and a 50 to 60 A fuse.

Controller to battery: 40 A, 4.9 ft, 3% drop.Open the wire calculator

The roof cable carries the string’s 10.42 A at 38.4 V, but in the US the National Electrical Code (690.8) bases PV wire sizing on the short-circuit current, with two 25% factors: 1.56 × 11.05 A = 17.3 A. Enter 1.25 × Isc, 13.81 A, as the load and the calculator adds the other 25%: for 16.4 ft it gives 12 AWG at 1.9% (check it); with metric sizes and their own ratings, 2.5 mm². Solar extension leads with MC4 plugs are usually thicker anyway; Victron notes MC4 connectors take 4 or 6 mm² solar cable. For panels you carry outside and plug in, Anderson Powerpole connectors are a common choice.

Fuses at the battery

Every positive cable that leaves the battery or the busbar gets a fuse at that end, rated for the cable, so that a short anywhere along it blows the fuse. With lithium, Victron also requires at least one fuse whose interrupt rating is at least the short-circuit current the battery can deliver: ask the battery maker for that figure.

Ranges from Victron’s Wiring Unlimited, with the Littelfuse MEGA datasheet and Blue Sea Systems’ MRBF block. Use blade fuses only for small branches, in a fuse box downstream.

Isolators

• PV isolator. Victron’s manual asks for a means to disconnect all current-carrying conductors of the PV array. Use a switch or breaker rated for DC at the array’s cold Voc and current, not an AC part.
• Battery switch. A switch on the positive lets you work safely; Victron recommends isolating the positive only. Victron’s Battery Switch is rated 275 A continuous at up to 60 V. Pull the main fuse instead if you have no switch.
• Order. Connect the battery to the controller first, then the panels: Victron’s MPPTs detect the system voltage from the battery.

Charging from the alternator: the DC-DC charger

Joining the two batteries with a relay (the classic split-charge relay) worked for lead-acid on older vehicles. With LiFePO4 it doesn’t: the lithium battery’s low resistance takes whatever current the alternator gives for as long as it can, and the charge voltage is the vehicle’s, not the battery’s. A DC-DC charger caps the current and applies the leisure battery’s own charge profile.

Many recent vehicles (the Euro 6 ones in Europe, as Victron’s manual puts it) have a smart alternator that varies its voltage and switches off when the engine doesn’t need it. A DC-DC charger copes with that if it can be told when the engine is running: Victron’s Orion-Tr Smart detects it from the voltage (starting at 14 V and stopping at 13.1 V in its smart-alternator setting), Renogy’s DCC30S takes an ignition signal wire. Check how your vehicle’s alternator is managed before you connect anything to it.

From the makers’ pages and manuals. On any DC-DC charger, check the output current against what your alternator can spare, a LiFePO4 profile, smart-alternator support, and the input fuse and cable the maker asks for.

Its input cable is the thick one: 360 W out at 87% is about 414 W in, some 33 A from the starter battery. Over 16.4 ft (5 m), the wire calculator gives 4 AWG; with metric sizes and their own ratings, 16 mm² and a fuse between 50 and 80 A, the same as Victron’s own table (16 mm², 60 A).

DC-DC charger input: 33 A over 16.4 ft.Check the cable

Frozen batteries. The documentation for these DC-DC chargers mentions no cut-off on battery temperature: with a battery that can freeze, the alternator would charge it frozen while you drive. Use a battery that blocks cold charging or heats itself (Renogy’s self-heating Pro, Victron’s SuperPack NG), or let a device that senses the battery temperature switch the charger off: the Orion-Tr Smart has a remote on/off input that can do this.

The inverter: pure sine, standby and surge

An inverter turns 12 V into mains-like AC. Choose a pure sine model, whose output has the same waveform as the grid that appliances are designed for, and size it for two numbers: the continuous power of everything you run at once, and the start-up surge of motors and compressors. Then look at a third, the standby draw.

Standby is the one people forget. Victron’s Phoenix 12/500 draws 6.0 W with no load, 144 Wh a day if it stays on, three times our router. Run the laptop through it instead of a USB-C charger and the day grows to 792 Wh, the battery to 124 Ah (see it in the calculator). In ECO mode (1.0 W, waking when a load appears) the day drops to 672 Wh and the battery to 105 Ah. Better still: run what you can on 12 V, and switch the inverter off.

From the makers’ datasheets. On any inverter, check continuous power at your temperature (Victron’s fall at 40 °C), peak power, no-load draw, the output voltage for your country, and the DC cable and fuse its manual asks for.

The inverter’s cable carries the largest current in the system: a 1000 W load is close to 100 A at 12 V. Follow the manual (Victron asks for 6 mm² up to 1.5 m on the 12/500, 10 mm² up to 3 m, and an extra fuse near the battery beyond 1.5 m), and check that the battery’s BMS allows that current.

A battery monitor

A LiFePO4 battery’s resting voltage barely moves between 20% and 90%, so a voltmeter can’t tell you the charge. A shunt monitor counts every amp-hour going in and out. The shunt goes in the negative: the battery’s negative terminal connects to the shunt alone, and every other negative cable lands on its other side, as in the diagram above.

From the makers’ pages. Check that the shunt is rated above your largest current and that nothing bypasses it.

The worked example, part by part

In the calculators: off-grid example, controller cable, DC-DC cable, roof cable.

A budget build of this example could be a LiTime 100 Ah, two RSP200D panels, an EPEver Tracer4210AN and Renogy’s monitor; a premium one, Victron’s SuperPack NG, SmartSolar 100/50 and Orion-Tr Smart 12/12-30: the SuperPack NG has a built-in BMS and battery monitor, so a SmartShunt is optional there. The budget build, on the other hand, has no low-temperature charge cut-off anywhere (this LiTime model has none, and EPEver lists none): if the battery can freeze, choose LiTime’s self-heating version or another battery with low-temperature protection. Mixing brands works, as long as every charger is set to the battery’s charge voltages. The calculator’s figures are the minimum: an extra panel or a 150 Ah battery costs less than running out of power.

Safety

• Fix the battery down. In a vehicle it must stay put in a crash; use the maker’s mounting points or a strapped box.
• Crimp, don’t twist. Use crimped lugs of the right size with a proper crimping tool, and torque the terminals as the manual says. A loose lug heats up.
• Route and protect cables. Grommets where they pass through metal, no sharp edges, and the roof entry sealed.
• Mains side. An inverter’s AC output is as dangerous as a wall socket. In a vehicle, check local rules and your insurance before changing the electrical system; in a building, have fixed AC wiring done or checked by a qualified electrician where your local rules require it.
• Cold and heat. No lithium charging below freezing without protection; keep the battery and the controller ventilated and away from the engine’s heat.
• One fuse per battery. With batteries in parallel, give each battery its own fuse, so that a fault in one isn’t fed by the others.
• Precharge the inverter. Its input capacitors draw a large inrush current when first connected: on a lithium battery that can cause an arc at the terminal or trip the BMS. Charge them first through a resistor for a few seconds, then make the main connection.
• AGM too. An AGM battery is sealed, but give it ventilation anyway, and never charge it frozen.

Common mistakes

• Sizing the panels for July and wondering why the battery is flat in October.
• A 75 V controller on three 12 V panels in series: fine at 25 °C, too high on a frosty morning.
• Leaving the inverter on for one small load: its standby can use more than the load.
• Fuses rated for the device, not the cable, or fitted at the wrong end of the cable.
• A relay between the vehicle battery and a LiFePO4 battery instead of a DC-DC charger.
• Negatives straight to the battery, bypassing the shunt, so the monitor misses that current.
• Trusting the voltage to tell a LiFePO4 battery’s charge.