# Camper Van Electrical System Guide: Planning a Build

> A camper van electrical system is designed backward from a measured daily amp hour load. That number sets battery bank size, which sets charge input from alternator and solar, which sets wire gauge, fusing and inverter capacity. Guessing at any stage compounds through the whole build.

**Type:** Guide  
**Canonical:** https://ocrv.pro/knowledge/guides/camper-van-electrical-planning-guide  
**Updated:** 2026-07-29

## Summary

Size a camper van electrical system from measured daily loads, not from battery marketing. Everything downstream, wire, fusing, charging and inverter, follows that one number.

Most camper van electrical systems that disappoint their owners were sized by working forward from a product instead of backward from a load. Somebody buys a 200 amp hour battery because that is what the forum said, adds 400 watts of solar because the kit was on sale, and then discovers in December that the induction cooktop and the diesel heater and the laptop and the refrigerator do not coexist. The components were fine. The sequence was wrong.

The correct sequence is boring and it works. Measure or calculate what you actually consume in a day, in amp hours at 12 volts. Size the battery bank to carry that through the number of days you intend to go without charging. Then size charge input to replace that consumption in the time and conditions you actually have. Then size conductors, fusing and the inverter to the resulting currents. Every one of those steps depends on the one before it.

This guide walks through that sequence with real numbers, then covers the parts of a van electrical build that are not about sizing at all: chassis integration on a Sprinter, Transit or ProMaster, grounding and bonding, safety and inspection considerations, and the documentation you need if the vehicle is ever in a collision. We build and repair these systems in the shop, and a meaningful share of the repair work is correcting decisions made in the first hour of a build.

## Start With a Real Load Audit

List every electrical device that will live in the van. For each one, record the current draw in amps at 12 volts and the realistic hours per day of operation. Multiply and sum. Devices that run on 120 volts through an inverter need their consumption converted back to the 12 volt side, and you have to include inverter inefficiency, which is generally 10 to 15 percent, plus the inverter idle draw for every hour it is switched on.

The numbers that surprise people are the continuous ones. A 12 volt compressor refrigerator drawing an average of 3 amps over a 24 hour duty cycle is 72 amp hours a day all by itself, and in summer heat that climbs. A diesel or gasoline air heater draws a few amps while its glow plug cycles and under one amp running, but it runs all night. A laptop through an inverter is far more expensive in amp hours than the same laptop on a 12 volt supply. Meanwhile the lights everyone worries about are trivial.

Build the audit as a table and be honest about hours. Then add a contingency, because every van gains devices after the build is finished. We typically design to the audited number plus 25 percent, and we tell owners which specific loads to drop first if the design has to be constrained by budget or by space. That conversation is much easier before anything is purchased.

- **Refrigerator dominates:** A 12 volt compressor fridge commonly runs 50 to 90 amp hours per day depending on ambient temperature, insulation and door discipline. It is usually the single largest continuous load in the van.
- **Inverter overhead is real:** Count 10 to 15 percent conversion loss plus the idle draw of the inverter for every hour it is powered. A large inverter left on all day consumes meaningful energy doing nothing.
- **Heating is hours, not amps:** An air heater draws little while running but runs for eight hours. Startup glow plug cycles are the current spike. Total daily consumption is modest but not zero.
- **High draw appliances:** Induction cooktops, electric kettles, hair dryers and air conditioning draw very large currents for short periods. They size the inverter and the conductors even if their daily energy is small.
- **Design margin:** Add 25 percent to the audited figure. Every van acquires devices after the build. Designing to exactly the audit guarantees a system that is undersized within a year.

## Sizing the Battery Bank

Battery capacity is daily consumption multiplied by days of autonomy, adjusted for usable depth of discharge. This is where lithium iron phosphate and lead acid diverge sharply. A LiFePO4 bank can be discharged to roughly 80 to 90 percent of nameplate capacity routinely without harm. An AGM bank should be held to about 50 percent to get a reasonable cycle life. So 200 amp hours of LiFePO4 delivers roughly what 350 amp hours of AGM delivers, at about a third of the weight.

LiFePO4 also charges faster and accepts high current without complaint, which changes the charging design downstream. AGM has a limited acceptance rate that tapers badly in the absorption stage, which means a big solar array cannot actually put its output into an AGM bank in the middle of the day. If your charging window is short, and on a van in winter it is, lithium chemistry is not a luxury, it is what makes the arithmetic work.

The counterweights are cost and cold. LiFePO4 costs more up front, though the cost per usable amp hour over the life of the bank generally favors it. And LiFePO4 must not be charged below freezing without either internal heating or a battery management system that blocks charge current when cold. In coastal Southern California that is rarely a factor. For anyone taking the van to the Sierra in winter, it is a design requirement rather than an afterthought.

- **Usable capacity, not nameplate:** LiFePO4 delivers 80 to 90 percent of nameplate routinely. AGM should be held near 50 percent for reasonable cycle life. Size on usable capacity or the bank will disappoint.
- **Charge acceptance:** LiFePO4 accepts high current across most of its charge curve. AGM tapers sharply in absorption, which wastes solar production in the middle of the day when it is available.
- **Weight and space:** Roughly a third of the weight of equivalent usable AGM capacity. On a van with payload limits and a build that is already heavy, this is a genuine design factor.
- **Cold charging limits:** LiFePO4 must not be charged below freezing without internal heating or a battery management system that blocks charge. Plan for it if the van will see winter altitude.
- **Battery management system:** A LiFePO4 bank without an appropriate management system is not a finished component. Cell balancing, temperature cutoff and over-current protection are part of the battery, not accessories.

## Charging: Alternator, Solar and Shore Power

Three sources charge a van house bank and they have completely different characteristics. Alternator charging through a DC-DC charger is the highest output and the most reliable, because it works at night, in the rain and in a parking garage, and it produces 30 to 60 amps whenever the engine runs. Anyone who drives regularly should treat this as the primary source and solar as the supplement, which is the reverse of how most builds are planned.

A DC-DC charger is required rather than optional on any modern van. Direct connection through a simple relay was acceptable when both banks were lead acid, but a lithium bank connected directly will demand more current than the alternator can safely sustain, and on vans with smart or temperature-compensated charging the vehicle will not produce the correct voltage anyway. The DC-DC charger isolates the two systems, controls current, and delivers the right charge profile for the house chemistry.

Solar is quiet, needs no driving, and produces far less than its nameplate suggests. A 400 watt array on a van roof in real conditions, with a good MPPT controller, will typically produce 1,200 to 1,800 watt hours on a clear summer day and a fraction of that in December marine layer. Shore power through a converter or an inverter-charger is the third source and it is the one that saves you when the other two have not kept up, which is why a properly sized shore charger is worth the panel space even on a van that rarely plugs in.

- **DC-DC charger:** Required for lithium house banks on modern vans. Isolates systems, limits current to protect the alternator, and delivers the correct profile. Size at 30 to 60 amps for most builds.
- **MPPT over PWM:** Maximum power point tracking harvests meaningfully more from the same panels, especially in partial shade and cool conditions. The cost difference is not worth arguing about.
- **Real solar output:** Expect roughly 3 to 4.5 watt hours per nameplate watt on a good summer day, much less in winter or under marine layer. Design to the winter number if you camp in winter.
- **Shore charging:** A converter or inverter-charger sized to the bank recovers the system quickly when solar and driving have not kept up. Worth the space even for occasional use.
- **Alternator capacity:** Verify what the vehicle alternator can actually spare with the engine at idle and the vehicle systems running. A DC-DC charger set beyond that capacity creates a mechanical problem, not an electrical one.

## Inverter Sizing and 120 Volt Design

Size the inverter to the largest simultaneous 120 volt load, not to the total of everything you own. A build with a 1,500 watt induction cooktop and nothing else large needs about a 2,000 watt inverter. A build that also runs a 1,200 watt air conditioner at the same time needs considerably more, and at that point the current draw on the 12 volt side becomes the real design constraint. A 3,000 watt inverter at full output pulls roughly 250 to 300 amps from the battery, which drives cable size, fuse rating and battery discharge capability all at once.

Pure sine wave rather than modified sine wave, without exception. Modified sine wave equipment causes trouble with anything containing a motor, a switching power supply or sensitive electronics, and the price gap has closed to the point where there is no reason to consider it. An inverter-charger that combines the inverter, the shore charger and a transfer switch in one unit is usually the better choice in a van because it consolidates three components and one set of connections.

On the 120 volt side, treat the van like a small building. A main breaker, individual branch circuits, ground fault protection at outlets, and correct conductor sizing. The transfer switch or automatic transfer switch must prevent the inverter and shore power from ever being connected at the same time, and the neutral to ground bonding must be handled correctly by the inverter when it is the source, which is a detail that a lot of otherwise good builds get wrong.

- **Size to simultaneous load:** The largest set of loads that will run at once, not the sum of everything owned. Oversizing costs money, space and idle consumption for capacity you never use.
- **Pure sine wave only:** Modified sine wave causes problems with motors, switching supplies and electronics. There is no meaningful price argument for it anymore.
- **DC current is the constraint:** A 3,000 watt inverter pulls roughly 250 to 300 amps at 12 volts. That figure sizes the cable, the fuse and the battery discharge rating, and it is where builds go wrong.
- **Transfer switching:** Shore power and inverter output must never be connected simultaneously. An automatic transfer switch or an integrated inverter-charger handles this correctly.
- **Neutral to ground bonding:** Bonding must exist at exactly one point for the active source. Inverters handle this internally when configured correctly. A double bond or no bond is a genuine safety fault.

## Conductors, Fusing and Protection

Wire gauge is determined by current and by acceptable voltage drop over the run length, and on a 12 volt system voltage drop is the binding constraint far more often than ampacity. A 3 percent drop budget is a reasonable target for most circuits and a 2 percent target for battery to inverter runs. Because 12 volt systems carry large currents, the cables are physically large, and builders who size by ampacity alone end up with dim lights, a refrigerator that cycles oddly and an inverter that shuts down on low voltage under load.

Every conductor gets overcurrent protection at the source end, sized to protect the wire, not the device. A fuse or breaker on the battery positive within a short distance of the terminal is not optional, because a battery cable that shorts to the chassis without protection will start a fire in seconds. Use appropriately rated Class T or similar fuses for lithium banks, which can deliver extremely high fault currents.

Physical protection matters as much as electrical protection. Grommets at every panel pass-through, strain relief at every termination, chafe protection where cable runs contact metal, and mechanical support so no connection carries the weight of the cable. A van vibrates continuously for its whole life, and every failure we trace in a van electrical system that was not a design error was a mechanical one: a chafed conductor, a loose lug, a terminal that fatigued.

- **Voltage drop sizes the wire:** Target 3 percent on most circuits and 2 percent battery to inverter. On 12 volt systems this governs long before ampacity does.
- **Fuse protects the wire:** Sized to the conductor, located at the source end, as close to the battery terminal as practical. This is the difference between a fault and a fire.
- **Class T for lithium:** LiFePO4 banks deliver very high fault currents. Main protection must have an interrupt rating suited to that, which ordinary blade fusing does not provide.
- **Chafe and strain relief:** Grommets at every pass-through, support so connections do not carry cable weight, protection wherever a run touches metal. Vans vibrate for their entire service life.
- **Shunt and monitoring:** A shunt-based battery monitor on the negative side is the only accurate way to know state of charge on a lithium bank. Voltage alone tells you almost nothing.

## Chassis Integration on Sprinter, Transit and ProMaster

Modern vans are networked vehicles and the house system has to coexist with that network. Sprinters in particular have a body controller that monitors electrical behavior, and connections made to the wrong circuits produce fault codes, disabled features and occasionally a vehicle that will not start. Mercedes provides defined auxiliary connection points and a battery management setup for upfitters, and using them is considerably easier than troubleshooting what happens when you do not.

Every chassis has its own upfitter documentation and every one of them is worth reading before drilling anything. Body builder guides define where you may drill, where you may weld, which structural members may not be penetrated, how to route through the floor without compromising corrosion protection, and which circuits are available for accessory use. Ignoring these documents is the origin of most of the coverage disputes and most of the corrosion problems we see on conversions.

Roof penetrations for solar and vents deserve particular attention because they are the most common source of water intrusion in a conversion. Use the correct fastener, seal with an appropriate polyurethane or butyl system, treat every cut edge for corrosion, and never rely on adhesive mounts alone in a vehicle that will see freeway speeds and crosswind loads. We repair the consequences of shortcuts here regularly and it is always more expensive than doing it correctly.

- **Use the upfitter connection points:** Mercedes, Ford and Stellantis all publish body builder guides with defined auxiliary circuits and connection points. Using them avoids body controller faults and disabled vehicle features.
- **Respect no-drill zones:** Body builder documentation identifies structural members and safety system routing that must not be penetrated. This is a crash safety matter, not a preference.
- **Treat every cut edge:** Bare steel at a drilled hole is a corrosion origin, and on the coast it starts immediately. Prime and seal every penetration on the day it is made.
- **Roof penetration method:** Mechanical fastening plus a correct sealant system. Adhesive-only mounts on a vehicle that sees freeway crosswind are a repair waiting to happen.
- **Alternator and idle capacity:** Confirm available alternator output at idle with vehicle loads active before setting DC-DC charger current. Some vans have far less spare capacity than their nameplate suggests.

## Documenting the Build for Insurance and Resale

A converted van is a cargo van with a large invisible asset inside it, and every valuation system in the insurance industry prices it by VIN. That means a $60,000 conversion on a 2022 Sprinter 170 will be valued as a 2022 Sprinter 170 cargo van unless you supply the evidence otherwise. We have watched this exact scenario play out on collision claims more than once, and correcting it after the fact takes weeks that a properly documented build avoids entirely.

Build the documentation as you go rather than reconstructing it later. Photograph the wiring before the walls close, with component labels visible. Keep every receipt in a cloud folder. Maintain a written schedule listing each component with model number, cost, install date and installer. Draw a one-line electrical diagram showing the sources, protection, distribution and loads, and keep a copy in the van and a copy in the cloud.

That same documentation is what makes future service possible. A van that arrives at our shop with a wiring diagram and labeled components gets diagnosed in an hour. A van with an undocumented build and no labels can take a full diagnostic day just to establish what is connected to what, at $285 for the first hour and $260 an hour after that. The diagram is the cheapest component in the entire system.

- **Photograph before closing walls:** Every run, every termination, every component with its label visible. Once the paneling is in, this record cannot be recreated without disassembly.
- **One-line diagram:** Sources, protection, distribution, loads, with wire gauges and fuse ratings noted. Copy in the van, copy in the cloud. Pays for itself the first time anything needs service.
- **Component schedule with receipts:** Model numbers, costs, install dates. This is the document that gets a conversion valued correctly on a claim rather than priced as a cargo van.
- **Confirm accessory coverage:** Ask your carrier specifically about coverage for permanently attached equipment and whether the build needs to be scheduled. Many policies cap it at a modest flat figure.
- **Label at the panel:** Every breaker, every fuse position, every switch. It costs an hour during the build and saves multiple diagnostic hours over the life of the vehicle.

## What a Van Electrical System Costs

A modest system with a 100 amp hour lithium bank, a DC-DC charger, 200 watts of solar, a small inverter and a proper distribution panel typically lands in the $3,500 to $6,500 range installed, depending on how much cabinetry work is involved. A comprehensive system with 400 to 600 amp hours of lithium, 600 watts or more of solar, a 3,000 watt inverter-charger, a full 120 volt panel and shore inlet, and an air conditioner runs $12,000 to $30,000 installed.

Van build outs overall run $5,000 to $80,000 and up at our shop, and electrical is usually between 15 and 35 percent of that. Mechanical and electrical labor is posted at $260 per hour, diagnostics at $285 for the first hour applied to the work. Parts under $100 carry 100 percent markup, parts over $100 carry 35 percent, and special order items require a non-refundable deposit because they cannot be returned.

The most useful thing we can say about budget is that correcting a poorly planned system costs more than building it correctly, and often costs more than the original system did. Undersized cable has to be replaced entirely. A battery bank that is too small cannot be extended with a mismatched second bank. An inverter chosen without regard for its DC current draw requires new cable, new fusing and sometimes a new battery. Spend the planning time first.

| Line item | Range | Hours |
| --- | --- | --- |
| Van build out, full conversion scope | $5,000 to $80,000+ | 30 to 600 |
| Solar array design and installation | $1,500 to $12,000+ | 6 to 45 |
| Lithium battery bank and management | $500 to $5,500 | 3 to 20 |
| Inverter or inverter-charger installation | $750 to $4,500 | 4 to 18 |
| Electrical diagnostics and troubleshooting | $285 to $5,000+ | 1 to 20 |

## What is included

- A load audit method that produces a real daily amp hour number
- Why the refrigerator and inverter idle draw dominate most van budgets
- Battery bank sizing on usable capacity rather than nameplate
- LiFePO4 versus AGM on charge acceptance, weight, cold limits and cost
- Why alternator charging through a DC-DC unit should be the primary source
- Realistic solar output figures for summer and winter conditions
- Inverter sizing to simultaneous load and the DC current that follows
- Conductor sizing by voltage drop, fusing that protects the wire, and chafe protection
- Chassis integration rules for Sprinter, Transit and ProMaster upfits
- Build documentation that protects claim value and future serviceability

## Questions

### How many amp hours do I need for a camper van?

Calculate rather than guess. Sum the daily amp hour consumption of every device at 12 volts, including inverter conversion loss and idle draw, then multiply by the days you want to go without charging, then divide by usable depth of discharge. A typical build with a compressor refrigerator, lights, fans, water pump and device charging lands between 80 and 150 amp hours per day, which usually means a 200 to 400 amp hour lithium bank.

### Do I need a DC-DC charger in a van conversion?

On any modern van with a lithium house bank, yes. A direct connection through a simple relay was acceptable when both banks were lead acid. A lithium bank will demand more current than the alternator can safely sustain, and vans with smart or temperature-compensated charging will not deliver the correct voltage anyway. The DC-DC charger isolates the systems, limits current to protect the alternator, and delivers the right charge profile.

### How much solar can I actually fit and produce on a van roof?

Most Sprinter, Transit and ProMaster roofs accommodate 400 to 800 watts once vents and a fan are accounted for. Real output is roughly 3 to 4.5 watt hours per nameplate watt on a clear summer day with a good MPPT controller, and substantially less in winter or under coastal marine layer. Solar is a valuable supplement. For most owners who drive regularly, alternator charging is the more reliable primary source.

### What size inverter should I put in my camper van?

Size to the largest set of loads that will run at the same time, not to the total of everything you own. A build with a 1,500 watt induction cooktop and nothing else large needs roughly 2,000 watts. Remember the DC side: a 3,000 watt inverter at full output draws 250 to 300 amps at 12 volts, which dictates cable size, fuse rating and battery discharge capability all at once.

### What does a camper van electrical system cost to install?

A modest system with a 100 amp hour lithium bank, DC-DC charger, 200 watts of solar, a small inverter and a proper distribution panel typically runs $3,500 to $6,500 installed. A comprehensive system with 400 to 600 amp hours, 600 watts or more of solar, a 3,000 watt inverter-charger and a full 120 volt panel runs $12,000 to $30,000. Mechanical and electrical labor is posted at $260 per hour.

### Will my insurance cover the electrical system in my van conversion?

Only if it is documented and, on many policies, only if it is scheduled. Carrier valuation prices by VIN, so a converted Sprinter is valued as a cargo van unless you supply a component schedule with model numbers, costs, install dates and receipts, plus photographs of everything installed. Ask your agent specifically about the coverage limit for permanently attached equipment before you need to know the answer.

## Related

- https://ocrv.pro/repairs/electrical-systems
- https://ocrv.pro/repairs/solar-and-power-systems
- https://ocrv.pro/sprinter-van-repair
- https://ocrv.pro/repairs/custom-fabrication
- https://ocrv.pro/knowledge/case-studies/sprinter-conversion-collision
- https://ocrv.pro/knowledge/guides/rv-insurance-claim-documentation-guide