How to size your van electrical system (battery, solar, etc.)

How to size your van electrical system (battery, solar, etc.)


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As an example, we'll go through the design of our own electrical system. Here is what we need to find out:
Daily Power Usage

? Ah


? W

Battery Bank

? Ah

Solar Power

? W

Solar Charger

? | ?

Alternator Charger

? A

Shore Charger

? A

Step 1: Daily Power Usage

The daily energy usage is pretty much running the whole show. Indeed, consuming more energy means that a bigger battery bank is needed; consequently, bigger charge sources are needed as well. So the very first thing to do is calculate how much energy we consume each day. To find out, the daily energy usage of each individual load is added:


  • DAILY ENERGY USAGE: Amount of energy required per day. The unit of measure is Ampere x hour, or Ah. The shower analogy would be how many gallons of water you need per day to shower.
  • CURRENT: Instantaneous flow of energy. The unit is Ampere, or A. The shower analogy would be how many gallons your shower is able to deliver each minute (gallon per minute, or gpm)
  • TIME: How many hours per day the load is used. The shower analogy would be how much time you need to wait for your loved one to get out of the shower so you can use the toilet 😛

To make things easy, we created a calculator.  Your job is to find the current draw for each of your loads (check the owner’s manual, spec sheet, product website, etc.), and the calculator will do the rest. The numbers below are based on true measurements, and we confirm that they’re pretty accurate (see our Real-World Data section for more info):

12V DC

FridgeNovaKool R58103.8A35% duty cycle3.8A * 35%*24h=31.9Ah20% duty cycle3.8A * 20%*24h=18.2Ah
Roof fanMaxxfan 6200K (10 Speed)0.1A@1, 0.2A@2, 0.3A@3, 0.4A@4, 0.6A@5, 0.9A@6, 1.1A@7, 1.5A@8, 2.0A@9, 2.8A@1020h per day @ 3 average0.3A*20h=6Ah12h per day @ 2 average0.2A*12h=2.4Ah
Wall FanSirocco II Elite0.2A@1, 0.3A@2, 0.5A@3, 0.9A@43h per day @ 20.3A*3h=0.9Ah   
Floor Vent FanSeaFlo 4″0.5A (dimmed)2h per day0.5A*2h=1Ah   
Ceiling Lights3W LEDs1.3A (total 10 lights @ 100% intensity)4h per day @ 80% intensity1.3A*80%*4h=4.2Ah8h per day @ 60% intensity1.3A*60%*8h=6.2Ah
Garage LightsLEDs Strip1.2A6min per day1.2A*0.1h=0.1Ah   
Water PumpShurflo6A12min per day6A*0.2h=1.2Ah6min per day6A*0.1h=0.6Ah
Composting Toilet FanNature’s Head0.1A24h per day0.1A*24h=2.4Ah24h per day0.1A*24h=2.4Ah
Propane SolenoidAFC1.2A2h per day1.2A*2h=2.4Ah2h per day1.2A*2h=2.4Ah
Webasto HeaterAirTop 2000 STC1.7A (average)   12h per day (estimate)1.7A*12h=20.4Ah
Propex HeaterHS20001.6A   Not Used (backup)  
Phone2 Smartphones1A (each)2h per day x 21A*2h*2=4Ah2h per day x 21A*2h*2=4Ah
TOTAL    54.1Ah  56.6Ah

120V AC

Laptop 1 Asus 45W Average 1.5h per day 45W x 1.5h/12V/0.85 6.6Ah
Laptop 2 HP 35W Average 1.5h per day 35W x 1.5h/12V/0.85 5.1Ah
Milk Frother Nespresso Aeroccino 550W 2 min per day 550W x 0.03h/12V/0.85 1.6Ah
Immersion Blender Braun 200W 30 sec per day 200W x 0.01h/12V/0.85 0.2Ah
Drone Charger DJI 40W Meh, not really using it
TOTAL 13.5Ah
Daily Power Usage
0 Ah

Step 2: Inverter

The size of our inverter might impact our battery bank. Indeed, inverters draw a huuuge amount of current, and our battery bank might not be able to handle it. So it’s a good thing to decide on our inverter size now. The goal here is to calculate what is the maximum power that we’ll ever need at any given time. To find out, we add the power of each load we plan on using simultaneously.

Thinking of using that induction cooktop (e.g. 1800W) and microwave (e.g. 1800W peak) at the same time? That would require an inverter of about 4000W! Don’t get us wrong, it can be done, but that’s an absurd amount of power for an off-the-grid system, and its quite dangerous (over 350A of current) if your connections are not perfect…  By using these appliances one after the other, you can go with a smaller inverter… 🙂

Simultaneous 120V AC Loads:
Milk FrotherNespresso Aeroccino550W max
Laptop 1Asus90W max
Laptop 2HP45W max
TOTAL 685W max

A “685W” inverter would be our absolute minimum, but it’s a good idea to keep some buffer so we went for:

1000W Samlex Pure Sine Inverter

Inverter Items List

#ItemDescriptionQuantityView on Amazon
11000W InverterSamlex PST-1000-12 PST Pure Sine1View
2Remote Control for InverterSamlex RC-15A for 600W/1000W Inverter1View
3Cable, 2 AWG, 5 ft Black + 5 ft RedWindyNation1View
4Lugs, 2 AWG Cable, 5/16″ RingConnect to Terminal Fuse Block and Bus Bar (Pack of 2)1View
5Terminal Fuse, 175ABlue Sea (To protect inverter’s cable)1View
6Terminal Fuse BlockBlue Sea (Connects directly on the Bus Bar. Holds the Terminal Fuse)1View
0 W

Step 3: Battery Bank

3.1- Sizing the battery based on Daily power usage

Sizing a battery bank is not an exact science because it’s influenced by parameters we don’t have control of (weather) and personal parameters (driving habits and access to shore power). Notice that battery size (capacity) is expressed in Ah. That’s why we calculated our daily power usage in Ah (not in Wh); it’s more intuitive that way!

3.1.1- Recommended Battery Bank

How many days without solar are you willing to accept? 4 days overcast can happen, so our battery bank should have enough power to go through this:

*Most economic Depth-of-dischage (DOD) for Lithium is 80%
*Most economic Depth-of-dischage (DOD) for AGM is 50%

Hughhh wait! That’s a MASSIVE battery bank for our needs… It’ll use quite a lot of space, and the initial cost will be HUGE! If we invest in a B2B charger (alternator charging), we won’t rely on weather alone, and it’s reasonable to aim for 1.75 factor. Why 1.75? Again, this is not exact science, but this value is a good rule of thumb and has worked for us…

Lithium Battery
Recommended Size
Recommended Size

That makes much more sense. And by reducing our battery bank size, the B2B charger just paid for itself!

3.1.2- Sun Seeker (Or Heavy Driver) Battery Bank

Planning on using your van only in summer or along the sun belt? Like to drive a lot? Use a factor of 1.5:

Sun Seeker Size
Sun Seeker Size

3.1.3- Snow Chaser Battery Bank

Planning on using your van to chase the snow? Ride on! Use a factor of 2:

Snow Chaser Size
Snow Chaser Size

We fit in the “snow chaser” category, so we need at least:


Battery Bank (min)

3.2- Sizing the battery base on the inverter

Here we want to make sure that the inverter is not drawing more current (amps) than the battery is able to deliver safely. For example, let’s take a Samlex 1,000W inverter hooked up to one (1) 100Ah Battle Born Lithium battery:

3.2.1- Worst Case Scenario
1,000W Inverter
Inverter Continuous Current
100Ah Battle Born Battery

Worst case scenario, a 1000W inverter draws more current (110A) than a 100Ah Lithium battery can take (100A max). BUT, it’s quite unlikely that the inverter will be used continuously within that “worst case” range… So maybe we can be more reasonable:

3.2.2- "Real-Life" Worst Case Scenario
1,000W Inverter
Inverter Continuous Current
100Ah Battle Born Battery

We conclude that a 1000W inverter can be used with a single 100Ah Lithium battery (2000W inverter with 200Ah battery bank, and so on) at the very minimum (we reached out to Battle Born Batteries and they concur with that conclusion). However, what if you need to use the inverter at full capacity AND other 12V loads simultaneously? You’ll most likely go over the limit of allowable continuous current (100A)… Therefore, to get a robust design, we would add an extra battery.

3.2.3- Rule of Thumb
Minimum Battery Size for Inverter:
(AGM can actually deliver more current without hurting them, but you would go down to 50% SOC way too fast)
Recommended Battery Size for Inverter:

We’re going for Lithium batteries, and we fit in the “snow chaser” profile, so we need at least 178Ah. We’d rather have excess power than not enough, so we went for:

2 x 100 Ah Battle Born Batteries

Battery Items List

#ItemDescriptionQuantityView on Amazon
1LiFePO4 200 AhBattle Born LiFePO4 100 Ah
22/0 Cable in 5/16 lugs, 1 feet Red + 1 feet BlackWindy Nation Copper
0 Ah
Battery Bank

Step 4: Solar Power

4.1 Solar Panel Wattage

Solar input varies with weather, seasons, location, and where you park (full sun VS shade). So again, it’s not a precise recipe…

Rule of Thumb:


It means that if we want to harvest 71Ah per day, we need (71Ah x 4W/Ah)= 284W of solar panels. Now keep in mind this is a broad rule, so we will harvest more in summer and less in spring/fall.

Sun Seeker Solar Power
Recommended Solar Power
Snow Chaser Solar Power

About winter: In December and January, solar is pretty much nonexistent (because of the sun angle, short solar day, and snow on the panels), so more solar panels won’t really help. But more solar panels do help a lot in late October/November and in February/March. That’s why we recommend more solar for snow chasers…

4.2- Solar Charge Controller

Victron makes excellent solar charge controllers at an honest price, so we don't see a reason to go anywhere else... So here are Victron recommendations:
Nominal Power (W) Victron Solar Charge Controller (click to see on Amazon)
0-145W 75|10
146-220W 75|15
221-290W 100|20
291-440W 100|30
441-700W 100|50
701-860W 150|60
861-1000W 150|70

We fit within the “Snow Chaser” profile, so we need at least 340W solar power.  In 2016, 2 x 160W panels is what we could find, but today we would go with:

- 2 x 175W Newpowa Solar Panels
- Victron 100|30 SmartSolar Solar Charge Controller

Solar Items List

#ItemDescriptionQuantityView on Amazon
1350W SolarNewPowa 175W Mono Panel2View
2Extension Cables, 8 AWG, 15 ft Red + 15 ft BlackWith MC4 Connectors1View
3Double Cable Entry GlandFor 8 AWG or 10 AWG Cable1View
440A Breaker/Switch, Surface MountBetween Panels and MPPT Charger1View
5MPPT Solar ChargerVictron 100|30 SmartSolar MPPT1View
640A Breaker/Switch, Surface MountBetween MPPT Charger & Battery1View
7Heat Shrink Terminal Ring, 8 AWG Cable, 5/16″ RingConnect to Bus Bar (Pack of 3)1View
8Heat Shrink Terminal Ring, 8 AWG Cable, 1/4″ RingConnect to Breakers (Pack of 3)2View
Solar Power
0 W


Victron Solar Controller

Step 5: Alternator Charger

We don’t like driving for no reason, but having the ability to charge from the alternator can drastically reduce our battery bank size, and we don’t depend on the weather anymore.

There are two requirements to fulfill:

1- Battery Recommended Charge Current
(per spec sheet)
Rule of thumb:
2- Don't exceed the Alternator Available Power (or "Unused Power")
Depends on vehicle & alternator option

In our case:

2 x BattleBorn Batteries wired in parallel
Ford Transit with heavy-duty alternator (230A)
Alternator Available Power
*80A value obtained from (unofficial source)

We could go up to 100A, but we went with:

60A Sterling Power Battery-to-Battery Charger

Alternator Items List

#ItemDescriptionQuantityView on Amazon
160A Battery-to-Battery Charger (B2B)Sterling Power BB12601View
2100A Breaker/Switch, Surface MountBlue Sea 285-Series2View
3Cable, 4 AWG, 15 ft Black + 15ft RedWindyNation1View
4Lugs, 4 AWG Cable, 5/16″ RingConnect to Bus Bar (Pack of 2)1View
5Lugs, 4 AWG Cable, 1/4″ RingConnect to Breakers (Pack of 2)2View


Alternator Charger

Step 6: Shore Charger

Together, we’re designing an electrical system for an off-the-grid vehicle. So, honestly, you probably don’t need shore power, except…

Consider adding shore power if:

It’s the same requirement as for the alternator charger, except this time we’re not limited by the vehicle alternator’s available unused power:

1- Battery Recommended Charge Current
(per spec sheet)
Rule of thumb:

We could go up to 100A (0.5 x 200Ah), but we went with:

50A Samlex Smart Battery Charger/Converter

Shore Items List

#ItemDescriptionQuantityView on Amazon
150A ChargerSamlex SEC-1250UL 12V1View
260A Breaker/Switch, Surface MountBetween Charger and Bus Bar1View
3Cable, 8 AWG, 5 ft Black + 5 ft RedWindyNation1View
4Heat Shrink Terminal Ring, 8 AWG Cable, 1/4 RingConnect to Breaker (Pack of 3)1View
5Heat Shrink Terminal Ring, 8 AWG Cable, 5/16″ RingConnect to Bus Bar (Pack of 3)1View


Shore Charger
Here is how we sized our electrical system:


Daily Power Usage




Lithium Battery


Solar Power


Alternator Charger


Shore Charger


It’s now time to size and make your own electrical system design. Of course, we have an AWESOME TOOL to make your life easier:

Feeling overwhelmed? Anxious? That's perfectly normal! To help, We created a group for DIY Van builders to connect and share their issues/concerns/solutions/ideas/etc... See you there!

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About us



Hello! We’re Isabelle and Antoine 🙂 In 2017, we sold our house (and everything in it), quit our engineering careers, and moved into our self-built campervan. Every day is an opportunity for a new adventure... We’re chasing our dreams, and hopefully it inspires others to do the same!

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Thanks to all of you, we managed to negociate group discount on these. Strength in numbers!