This is the second part of our DIY Van Electrical Guide trilogy:
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Daily Power Usage
? Ah
Inverter
? 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 (Ah) = CURRENT (A) x TIME (h)
- 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
| SUMMER | WINTER | |||||||
|---|---|---|---|---|---|---|---|---|
| LOAD | DESCRIPTION | CURRENT | CALCULATION ASSUMPTION | CALCULATION | DAILY POWER USAGE | CALCULATION ASSUMPTION | CALCULATION | DAILY POWER USAGE |
| Fridge | NovaKool R5810 | 3.8A | 35% duty cycle | 3.8A * 35%*24h= | 31.9Ah | 20% duty cycle | 3.8A * 20%*24h= | 18.2Ah |
| Roof fan | Maxxfan 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@10 | 20h per day @ 3 average | 0.3A*20h= | 6Ah | 12h per day @ 2 average | 0.2A*12h= | 2.4Ah |
| Wall Fan | Sirocco II Elite | 0.2A@1, 0.3A@2, 0.5A@3, 0.9A@4 | 3h per day @ 2 | 0.3A*3h= | 0.9Ah | |||
| Floor Vent Fan | SeaFlo 4″ | 0.5A (dimmed) | 2h per day | 0.5A*2h= | 1Ah | |||
| Ceiling Lights | 3W LEDs | 1.3A (total 10 lights @ 100% intensity) | 4h per day @ 80% intensity | 1.3A*80%*4h= | 4.2Ah | 8h per day @ 60% intensity | 1.3A*60%*8h= | 6.2Ah |
| Garage Lights | LEDs Strip | 1.2A | 6min per day | 1.2A*0.1h= | 0.1Ah | |||
| Water Pump | Shurflo | 6A | 12min per day | 6A*0.2h= | 1.2Ah | 6min per day | 6A*0.1h= | 0.6Ah |
| Composting Toilet Fan | Nature’s Head | 0.1A | 24h per day | 0.1A*24h= | 2.4Ah | 24h per day | 0.1A*24h= | 2.4Ah |
| Propane Solenoid | AFC | 1.2A | 2h per day | 1.2A*2h= | 2.4Ah | 2h per day | 1.2A*2h= | 2.4Ah |
| Webasto Heater | AirTop 2000 STC | 1.7A (average) | 12h per day (estimate) | 1.7A*12h= | 20.4Ah | |||
| Propex Heater | HS2000 | 1.6A | Not Used (backup) | |||||
| Phone | 2 Smartphones | 1A (each) | 2h per day x 2 | 1A*2h*2= | 4Ah | 2h per day x 2 | 1A*2h*2= | 4Ah |
| TOTAL | 54.1Ah | 56.6Ah | ||||||
120V AC
| LOAD | DESCRIPTION | POWER | CALCULATION ASSUMPTION | CALCULATION | DAILY POWER USAGE |
|---|---|---|---|---|---|
| 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 |
- Keep in mind that the numbers above vary a lot with weather. On a hot summer day, we use more power (because of the fridge and fans), and on a cool day, we use less power than what we calculated above.
- In winter, the fridge and the fans use much less power, but the Webasto uses more. Therefore, our daily power usage is quite similar!
- From now on, we will work with the largest value only (winter in our case):
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:
| LOAD | DESCRIPTION | MAX POWER (W) |
| Milk Frother | Nespresso Aeroccino | 550W max |
| Laptop 1 | Asus | 90W max |
| Laptop 2 | HP | 45W 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
| # | Item | Description | Quantity | View on Amazon |
| 1 | 1000W Inverter | Samlex PST-1000-12 PST Pure Sine | 1 | View |
| 2 | Remote Control for Inverter | Samlex RC-15A for 600W/1000W Inverter | 1 | View |
| 3 | Cable, 2 AWG, 5 ft Black + 5 ft Red | WindyNation | 1 | View |
| 4 | Lugs, 2 AWG Cable, 5/16″ Ring | Connect to Terminal Fuse Block and Bus Bar (Pack of 2) | 1 | View |
| 5 | Terminal Fuse, 175A | Blue Sea (To protect inverter’s cable) | 1 | View |
| 6 | Terminal Fuse Block | Blue Sea (Connects directly on the Bus Bar. Holds the Terminal Fuse) | 1 | View |
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:
Lithium
- 4 days autonomy
- (Daily Power Usage x 4)/0.8*=
- 355Ah
*Most economic Depth-of-dischage (DOD) for Lithium is 80%
AGM
- 4 days autonomy
- (Daily Power Usage x 4)/0.5*=
- 568Ah
*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
- (Daily Power Usage x 1.75)/0.8 =
- 155Ah
Recommended Size
AGM
- (Daily Power Usage x 1.75)/0.5 =
- 249Ah
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:
Lithium
- (Daily Power Usage x 1.5)/0.8 =
- 133Ah
Sun Seeker Size
AGM
- (Daily Power Usage x 1.5)/0.5 =
- 213Ah
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:
Lithium
- (Daily Winter Power Usage x 2)/0.8 =
- 178Ah
Snow Chaser Size
AGM
- (Daily Winter Power Usage x 2)/0.5 =
- 284Ah
Snow Chaser Size
We fit in the “snow chaser” category, so we need at least:
178Ah
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
- Max power / Lowest voltage / Peak Efficiency (see spec sheet to find these values)
- 1,000W/10.7V/0.85 =
- 110A
Inverter Continuous Current
100Ah Battle Born Battery
- Allowable discharge current (per spec sheet):
- 100A continuously
- 200A for 30 seconds
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
- Power / Voltage / Efficiency (based on "common sense" values)
- 950W/11.5V/0.85 =
- 97A
Inverter Continuous Current
100Ah Battle Born Battery
- Allowable discharge current (per spec sheet):
- 100A continuously
- 200A for 30 seconds
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:
- = Inverter Power / 10
- (example: 2000W / 10 = 200Ah)
(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:
- = Inverter Power / 10 + extra battery
- (example: 2000W / 10 + 100ah = 300Ah)
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
| # | Item | Description | Quantity | View on Amazon |
| 1 | LiFePO4 200 Ah | Battle Born LiFePO4 100 Ah 12V | 2 | amzn.to/2l6YuQQ |
| 2 | 2/0 Cable in 5/16 lugs, 1 feet Red + 1 feet Black | Windy Nation Copper Cable | 1 | amzn.to/2D6EGFX |
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:
EACH 4W OF SOLAR PANEL IS ABLE TO HARVEST 1Ah PER DAY.
| SOLAR PANELS | DAILY HARVEST |
| 100W | 25Ah |
| 175W | 44Ah |
| 200W | 50Ah |
| 300W | 75Ah |
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
- Daily Power Usage x 4W/Ah x 0.8 =
- 227W
Recommended Solar Power
- Daily Power Usage x 4W/Ah x 1 =
- 284W
Snow Chaser Solar Power
- Daily Winter Power Usage x 4W/Ah x 1.2 =
- 340W
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:
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
| # | Item | Description | Quantity | View on Amazon |
| 1 | 350W Solar | NewPowa 175W Mono Panel | 2 | View |
| 2 | Extension Cables, 8 AWG, 15 ft Red + 15 ft Black | With MC4 Connectors | 1 | View |
| 3 | Double Cable Entry Gland | For 8 AWG or 10 AWG Cable | 1 | View |
| 4 | 40A Breaker/Switch, Surface Mount | Between Panels and MPPT Charger | 1 | View |
| 5 | MPPT Solar Charger | Victron 100|30 SmartSolar MPPT | 1 | View |
| 6 | 40A Breaker/Switch, Surface Mount | Between MPPT Charger & Battery | 1 | View |
| 7 | Heat Shrink Terminal Ring, 8 AWG Cable, 5/16″ Ring | Connect to Bus Bar (Pack of 3) | 1 | View |
| 8 | Heat Shrink Terminal Ring, 8 AWG Cable, 1/4″ Ring | Connect to Breakers (Pack of 3) | 2 | View |
100|30
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
- LiFePO4: Up to 0.5C (50% of capacity)
- AGM: Up to 0.2C (20% of capacity)
2- Don't exceed the Alternator Available Power (or "Unused Power")
In our case:
2 x BattleBorn Batteries wired in parallel
- 50A recommended charge current for each battery wired in parallel.
- 50A x 2 =
- Up to 100A
Ford Transit with heavy-duty alternator (230A)
- Alternator power - Ford Transit power needs.
- 230A - 80A* =
- 150A
Alternator Available Power
We could go up to 100A, but we went with:
60A Sterling Power Battery-to-Battery Charger
Alternator Items List
| # | Item | Description | Quantity | View on Amazon |
| 1 | 60A Battery-to-Battery Charger (B2B) | Sterling Power BB1260 | 1 | View |
| 2 | 100A Breaker/Switch, Surface Mount | Blue Sea 285-Series | 2 | View |
| 3 | Cable, 4 AWG, 15 ft Black + 15ft Red | WindyNation | 1 | View |
| 4 | Lugs, 4 AWG Cable, 5/16″ Ring | Connect to Bus Bar (Pack of 2) | 1 | View |
| 5 | Lugs, 4 AWG Cable, 1/4″ Ring | Connect to Breakers (Pack of 2) | 2 | View |
60A
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:
- You're a snow chaser with an AGM battery (finishing the absorption phase takes a long time and it's important for the battery's life cycle).
- If spending extended time at a friend's house, a relative's house, or at a full-service campground (it's nice to just plug in and forget about the battery).
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
- LiFePO4: Up to 0.5C (50% of capacity)
- AGM: 0.2C (20% of capacity)
We could go up to 100A (0.5 x 200Ah), but we went with:
50A Samlex Smart Battery Charger/Converter
Shore Items List
| # | Item | Description | Quantity | View on Amazon |
| 1 | 50A Charger | Samlex SEC-1250UL 12V | 1 | View |
| 2 | 60A Breaker/Switch, Surface Mount | Between Charger and Bus Bar | 1 | View |
| 3 | Cable, 8 AWG, 5 ft Black + 5 ft Red | WindyNation | 1 | View |
| 4 | Heat Shrink Terminal Ring, 8 AWG Cable, 1/4 Ring | Connect to Breaker (Pack of 3) | 1 | View |
| 5 | Heat Shrink Terminal Ring, 8 AWG Cable, 5/16″ Ring | Connect to Bus Bar (Pack of 3) | 1 | View |
50A
Here is how we sized our electrical system:
71Ah
Daily Power Usage
1000W
Inverter
200Ah
Lithium Battery
350W
Solar Power
60A
Alternator Charger
50A
Shore Charger
PART C...
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:
