Loading...
Fact-checked on Apr 8, 2026
An amp hour is just current multiplied by time. A device drawing 1 amp for 1 hour uses 1Ah. That's it — that's the whole concept. Where people get tripped up is translating watts to amps (divide by voltage) and understanding that a "200Ah battery" doesn't actually give you 200Ah of usable power. With LiFePO4 at 85% depth of discharge, you get about 170Ah. With AGM at 50% DoD, you only get 100Ah. Same label, wildly different real-world capacity.
The formula is dead simple: Watts / Volts = Amps, then Amps x Hours = Amp-hours. A 60W fridge compressor running 40% of the time draws 5A x 9.6h = 48Ah/day from a 12V battery. LED lights at 30W for 5 hours: 2.5A x 5h = 12.5Ah. Phone charging at 20W for 3 hours: 1.67A x 3h = 5Ah. A typical van day adds up to 60-120Ah depending on your setup. That's why I always tell people: start by listing every device, its wattage, and how many hours you'll actually use it. Then do the math before buying batteries.
⚡ Expert tip
Peukert's effect makes AGM capacity drop under high loads — a 100Ah AGM at 20A draw might only deliver 80Ah total. LiFePO4 barely suffers from this. If you're running an inverter with big loads, lithium's rated capacity is much closer to what you actually get.
Example calculation for your setup
Results based on a typical use case
| Appliance | Power | Usage/day | Wh/day |
|---|---|---|---|
| Compression fridge | 45W | 24h | 1080 |
| LED lighting | 20W | 4h | 80 |
| Water pump | 30W | 0.5h | 15 |
| Phone charging | 15W | 2h | 30 |
| Daily consumption | 1205 Wh | ||
Recommended LiFePO4 battery
126 Ah
(80% depth of discharge)
Recommended AGM battery
201 Ah
(50% depth of discharge)
Recommended solar panels
302 Wc
4h average sun hours
Recommended inverter
No 220V appliances — no inverter needed
Adjust these values with the calculator below
VANPOWERCALC PROYOUR ENERGY PROFILE
YOUR ENERGY PROFILE.
This document contains the sizing of your future electrical installation, calculated based on your appliances.
Minimum Daily Consumption
0 WH / DAY
Inventory:
SYSTEM SIZINGPage 2
Battery
To guarantee 0WH without damaging your bank (80% max discharge):
0 AH
LiFePO4 12V
Solar
Minimum power required to recharge your consumption:
0 W
via MPPT
220V AC
Maximum power (with 25% safety margin).
0 W
No Need
Safety & CablingPage 3
12V Cable Sizing Guide
Use this professional reference table to select the correct gauge (mm²) for your cables. For 12V in a van, the maximum tolerated voltage drop is 3%. Always use multi-stranded flexible automotive wire.
| Current (A) | Round trip < 2m | Round trip 4m | Round trip 6m |
|---|---|---|---|
| 5A (LEDs, USB) | 1.5 mm² | 2.5 mm² | 4 mm² |
| 10A (Fridge, Pump) | 2.5 mm² | 4 mm² | 6 mm² |
| 20A (Heater) | 4 mm² | 10 mm² | 10 mm² |
| 50A (DC/DC Booster) | 10 mm² | 16 mm² | 25 mm² |
| 100A (Inverter) | 25 mm² | 35 mm² | 50 mm² |
Fuse Sizing
The fuse protects the wire, not the appliance. Always place it as close to the power source as possible (battery or busbar).
- Wire 1.5 mm² → Max fuse 10A
- Wire 2.5 mm² → Max fuse 20A
- Wire 4 mm² → Max fuse 30A
- Wire 6 mm² → Max fuse 40A
- Wire 10 mm² → Max fuse 60A
SCHÉMA ÉLECTRIQUE
PANNEAUX SOLAIRES
0W
RÉGULATEUR MPPT
BATTERIE AUXILIAIRE
0 Ah
Lithium LiFePO4
FUSIBLE
FUSIBLE
BOÎTE À FUSIBLES 12V
Pompe, Leds, Frigo...
CONVERTISSEUR 220V
NON REQUI
PRO GEARPage 4
SHOPPING LIST
Where to find this equipment? Here is the community-approved selection.
12V 6-way Fuse Box
Mandatory protection
Digital Multimeter
Test your connections
Heavy Duty Crimping Tool
For perfect lugs
Heat Shrink Tubing
Insulation and safety
GENERATED BY VANPOWERCALC PRO - UNLIMITED.
About this tool
I see this confusion constantly in van build forums: someone buys a 100Ah AGM battery thinking they have 100Ah to play with, then wonders why their fridge dies after one night. Let me break down why.
First, depth of discharge. AGM batteries should only be drained to 50% regularly, or you'll kill them in under a year. So your 100Ah AGM is really a 50Ah battery. LiFePO4 handles 80-95% DoD across 3,000-5,000 cycles, so a 100Ah lithium gives you 80-95Ah of real power. Right there, same "100Ah" label, but lithium gives you nearly double the usable energy.
Second, voltage sag matters. As an AGM discharges, voltage drops from 12.6V down toward 12.0V. Some devices — especially inverters — start acting weird below 12.0V. LiFePO4 holds a flat 12.8-13.2V for most of its discharge curve, then drops off a cliff near empty. That flat curve means your inverter runs more efficiently across the whole cycle.
Here's a real day from my setup. Compressor fridge: ~45Ah. LED lights (6 puck lights, 3W each, 5 hours): ~7.5Ah. Laptop charging: ~15Ah. Phone and misc USB: ~5Ah. Diesel heater fan in winter: ~8Ah. Total: roughly 80Ah on a normal day. On a lazy summer day with no laptop, maybe 55Ah. Cooking day with the induction plate through the inverter? Could hit 130Ah.
The inverter introduces another wrinkle. Inverters are typically 85-90% efficient, so if you're running a 500W appliance through an inverter, you're actually pulling about 570W from your battery. Always add that 10-15% inefficiency tax when calculating Ah for AC loads.
My rule of thumb: take your calculated daily consumption, multiply by 1.2 for safety margin, and that's your minimum usable battery capacity. Want two days of autonomy without solar? Double it.
Frequently asked questions
How many amp hours do I need for van life?▼
Most full-time vanlifers end up between 200-400Ah of LiFePO4 at 12V. A minimal setup (fridge, lights, phone charging) can get by on 100Ah lithium. Once you add a laptop, diesel heater, and occasional inverter use, you'll want 200Ah. Heavy users with induction cooking or AC need 300-400Ah or a move to 24V/48V.
What's the difference between amp hours and watt hours?▼
Watt-hours = Amp-hours x Voltage. A 12V 200Ah battery holds 2,400Wh. Watt-hours are actually more useful for comparing batteries because they account for voltage. A 24V 100Ah battery and a 12V 200Ah battery both hold 2,400Wh — identical energy, just different voltage and current.
Why does my 200Ah AGM battery die so fast?▼
Because you only have 100Ah of usable capacity. AGM batteries should only be discharged to 50% to maintain decent cycle life (400-600 cycles). Drain them to 80% regularly and they'll be dead in a few months. If you switched to a 200Ah LiFePO4, you'd get 170Ah usable — almost double — and it would last 3,000-5,000 cycles.
Do I calculate amp hours at 12V or at the appliance voltage?▼
Always calculate at battery voltage (usually 12V) because that's what's being depleted. For 12V devices, it's straightforward: watts / 12 = amps. For AC devices through an inverter, divide the AC wattage by 12 and then multiply by about 1.12 to account for inverter efficiency losses.
How many amp hours does a van fridge use per day?▼
A typical 12V compressor fridge (like a Dometic CFX or Vitrifrigo) uses 25-45Ah per day, which is roughly 300-550Wh. It depends on ambient temperature, how often you open it, and the thermostat setting. In summer heat, expect the higher end. I measured my 50L compressor fridge at 38Ah/day averaged over a week of mixed weather.
The links below are Amazon affiliate links. If you purchase through these links, I earn a small commission — at no extra cost to you. This is what allows me to keep this site free.
Recommended gear
Editor's PickLiFePO4 Battery 100Ah100% usable capacity, 3000+ cycles, built-in BMS.Leo recommendsThis is the battery I've been using in my van for 2 years. Reliable, lightweight, and the best value for money out there.$418 on Amazon →Digital MultimeterMeasures voltage, current, resistance — essential for 12V wiring.Leo recommendsFirst tool to buy before you even start your install. You'll pull it out 100 times to check voltages and connections.$107 on Amazon →Pure Sine Wave Inverter 1000WPure sine wave, sensitive devices compatible, 12V→110V/230V.Leo recommendsIf you need to run a microwave or coffee machine, this is the one. Pure sine wave = zero risk for your gear.$176 on Amazon →