How to Calculate Your 4WD Power Needs Before Buying a Battery
Why Sizing Matters More Than Brand
The most common mistake in 4WD auxiliary power setups is buying a battery based on price or brand recommendation rather than actual power consumption. A 100Ah battery in one rig might be perfect. In another with identical equipment it might leave you with a warm fridge by day two. The difference is almost always in the calculation — or the lack of one.
This guide walks through the exact method for calculating your daily power consumption before you spend a dollar on a battery.
Step 1 — List Every Device You Plan to Run
Write down every 12V device you will run from your auxiliary battery. Include everything — fridge, lights, phone and device chargers, water pump, CPAP if you use one, laptop, camera batteries, fan. Don't leave anything out.
Step 2 — Find the Current Draw of Each Device
Current draw is measured in amps (A). Find it on the device's label, in the manual, or from the manufacturer's specifications. If you only have watts, divide by 12 to convert to amps: Amps = Watts ÷ 12.
Common 12V device current draws:
| Device | Typical Draw | Notes |
|---|---|---|
| 40L 12V compressor fridge | 2.5-4A average | Cycles on/off; depends on ambient temp |
| 60L 12V compressor fridge | 3.5-5A average | Higher in summer |
| LED strip lighting (3m) | 1-2A | Per strip |
| Phone charging (USB) | 1-2A | Per device |
| Laptop charging | 3-5A | Via 12V adapter or inverter |
| 12V water pump | 5-8A | Only while running (minutes per day) |
| CPAP machine | 4-6A | Per night of use |
| 12V tyre compressor | 15-30A | Short duration only |
| 60W inverter load | 5-6A | Plus inverter inefficiency |
Step 3 — Calculate Daily Amp Hours
For each device, multiply its current draw by the number of hours per day you run it. This gives you the daily amp-hour (Ah) consumption for that device.
Example calculation for a typical weekend touring setup:
- 40L fridge at 3A average × 24 hours = 72Ah
- LED lighting at 1.5A × 4 hours = 6Ah
- Phone charging at 1.5A × 2 hours = 3Ah
- Water pump at 6A × 0.25 hours = 1.5Ah
Total daily consumption: 82.5Ah
Step 4 — Factor in Battery Chemistry
AGM batteries should not be discharged below 50% — so the usable capacity is half the rated Ah. To cover 82.5Ah per day from AGM, you need a battery rated at 165Ah or higher.
LiFePO4 batteries can be discharged to 80% — usable capacity is 80% of the rated Ah. To cover 82.5Ah per day from LiFePO4, you need a battery rated at approximately 103Ah or higher. A 100Ah LiFePO4 is borderline — a 120Ah gives you comfortable headroom.
Step 5 — Factor in Charging Input
If you drive each day, your DC-DC charger tops up the battery while you travel. A 25A DC-DC charger running for 4 hours of driving puts back approximately 80-100Ah, which nearly covers a typical daily camping load.
If you stay stationary for multiple days, solar becomes the primary charge source. A 200W solar panel in good Australian conditions generates approximately 40-70Ah per day. For a 82.5Ah daily load without driving, you would need 200-300W of solar to maintain charge balance.
The formula for solar sizing: Daily consumption Ah ÷ Average sun hours (4-5 hours in most of Australia) = Minimum panel wattage required.
What Capacity Do You Actually Need?
Based on the calculation above:
- Weekend trips, driving daily — 100Ah LiFePO4 or 200Ah AGM is sufficient
- Long weekends, occasional stationary days — 120-150Ah LiFePO4 with 200W solar
- Week-long remote touring, multi-day stationary — 200Ah LiFePO4 with 300-400W solar
- Extended travel, full-time touring — 200-300Ah LiFePO4 with 400W+ solar and a monitored system
Don't Forget Circuit Protection
Whatever size battery you install, every positive cable in the system needs a fuse rated to the cable's current capacity, installed as close to the battery as possible. Every connection point should use waterproof heat shrink butt connectors or sealed automotive connectors. Undersized cables and unprotected joins are the two most common causes of 4WD electrical fires.
Shop 4WD Electrical Accessories at Auto Relay
Auto Relay stocks Anderson plugs, 50A quick connect leads, waterproof butt connectors, inline fuse holders, and wiring loom tape — everything needed to correctly wire a dual battery system. Fast shipping across Australia.