A mathematical, field-tested worksheet to accurately determine your campervan battery capacity and solar replenishment needs.
The Amp-Hour Formula: Converting Watt-Hours to 12V Amp-Hours
All vehicle energy audits must be conducted in Amp-Hours (Ah) at the nominal system voltage (12.8V for LiFePO4). To convert appliance wattage to Ah: Divide appliance Watts by 12V, then multiply by daily runtime in hours. For AC appliances running through an inverter, factor in an additional 15% efficiency loss (85% inverter efficiency).
Categorizing Baseloads vs Variable High-Draw Appliances
Campervan baseloads run 24 hours a day: 12V chest fridges, diesel heater combustion fans, USB charging ports, and water pump pressure switches. Variable loads include induction hot plates, electric kettles, and high-spec editing laptops. Overestimating solar generation during overcast winter days is the number one cause of dead batteries.
The 3-Day Autonomous Safety Factor for Off-Grid Reserves
True off-grid independence requires a minimum of 2.5 to 3 days of autonomy without solar or alternator replenishment. If your calculated daily consumption is 100 Ah, a 300 Ah lithium bank provides peace of mind during prolonged inclement weather or shaded camping spots.
Technical Specifications & Reference Matrix
| Appliance | Power Draw (Watts) | Daily Hours | Daily 12V Ah Consumed |
|---|---|---|---|
| 12V 65L Compressor Fridge | 45W (33% duty cycle) | 24 hrs | 30 Ah |
| Diesel Air Heater Fan | 25W (running) | 8 hrs | 16.6 Ah |
| Induction Cooktop (Cooking) | 1400W (via Inverter) | 0.5 hrs | 68.6 Ah |
| Laptop & Phone Charging | 65W USB-C PD | 4 hrs | 21.6 Ah |
💡 Expert Field Recommendation
Ditch 120V laptop power bricks in favor of 12V USB-C Power Delivery (PD) native vehicle sockets. Converting 12V DC to 120V AC through an inverter and back to 20V DC through a laptop charger wastes up to 25% of your battery power in thermal dissipation.
Diagnostic Troubleshooting Checklist
- Fridge runs continuously and drains battery overnight: Poor fridge compartment ventilation. Install a low-RPM 12V exhaust fan behind the condenser coils to drop run time by 40%.
Frequently Asked Questions
Can I mix old and new lithium batteries in parallel?
Only if they are of identical chemistry, nominal voltage, and similar internal cycle wear. Mixing mismatched ages causes uneven current distribution and premature BMS shutoffs.
Article researched and verified by Liam Sterling, dedicated technical specialist and editorial contributor.
