Quick answer
Work out how much power you use in a day, then size your solar to replace it, with a margin for cloudy days and less-than-ideal panel angles. Panels rarely produce their full rated output: shading, heat, dust, panel angle, time of year and where you are in Australia all reduce it. A common planning approach is to estimate daily solar harvest as panel watts × an assumed number of peak sun hours × a system efficiency factor, then compare it with your daily use. Your battery size decides how long you can last when the sun doesn’t cooperate.
Is this a DIY job?
- Difficulty: Easy to calculate; Medium to install portable panels; Pro for roof-mounted panels, regulators and wiring into an existing system
- Time: 30 minutes to work out
- Tools: appliance labels or manuals, a battery monitor (if you have one), calculator
- Skills: simple maths
Step by step
- List your 12V appliances. Fridge, lights, water pump, fans, USB charging, diesel heater, TV, and anything run through an inverter.
- Find each one’s power draw. Look for amps (A) or watts (W) on the label or in the manual. For 12V, amps ≈ watts ÷ 12.
- Estimate hours of use per day. A fridge cycles on and off, so use the maker’s daily energy figure if it gives one, or measure it with a battery monitor.
- Work out daily use in amp-hours (Ah). For each appliance: amps × hours per day. Add them up.
- Add inverter losses. Anything run through an inverter uses more than its rated power from the battery. Allow extra, or check the inverter maker’s efficiency figure. (see How do I choose and install an inverter?)
- Estimate daily solar harvest. For planning: panel watts ÷ 12 × assumed peak sun hours × efficiency factor = Ah per day. The peak sun hours you can expect depend on location and season; use a conservative figure for winter or southern trips. An efficiency factor (for heat, dust, angle and cable and regulator losses) is often assumed to be around 0.7, but treat it as a rough guide.
- Compare and add a margin. Your solar harvest on a typical day should comfortably exceed your daily use if you want to stay put. Cloudy days can produce much less.
- Size the battery for the gap. Battery capacity covers nights and cloudy days. Lithium lets you use more of its rated capacity than AGM.
- Choose a suitable regulator. MPPT regulators generally get more from the panels than PWM regulators, especially in low light and on larger systems. PWM can suit a small system on a budget. Many DC-DC chargers include one. (see DC-DC charger or VSR isolator: which do I need?)
- Check with real numbers. Once set up, a battery monitor shows what you actually use and harvest. Adjust from there.
Specs table
Worked example (illustration only, not a recommendation)
| Item | Draw | Hours/day | Ah/day |
|---|---|---|---|
| 12V compressor fridge (average) | 2.5 A | 24 (average, including cycling) | 60 |
| LED lights | 2 A | 4 | 8 |
| Water pump | 5 A | 0.5 | 2.5 |
| USB devices | 2 A | 3 | 6 |
| Total | about 77 Ah |
Solar estimate using the planning formula, assuming 5 peak sun hours and an efficiency factor of 0.7:
| Panel size | Estimated Ah/day |
|---|---|
| 200 W | 200 ÷ 12 × 5 × 0.7 ≈ 58 Ah |
| 300 W | ≈ 88 Ah |
| 400 W | ≈ 117 Ah |
In this example, 200 W wouldn’t keep up on a good day; 300 W would cover the daily use with a small margin; 400 W gives more room for cloud or extra loads. Fewer sun hours (for example in winter) reduce every figure. Your own appliance draws and conditions will differ.
What you’ll need
- solar panel 12V or portable folding solar panel
- MPPT solar regulator (or a DC-DC charger with solar input)
- Anderson plug 50A and solar extension lead
- battery monitor
- inline fuse holder / circuit breaker
When to call an auto electrician
- You’re installing roof panels and running cable into the van.
- You’re adding solar to a system with a DC-DC charger, lithium battery or inverter.
- The battery isn’t charging as expected from solar.
- Any 240V work. Fixed 240V wiring in a caravan must be done by a licensed electrician.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Portable or roof-mounted solar? A: Roof panels are always connected and need no setting up. Portable panels can be aimed at the sun and placed in sun while the van is in shade. Many people use both.
Q: Why am I getting less solar than my panels are rated for? A: Panel ratings are measured in ideal lab conditions. Heat, angle, dust, shading, time of year and cable losses all reduce real output.
Q: Do I need an MPPT regulator? A: It’s not essential, but MPPT generally harvests more energy than PWM. Many DC-DC chargers have MPPT built in.
Q: How long will my battery last without sun? A: Divide the battery’s usable capacity by your daily use, after allowing for any partial solar. Lithium lets you use more of its rated capacity than AGM.
Sources
- Signature Camper Trailers, “How much solar do you need?” – planning formula (panel watts ÷ 12 × 5 peak sun hours × 0.7 efficiency); daily use in Ah; cloudy-day reduction (manufacturer guide; assumptions not independently sourced). https://signaturecampertrailers.com.au/how-much-solar-do-you-need/ (checked October 2026)
- REDARC, “Regulating the sun” – MPPT regulators harvest more from the panels, especially in low light and on larger systems; PWM suits small, budget systems. https://www.redarcelectronics.com/au/discover/regulating-the-sun/ (checked October 2026)
- WorkSafe ACT, “Electrical incident” – 240V installations in caravans and transportable structures should be done by a licensed electrician and certified to AS/NZS 3001. https://www.worksafe.act.gov.au/health-and-safety-portal/safety-alerts/electrical-incident (checked October 2026)
- Consumer, Building and Occupational Services Tasmania, “Using electricity safely in caravans and tents” – don’t do DIY electrical work on a caravan; only a licensed electrician can. https://cbos.tas.gov.au/topics/technical-regulation/electrical-standard-safety/using-electricity-safely-in-caravans-and-tents (checked October 2026)