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Once your battery hits 100%, your MPPT controller throttles the panels and all that free solar energy goes to waste. On a long sunny day, you could be "losing" 3-4 hours of production. There are ways to use that surplus.
The most practical diversion: a 12V water heater element (200-300W) wired to a relay that activates when battery voltage hits 14.4V+. Battery full → relay closes → surplus heats your water tank. The Victron SmartSolar MPPT has a programmable relay output that can trigger at a set voltage — perfect for this. Total parts cost: $30-50.
⚡ Expert tip
Use a relay with a hysteresis range — activate at 14.4V, deactivate at 13.8V. Without hysteresis, the relay chatters on/off rapidly as the MPPT adjusts voltage around the trigger point. A $5 voltage-controlled relay with adjustable hysteresis from Amazon solves this.
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 solar panels
302 Wc
4h average sun hours
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
REGULATEUR 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
Other options: charge a second battery bank (house battery full → divert to starter battery via a simple relay), run a Peltier cooler for extra fridge cooling, or heat a thermal mass (water tank with immersion heater) that releases warmth overnight. The water heater approach is the most popular because hot water is always useful — free showers powered by surplus solar. A 5-gallon tank with a 200W element heats from cold to 45°C in about 2-3 hours of surplus production. The reality check: this only works when you have genuine surplus — meaning summer months with long days and a battery that reaches 100% before noon. In spring, fall, and winter, you rarely have surplus. For most vanlifers, solar diversion is a nice-to-have summer bonus, not a core system feature.
Frequently asked questions
How do I know if I have surplus solar?▼
If your battery monitor shows 100% SOC and your MPPT display shows the panels producing less than their rated output — your MPPT is throttling because there's nowhere to send the energy. In VictronConnect, this shows as 'absorption' or 'float' mode with reduced panel current.
Can I divert surplus solar to heat water?▼
Yes. Wire a 200-300W 12V immersion heater to a relay triggered by battery voltage (14.4V+). When the battery is full, the relay sends surplus to the heater. The Victron SmartSolar has a built-in relay output for exactly this purpose.
How much surplus solar is typical?▼
In summer with 400W panels and 200Ah battery: you might hit full charge by 11 AM, wasting 4-5 hours of potential production (800-1,000Wh). In winter or with heavy consumption: zero surplus. It's a summer phenomenon.
Is it worth adding a dump load?▼
For full-time vanlifers in sunny climates: yes, free hot water is worth $30-50 in parts. For weekenders or winter travelers: probably not — you rarely have surplus. It's a low-cost, low-priority upgrade.
Can surplus solar damage my battery?▼
No. The MPPT controller handles this automatically by reducing panel current when the battery is full. The battery voltage is maintained at float level (13.6V for LiFePO4). Diversion is about using energy that would otherwise be wasted, not about protecting the battery.