Questions answered
Van and RV solar questions, answered
The solar questions builders ask before they drill the roof, answered from Victron's technical papers and our state-by-state solar model.
Updated October 24, 2026. Every answer lists the sources we read it from. Prices are the makers' own, from the day we last checked.
Disclosure: We may earn a commission if you buy through these links. It never changes what we recommend or what the tools calculate. How we make money.
MPPT or PWM: which solar charge controller do I need?
Short answer
Usually MPPT. In Victron's worked example, a PWM controller harvests 81 W from a 100 W panel at 25°C cell temperature, 19% less than an MPPT controller. Victron calls PWM a good low-cost choice only for small systems with warm panels, and recommends MPPT once the array passes a few hundred watts on a 12 V battery.
Why: a PWM controller pulls the panel down to near the battery's voltage, so the panel rarely works at its best point. An MPPT controller finds that point and converts the extra voltage into charging current.
When the gap closes: at 75°C cell temperature, Victron's example shows almost no difference. MPPT pulls ahead when panels are cool, in low light, and with partial shading.
Redodo 40A MPPT Solar Charge Controller (12/24V)
40 A MPPT for 12 V or 24 V banks, LiFePO4 mode by default, Bluetooth built in.
$144.99$179.99
See at Redodo (opens in a new tab)Renogy Rover Lite 60A MPPT
60 A MPPT for larger roofs; works at 12, 24, 36 or 48 V.
Should I wire solar panels in series or parallel?
Short answer
Series raises the voltage, parallel raises the current, and the total watts are the same either way. With an MPPT controller, series lets you use thinner cable, because doubling the voltage halves the current. The limit is the controller's voltage rating: the array's open-circuit voltage must stay below it.
Victron's example: a 12 V panel has an open-circuit voltage around 22 V, so a 75 V controller can take up to three of them in series.
Leave margin. Victron notes that panel voltage drops about 4.5% for every 10°C of warming, which means it climbs when panels are cold.
With a PWM controller, series wiring doesn't help: Victron points out that adding cells in series reduces a PWM controller's performance at low temperature.
What size charge controller do I need for 400 watts of solar?
Short answer
On a 12 V battery, a 30 A MPPT. Controllers are rated by the current they send into the battery, so the same controller handles about twice the watts on 24 V. Victron's SmartSolar 100/30, for example, is rated for 440 W of panels on 12 V and 880 W on 24 V.
The next size up, Victron's 100/50, is rated for 700 W on 12 V and 1,400 W on 24 V.
Check two numbers on any controller: the charge current (amps into the battery) and the maximum PV voltage, which your panels' combined open-circuit voltage must stay under.
Redodo 40A MPPT Solar Charge Controller (12/24V)
40 A MPPT for 12 V or 24 V banks, LiFePO4 mode by default, Bluetooth built in.
$144.99$179.99
See at Redodo (opens in a new tab)Do I need a fuse between the solar panels and the charge controller?
Short answer
Yes. Victron's wiring guide says a fuse needs to sit between the PV array and the solar charger, and local rules may ask for a circuit breaker or a ground-fault device as well. Fuse the controller-to-battery cable too: ABYC's boat standard asks for protection within 7 inches of the battery on the wire from any charging source.
Size each fuse to protect the wire it sits on, per Blue Sea's method: at least 125% of the current, and no more than the wire is rated for.
For your own rig: Wire and fuse sizes by amps and length
Can I connect a solar panel directly to a battery?
Short answer
No. Victron puts it plainly: if a solar panel is connected directly to a battery, the battery will get damaged. A charge controller converts the panel's higher voltage into a safe charging voltage and stops pushing current once the battery is full.
Even the simplest controller does this. Victron describes a PWM controller as a switch that disconnects and reconnects the panel once the battery reaches its absorption voltage, to prevent overcharging.
Redodo 40A MPPT Solar Charge Controller (12/24V)
40 A MPPT for 12 V or 24 V banks, LiFePO4 mode by default, Bluetooth built in.
$144.99$179.99
See at Redodo (opens in a new tab)How much solar do I need for a van?
Short answer
Divide your daily watt-hours by what one watt of panel makes per day in your worst month. In our PVWatts model, a watt of flat roof panel makes about 6.3 Wh a day in Phoenix in June but only 0.7 Wh in Seattle in December, so 400 W makes roughly 2,505 Wh against 277 Wh. The planner runs this for your state and month.
The model: NREL's PVWatts v8 on NASA POWER hourly weather for 2022 to 2024, flat (0 degree) panels, 14% system losses, one reference city per state (Phoenix for Arizona, Seattle for Washington).
In winter, driving can beat the roof: an hour with a 40 A DC-DC charger puts back up to about 512 Wh, more than 400 W of flat panels makes all day in Seattle in December (about 277 Wh in our model).
For your own rig: Your state, month by month · Solar sized for your loads (planner)
Safety first
Fuse every positive wire as close to the battery or bus bar as you can, and use the wire and fuse sizes from a standard table, not a guess. Boats should follow ABYC E-11; anything that touches 120/240 V AC should follow the NEC. Have a qualified installer check any 120/240 V AC or shore power work.
Run the power planner
Battery, solar, charger, inverter, every wire and fuse, and a priced parts list.
Draw your wiring
Drag parts onto a diagram and get gauge and fuse labels on every run.
More questions answered
Keep going
Van electrical basics
The six parts every camper system has, how they connect, and a diagram to download.
Wire gauge and fuse chart
12 V and 24 V wire sizes and fuses by amps and run length, from the ABYC tables.
How many solar panels
What 200, 400 and 600 W of flat roof panels make in your state, month by month.