The short answer. Panel watts = the watt-hours you use a day, divided by your peak sun hours, divided again by how much of their label your panels really make. Then pick the controller whose rated panel watts at your battery voltage covers that, and check the panel voltage and current fit it.
Enter the amp-hours and your sun hours.
Shade, dirt, heat and the angle of the panels all cut output. Victron’s own MPPT sizing tool on each controller’s product page is the final check. Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
Guidance, not a substitute for a qualified electrician. On a boat, when in doubt, ask an ABYC-certified marine electrician.
What you need before you start
- Amp-hours a day. The Battery bank size calculator gives you this. If solar only has to cover part of it, enter that part.
- Peak sun hours. This depends on where you are and the month. The free PVWatts tool (pvwatts.nlr.gov) gives it for any address. Use the worst month you’ll be out, not the yearly average.
- Your panel’s datasheet, for two numbers: its output at normal operating temperature (listed as NMOT or NOCT watts), and its Voc, Vmp and Isc if you want the panel check.
How the math works
58 Ah a day on 12V is 696 watt-hours. With 4 sun hours, a perfect panel would need 174 W. Real panels make less than their label once they heat up in the sun, so the tool asks for that share. At 75% you need about 237 W. It also counts the controller’s 98% peak efficiency from Victron’s datasheet.
Leave panel output at 100% and the tool warns you: that’s the label, and panels don’t hold it all day.
Picking the controller
Victron rates each SmartSolar MPPT for a set number of panel watts at each battery voltage. The 100/20 handles 290 W on a 12V battery and 580 W on 24V; the 100/30 handles 440 W and 880 W. The tool picks the smallest one on our list that covers your array.
Going over isn’t a disaster. Victron says the controller just limits its input power. You lose the extra watts at midday, which can still be worth it on cloudy days.
The panel check
Open “Check my panels against the controller” and enter the panel’s Voc, Vmp and Isc, plus how they’re wired. The tool checks three of Victron’s rules:
- Voltage. Panels in series add their voltage, and cold panels make more. Victron says to keep a 10% margin where nights get near or below 10°C, and that going over the controller’s limit stops charging and can damage it. The 150-volt models have a 145 V start-up limit.
- Start-up. The panel voltage has to be at least 5 V above the battery before charging starts, then stay at least 1 V above. A charging 12V battery can sit near 14.9 V, so the tool checks against that.
- Current. Strings in parallel add their current. Victron warns that more short-circuit current than the controller’s limit may damage it.
Bundles for this job: Van solar starter kit
Quick answers
What size solar panel do I need to charge a 100Ah battery? It depends on how much you take out each day, not on the battery’s size. Put back what you use: a day’s amp-hours times the voltage, divided by your sun hours and your panels’ real output.
Can I connect more solar than the controller is rated for? Victron says yes, within limits: the controller caps its input power. Panel voltage and short-circuit current must still stay under the controller’s maximums.
Why does my controller say error 33? Victron’s manual says that’s PV voltage too high. Put fewer panels in series, and keep a 10% margin where it gets cold.
12V or 24V for solar? On 24V each controller handles twice the panel watts, per Victron’s ratings, and the cables carry half the current.
Sources
Victron SmartSolar MPPT datasheets and manuals (75/15-100/20, 100/30-100/50, 150/35-150/45, 150/60-150/70); Victron Gel and AGM datasheet; PVWatts. Links in calculators/data/mppt.json and Picking an MPPT: 100/20 to 150/60 in one table.
