Guidelines to Adding One More Solar Panel to Same MPPT?

Been thinking about this a lot lately with my garden office setup. Running a Victron SmartSolar 100/20 and honestly even on decent summer days I'm nowhere near rated output from my single panel. UK weather being what it is, you're lucky to see 60-70% of rated watts on a clear day, never mind cloud cover.

So I'm looking at adding a second panel in series to squeeze more out of the controller before upgrading anything. Few things I'm trying to get my head around though:

• Voltage headroom — does anyone actually calculate the cold-temperature Voc before wiring up? Easy to accidentally overvolt the MPPT on a crisp January morning
• Mismatched panels — is mixing wattages genuinely as bad as people say, or are forums just scaremongering?
• Shading scenarios — series vs parallel when partial shading is likely (mine gets tree shadow late afternoon)

My instinct is series gives better low-light performance since the MPPT has more voltage to work with, but I've seen conflicting advice.

Anyone here actually done this on a smaller Victron or Renogy controller? Curious whether the real-world gains were worth the faff, especially for backup power use rather than full-time off-grid.

Also — does the Victron app actually show enough data to verify it's working properly after adding a second panel, or would you recommend a separate meter inline?

Proper practical experience welcome, not just spec-sheet chat. 🙂

FormerTeacher50 | 234 posts | ⚡ Off-Grid Enthusiast

@SolarTrevor Before you dive in, the key figures to check are your MPPT's maximum open circuit voltage (Voc) and short circuit current (Isc) limits. The 100/20 handles up to 100V input, so if you're adding a second panel in series, add the two Voc figures together and make sure you've got comfortable headroom — cold mornings can push Voc noticeably higher than the rated figure. Worth applying a temperature correction for your coldest likely conditions.

If you go parallel instead, watch the combined Isc doesn't exceed 20A input limit.

Also double-check your existing cable sizing can handle the additional current. Easy to overlook that bit!

What panel are you currently running — make and wattage? That'd help narrow things down properly. 🙂

Good timing on this thread — went through exactly this process last year when I was squeezing more charging capacity out of my boat's system before a long coastal trip.

The bit most people miss: it's not just about whether your total wattage fits the 20A output limit. You need to check your panels' combined Voc against that 100V input ceiling, especially on a crisp winter morning when cold panels spike voltage higher than the datasheet suggests.

On my setup I've got two panels wired in parallel specifically to keep Voc sensible while doubling the current. Victron's MPPT Calculator tool does the heavy lifting — stick your panel specs in and it'll flag any cold-weather Voc issues before you've bought anything.

What panels are you currently running? Series versus parallel wiring changes the conversation entirely.

The golden rule nobody mentions: check your existing panel's Voc first, double it (roughly), and make sure you're still under that 100V input limit on the SmartSolar — parallel wiring keeps voltage the same but series will shoot you over faster than a dodgy eBay listing 😄

Also worth firing up the Victron MPPT calculator online before you commit — saved me from a very expensive mistake on my van build when I nearly wired two Renogy 175W panels in series like an absolute muppet.

Really useful thread this — I went through something similar when I was trying to squeeze more charge into my narrowboat batteries on cloudy days.

One thing nobody's touched on yet: are your two panels going to be series or parallel? On the 100/20, you've got that 100V max input to respect, so if your panel Voc is already sitting around 45-48V, series is immediately off the table and you're going parallel.

Also worth checking — is your existing panel shaded at any point during the day? Adding a second panel in parallel when one's getting partial shade can actually drag the whole array down. Learned that the hard way with my Fogstar setup before I understood how MPPT actually handles mismatched inputs.

What's your existing panel's wattage and Voc? That'd make it much easier to work out whether the 100/20 can actually handle both without throttling.