TL;DR
- Keep one roof pitch & one azimuth per MPPT input. Mixing tilts or directions on a single MPPT creates mismatched IV curves and sub-optimal tracking.
- MPPT ≈ “smart DC-DC converter.” It constantly adjusts panel voltage to find the sweet spot (maximum power point) and then converts that power efficiently to the battery’s charging voltage.
- Hawai‘i off-grid: A simple, robust practice is a fixed rack facing South (or East if you value earlier charging), at roughly ~20° tilt. A common 4/12 roof (~18.4°) is close enough for winter-biased performance.
Why put one pitch & one direction on a single MPPT?
An MPPT input assumes all modules it manages share a similar IV curve. When you combine strings on different tilts or azimuths on the same MPPT, each string produces current differently throughout the day. In series strings, the current is limited by the “weakest” string at that moment; in parallel, the overall power curve can develop multiple peaks. The result is mismatch losses and a controller that may track the wrong peak under clouds, partial shade, or mixed orientations.
- Mixed azimuth/tilt on one MPPT ⇒ distorted power curve, recurring sub-optimal tracking.
- Same azimuth/tilt per MPPT ⇒ clean, single-hump power curve that’s easy to track.
- If you must mix orientations, use separate MPPT inputs or separate controllers for each group.
Rule of thumb: Group modules by same model, same tilt, same azimuth, similar shade profile per MPPT input.
How an MPPT charge controller works (plain + technical)
Plain language
Think of your panels like a bicycle rider: there’s a cadence where they’re strongest. An MPPT “listens” for that cadence by nudging the panel voltage up/down and watching the watts. When watts rise, it keeps nudging that direction; when watts fall, it backs off. Once it finds the sweet spot, it converts that panel power to whatever voltage the battery needs right now for its charge stage.
Technical snapshot
- Search: The controller samples P(V)=V×I while perturbing voltage (P&O) or using incremental conductance to locate the MPP.
- Convert: A high-efficiency DC-DC stage (buck/boost or buck-only, model-dependent) translates the panel’s MPP voltage to the battery’s required voltage/current.
- Charge stages: Bulk (current-limited), Absorb (voltage-limited for a held time), Float (maintenance), and sometimes Equalize (where applicable and supported).
- Dynamics: Under fast clouds or mixed strings, the controller’s search sees a changing landscape; keeping one orientation per MPPT keeps the landscape simple.
Hawai‘i fixed-rack: South or East at ~20° (winter-friendly)
For off-grid systems on the islands, the practical target is reliable winter production: days are shorter, cloudier, and rainy. A fixed rack at roughly ~20° facing South hits a strong mid-day window without mechanical complexity. Many homes have a 4/12 pitch roof (~18.4°), which is close enough and performs well year-round with a winter bias.
If your loads or weather patterns favor earlier energy (kick-starting battery charging in the morning and getting ahead of afternoon clouds), an East-facing array is a solid alternative for off-grid practicality. West favors late afternoon; grid-tie sites may prefer that, but for battery systems the earlier charge can be more valuable.
Quick orientation notes
- South: balanced daily curve, strong mid-day charging.
- East: earlier ramp-up; useful for batteries ahead of afternoon showers.
- Keep all modules per MPPT on the same tilt/azimuth.
- Prioritize durable mounting and clean wire management in coastal environments.
Build notes & checklist
- Module grouping: Same make/model, same tilt/azimuth, similar shade profile per MPPT input.
- String sizing: Keep worst-case cold Voc below the controller’s max input. Aim for a Vmp window the controller tracks efficiently. Verify with manufacturer string tools or datasheets.
- Overcurrent protection: Fuse/breaker each string in the combiner as required. Consider DC surge protection devices where appropriate.
- Wiring: Size conductors for temperature and voltage drop. As a target, keep PV homerun voltage drop modest (often ~2–3%). Use UV-resistant hardware and tidy drip loops.
- Charge profile: Program battery-specific Bulk/Absorb/Float (and Equalize if supported/needed) per the battery manufacturer.
- Shading: Avoid vent stacks and trees casting moving shadows; even brief string shading can pinch current.
- Maintenance: Rinse debris and salt film as needed; always de-energize appropriately and follow safety procedures.
FAQ
Can I mix East and South on the same MPPT?
Not recommended. You’ll create a multi-peaked power curve and invite tracking errors. Use separate MPPT channels/controllers for different orientations.
Why does “series” amplify mismatch?
In series, the same current must flow through every module. If one string wants to deliver less current (cooler, shaded, or a different tilt), it drags the entire chain to its lower current.
Is ~20° the only correct tilt for Hawai‘i?
No—think of it as a practical, winter-biased target for fixed racks. Anywhere in that neighborhood performs well. Roof pitch and racking logistics often decide the exact angle.
What about microinverters?
This page focuses on off-grid battery charging with MPPT charge controllers. Module-level inverters are a different architecture (AC at the roof) and follow different design rules.
Safety & disclaimer
The information on this page is shared for educational purposes. Electrical systems can be hazardous. Always follow manufacturers’ instructions, applicable codes, and safe work practices. When in doubt, consult a qualified professional.