How many panels can you install?
Three limits decide it: the inverter's voltage window sets a minimum and maximum per string, each MPPT's current rating and the PV-watt rating set a total, and your roof or yard sets a physical ceiling. Pick a panel and inverter from the spec sheets we've collected, or type your own numbers.
Start here: pick a panel
Worked out from today's prices across every panel we track. Take one of these into the calculator below, or skip down and choose any panel from the list. Standard panels and pallets are split by face, because a bifacial panel only earns its rear-side gain over a light surface and comparing the two on price alone rewards the wrong panel for a dark roof.
Standard Rigid framed panels for a roof or ground mount.
Flexible Bendable panels for curved roofs on vans, boats and RVs.
Portable Folding panels for charging a power station away from the house.
No footprint pick here on purpose. Makers publish a folding panel's folded size, not the area it covers when it is open, so comparing it against a rigid panel would be measuring the wrong thing.
Pallets Ranked separately, because a pallet always beats a single panel per watt and would win every category above. You are buying the whole pallet.
35 panels in this pallet, 450 W each. Only worth it if you need that many and have somewhere to put them.
36 panels in this pallet, 420 W each. Only worth it if you need that many and have somewhere to put them.
22 pallets tracked.
Per-watt figures use the panel's rated STC output, which is a laboratory number. Real output is lower. A panel with no published dimensions is left out of the footprint comparison rather than guessed at.
Panels vs. inverter and space
STC values from the panel's data sheet and the PV-input section of the inverter's data sheet. Picking a product fills the fields; you can still edit them.
Panel
Inverter or charge controller
| MPPT | Max A | Max Isc A | Inputs | Max W | Max V | MPPT range | Strings | Per string | Panels | Watts |
|---|
Each MPPT is sized on its own: some inverters have MPPTs of different ratings (the EG4 18kPV has one 25 A and two 15 A inputs). "Inputs" is how many physical PV pairs it exposes; leave blank if unknown. The last four columns fill in once a panel is chosen: how many parallel strings that MPPT can take, the min–max panels per string, and the panel and watt range it can carry. Panels = strings × panels per string. An MPPT that takes 2 strings of 10 carries 20 panels, but no single string may pass the per-string maximum - wire it as 2 × 10, never one run of 20.
Your site
Facts about where the array is going, not about the gear. Cold raises panel voltage, which is what sets the maximum you can put in one string, so use the record low for the system's whole life rather than last winter's. The area is optional and sets a hard ceiling on the count.
Choose or enter a panel (Voc, Vmp, Isc, Imp) and an inverter (voltage window plus at least one MPPT current or watt limit).
Per string (panels wired in series)
Whole inverter
What you can build
Wiring plan
| MPPT | Strings in parallel | Panels per string | Panels | String Voc, cold | Current at Imp | Watts |
|---|
Series (one string): voltages add, current stays at one panel's Imp - this is what fills the MPPT voltage window. Parallel (two strings on the same MPPT): currents add, voltage stays - only do it when the strings are identical in length and orientation, and check the MPPT's current limit. Strings on different MPPTs are independent and may differ; strings on the same MPPT must match.
Each MPPT is its own DC input. The voltage limit applies to each one separately: 9 panels on MPPT 1 and 9 on MPPT 2 means each input sees 9 × Voc, not 18 × Voc - voltage never adds between MPPTs. What adds across MPPTs is watts (the PV input allowance), and within one MPPT, the current of its parallel strings.
How this is calculated
- Cold voltage. Panel voltage rises as temperature falls. With a Voc temperature coefficient: Voc × (1 + coef ÷ 100 × (Tmin − 25 °C)). Without one, the NEC 690.7(A) table factor (1.02 at 20 °C … 1.25 at −40 °C).
- Maximum per string. The smaller of ⌊max PV voltage ÷ cold Voc⌋ and ⌊MPPT high ÷ cold Vmp⌋, rounded down. Exceeding the max voltage destroys the input; exceeding the MPPT range just stops production on cold days.
- Minimum per string. The largest of the floors that apply: the sheet's own "MPPT full power voltage" where it publishes one (the best evidence there is), otherwise the larger of ⌈MPPT low × 1.2 ÷ Vmp⌉ from the data sheet, and ⌈target ÷ Voc⌉ from the practical minimum string voltage (220 V by default for a 48 V system - well above the data-sheet floor, so the MPPT starts early, runs efficiently and still works when hot panels sag). The target is ignored when the MPPT window cannot hold it, e.g. a 150 V solar generator.
- Reference temperature. Voc is defined at STC, 25 °C, so the cold correction runs from 25 °C. Some calculators and install guides correct from the 45 °C NOCT row instead, which is about 5 % more conservative and can cost one panel per string - if you want that margin, enter a temperature 20 °C lower than your real record low.
- Strings per MPPT. For each MPPT separately: the smaller of ⌊its max input current ÷ Imp⌋, ⌊its max short-circuit current ÷ Isc⌋ and its number of string inputs, with bifacial gain applied to the currents.
- Per MPPT vs whole inverter. Voltage and current ratings are per MPPT input - each is checked on its own, and voltages do not add between inputs. Only the watt ratings are inverter-wide totals (with a per-MPPT watt limit too on some models).
- PV watts - two different numbers. "Maximum utilized solar power" is what the inverter converts; output clips above it. "Recommended maximum solar input" is how much array you may connect, and that is what caps the panel count here. Filling the gap between them is deliberate over-panelling: it costs a little clipping around noon in summer and buys real production on cloudy days, early mornings, late afternoons and through the winter.
- Fit. Panels laid in a grid with your gap, trying both orientations, on a plain rectangle. Real roofs have vents, setbacks (often 18–36 in from ridges and edges) and shade; treat the number as an upper bound.
Planning figures, not a design. These calculators give you ballpark numbers to shop and plan with. They are not an electrical design, they cannot see your site, your loads or your local code, and they may be wrong. String voltage is the number that damages hardware when it is wrong: check it against the inverter's own data sheet before wiring. Wire sizing, over-current protection, grounding, rapid shutdown and interconnection must follow NEC 690 and your local code. Have any design checked by a licensed electrician before you buy or energize anything. You use these tools at your own risk.More on safety.