Two panels can be joined two ways: in series, which adds their voltages, or in parallel, which adds their currents. Both work. Which one is right is decided by your charge controller, not by your panels, and getting it wrong costs you either half your power or the controller itself.
Here is the whole decision in one line: if you have a PWM controller, wire in parallel. If you have an MPPT controller, wire in series. Everything below is why, and the arithmetic you need to do before you buy anything.
What series and parallel actually do
Series means the positive lead of panel one goes to the negative lead of panel two. Voltage adds, current stays the same. Parallel means positive joins positive and negative joins negative, usually through a Y-branch connector. Current adds, voltage stays the same.
Using a real datasheet — the Renogy 100 W 16BB N-type panel, rated Vmp 19.97 V, Imp 5.01 A, Voc 22.79 V, Isc 5.31 A — two of them come out like this:
| Measurement | One panel | Two in series | Two in parallel |
|---|---|---|---|
| Vmp (working voltage) | 19.97 V | 39.94 V | 19.97 V |
| Imp (working current) | 5.01 A | 5.01 A | 10.02 A |
| Voc (open circuit, at 77°F) | 22.79 V | 45.58 V | 22.79 V |
| Isc (short circuit) | 5.31 A | 5.31 A | 10.62 A |
| Power at STC | 100 W | 200 W | 200 W |
Same 200 watts either way. The difference is what shape those watts arrive in, and whether your controller can use that shape. Our longer series vs parallel explainer covers the topology in more depth.
Your charge controller decides, and PWM and MPPT want opposite things
A PWM controller is a switch. It connects the array straight to the battery, so the array gets dragged down to whatever the battery is sitting at. Victron’s own technical paper puts numbers on the damage: a 100 W panel charging a 13 V battery through a PWM controller delivers 81 W, which is 19% less than the same panel on an MPPT controller at the same moment. Morningstar states the PWM rule plainly — for a 12 V battery the array’s Vmp must be above 15 V, and should be “higher than, but as close to, the maximum battery voltage as possible.”
Put two 12 V panels in series on a PWM controller and you hand it roughly 40 V, which it will happily crush down to 13 V. You just paid for 200 W and threw away most of it. That is why the common advice to “wire in series, it’s better” is actively wrong for half the people reading it.
An MPPT controller is a DC-to-DC converter. It holds the array at its maximum power point and converts the surplus voltage into extra charging current. Morningstar puts the typical real-world gain at 10–15% over PWM; Victron shows the advantage is largest in the cold and shrinks to nothing when panels are hot.
| Cell temperature | Who wins | Why |
|---|---|---|
| Below 113°F (45°C) | MPPT, clearly | Panel Vmp is high; PWM throws the surplus away |
| 113–167°F (45–75°C) | Roughly a tie | Victron measures PWM within 10% of MPPT across this band |
| Above 167°F (75°C) | MPPT, if the array voltage is high enough | Vmp collapses toward battery voltage and PWM stops charging properly |
Those are cell temperatures, not air temperatures. A free-standing array with no wind runs about 40°C (72°F) above ambient, so a 90°F afternoon in Garden Grove is a 160°F panel. A panel with its back fully boxed in can pass 212°F, and at that point a PWM controller cannot reach absorption voltage at all. If you are still choosing hardware, read our MPPT vs PWM comparison and our charge controller picks before you buy connectors.
Do the cold-weather voltage math first
This is the step that destroys equipment, and not one page ranking for this search performs it. A panel’s open-circuit voltage rises as it gets colder. Datasheets quote Voc at 77°F, and every panel carries a temperature coefficient of Voc — a negative number, typically between −0.26 and −0.35 percent per °C.
The Renogy panel above is −0.26%/°C. Two in series, starting from 45.58 V:
| Coldest expected morning | Voc rise | Actual array Voc |
|---|---|---|
| 77°F (25°C, datasheet) | — | 45.6 V |
| 32°F (0°C) | +6.5% | 48.5 V |
| 14°F (−10°C) | +9.1% | 49.7 V |
| −4°F (−20°C) | +11.7% | 50.9 V |
| −22°F (−30°C) | +14.3% | 52.1 V |
A controller rated for 50 V of PV input reads a comfortable 45.6 V when you test it in the driveway in July. At 14°F it is over the limit, and at −4°F it is dead. Voc is what the array sits at before the controller starts drawing current, which is exactly the first clear, freezing minute after sunrise — the panel is at its coldest and its voltage at its highest at the same instant.
The arithmetic is: Voc(cold) = Voc(STC) × [1 + (25 − Tmin°C) × coefficient ÷ 100]. Morningstar’s own worked example uses the record low for the site, and that is the number to use, not the average January temperature. Victron’s model numbers state the ceiling directly: a SmartSolar 75/15 accepts 75 V of PV and a 100/20 accepts 100 V. Exceed it and you are outside warranty and usually outside a working controller.
Wiring in parallel sidesteps this almost entirely — the same two panels in parallel reach only 26.1 V at −22°F. If you cannot find a coefficient on the label, or the panel is salvaged or home-built and has no nameplate at all, you have no way to do this calculation and should wire in parallel. You can confirm the real numbers with a multimeter; our guide to testing solar panels walks through measuring Voc and Isc safely. Cold weather helps output in other ways too, which we cover in do solar panels work in winter.
Check the current against the controller and the wire
The National Electrical Code assumes a PV circuit runs at its short-circuit current continuously, then adds margin on top. NEC 690.8(A) sets maximum circuit current at 125% of Isc, because irradiance can reach 1,250 W/m² — 125% of standard test conditions — for extended periods. NEC 690.8(B) then requires conductors sized at 125% of that, so the combined factor is 1.56.
For our two panels in parallel, Isc is 10.62 A:
- Maximum circuit current: 10.62 × 1.25 = 13.3 A. The controller must be rated at or above this, so a 15 A unit is fine and a 10 A unit is not. Morningstar states the same rule from the other direction: a 30 A controller accepts an array of at most 24 A Isc.
- Minimum conductor ampacity: 10.62 × 1.56 = 16.6 A.
In series, Isc stays at 5.31 A, so those numbers fall to 6.6 A and 8.3 A. This is the quiet advantage of series on an MPPT system: everything downstream gets smaller and cheaper.
A portable panel clipped to a battery in a truck bed is not an NEC installation and no inspector is coming. Do the arithmetic anyway. The code numbers are not bureaucracy, they are the margin between a hot connector and a fire.
Wire gauge over a real run
Ampacity tells you what will not melt. Voltage drop tells you what will actually work, and it is almost always voltage drop that sets the gauge. The standard single-phase formula is VD = (2 × K × I × L) ÷ CM, where K is 12.9 for copper, I is current in amps, L is the one-way run in feet and CM is the conductor’s circular mils.
Here is a 25-foot run from array to controller, with our two panels wired both ways:
| Wire | Parallel: 10.0 A at 20 V | Series: 5.0 A at 40 V |
|---|---|---|
| 14 AWG | 1.57 V lost — 7.9% | 0.78 V lost — 2.0% |
| 12 AWG | 0.99 V lost — 4.9% | 0.49 V lost — 1.2% |
| 10 AWG | 0.62 V lost — 3.1% | 0.31 V lost — 0.8% |
| 8 AWG | 0.39 V lost — 2.0% | 0.25 V lost — 0.6% |
The NEC’s informational notes suggest keeping a branch circuit under 3% and the total under 5%. They are recommendations rather than mandatory rules, but they are a sensible target. Notice that 14 AWG clears the 16.6 A ampacity requirement calculated above and still loses nearly 8% — which is why sizing on ampacity alone leaves you wondering where your watts went. Notice too that doubling the voltage cuts the percentage loss by four, for the same copper.
Two panels that are not identical
“Just use identical panels” is good advice hiding a real explanation. Series and parallel fail differently when panels are mismatched, and the difference is large enough to change which wiring you choose.
In series, current is the bottleneck. The whole string runs at the lowest current any panel in it can supply. Pair a 100 W panel (Imp about 5 A) with a 200 W panel (Imp about 10 A) in series and the string carries 5 A at the sum of their voltages — roughly 200 W from 300 W of nameplate. You are paying for a 200 W panel and receiving 100 W of it. The same mechanism applies to a shaded, soiled or aged panel: in series it drags the good one down to its level. Worse, as the UNSW PVEducation reference explains, the weak panel can end up dissipating the good panels’ power as heat rather than delivering it, and that localised heating causes permanent damage.
In parallel, voltage is the bottleneck, and panels vary far less in Vmp than in Imp. Two panels of different wattage in parallel still share one operating voltage, so the one with the higher Vmp gets pulled slightly off its maximum power point, but the loss is a few percent rather than a third. PVEducation is explicit that parallel mismatch is the less severe case.
The honest rules: mixing different wattages in parallel is usually fine if the Vmp figures are within about a volt of each other. Mixing them in series is usually a waste of money unless the Imp figures genuinely match. Different orientations — one panel on a south face, one on a west face — behave like permanent mismatch, and belong in parallel for the same reason. If you want both panels to work independently, parallel is the configuration that lets them.
Fuses, connectors and grounding
The battery cable always gets a fuse. This is not optional and it is not about the panels. A 12 V battery can deliver hundreds of amps into a short, and the controller-to-battery cable is the one place in the system where that energy can reach. Size the fuse to the controller’s maximum charge current, size the wire to the fuse, and put the fuse as close to the battery’s positive terminal as the cable physically allows. The stub between terminal and fuse is the only unprotected wire in your system, so make it short.
Two panels in parallel almost certainly do not need string fuses. NEC 690.9(A) says an overcurrent device is not required for PV modules or source-circuit conductors sized per 690.8(B) where the available fault current cannot exceed the module’s ratings. With two strings, the worst case is one string back-feeding the other at 1.25 × Isc, which for our panels is 6.6 A against a 15 A maximum series fuse rating printed on the label. That passes comfortably. At three or more parallel strings the arithmetic tightens quickly, and that is the real origin of the “three strings and up need fuses” rule of thumb.
Sources disagree on the exact test. SunWize’s tech note applies the full 1.56 factor to the whole array — 5.31 A × 2 × 1.56 = 16.6 A, which would exceed our panel’s 15 A fuse rating and demand fuses at two strings. The back-feed method says no fuses needed. Inline MC4 fuse holders cost about eight dollars each. When two credible methods disagree and one outcome is a fire, fit the fuses.
Connectors: use MC4 or the exact connector your panels ship with, and do not mix brands that merely look alike — dimensional differences between lookalike connectors cause high-resistance joints that run hot. Crimp with the right die rather than pliers, and never make or break a PV connection under load. Open the array first.
Grounding: NEC Article 690 requires exposed metal module frames and racking to be bonded to an equipment grounding conductor. On an RV or boat that means bonding frames to the chassis or bonding system; on a building it means a proper equipment grounding conductor back to the system ground. Use the marked grounding hole on the frame with the specified hardware, not a self-tapper through the anodizing. If the panels are going on a structure, our guide on how solar panels are installed on a roof covers the mounting and bonding side.
Never connect panels to a battery without a charge controller
A panel has no idea when a battery is full. It keeps pushing current until something stops it, and at that point the battery voltage climbs past its charging limit. Trojan states the outcome for lead-acid directly: overcharging causes excessive gassing as water breaks down, heat build-up and accelerated aging, and in sealed batteries it dries out the electrolyte and ruins the cell. The gas coming off is hydrogen, which is why you also do not do this in a sealed compartment. With lithium the failure is faster and less forgiving, because a LiFePO4 pack held above roughly 14.6 V relies on its BMS to disconnect, and a BMS is a last-resort protection device, not a charging strategy.
You will see a rule quoted that a panel under C/10 watts is safe to connect directly. That describes a 2–5 W trickle maintainer on a large battery, where self-discharge absorbs the current. It does not describe two 100 W panels, and it is not a licence to skip the controller. The correct order of assembly is battery to controller first, then panels to controller, so the controller can detect the system voltage before it sees PV input. We cover the single-panel version of this in using a solar panel to charge a battery, and the multi-component version in connecting panels to a battery bank, controller and inverter. For vehicles, see hooking a solar panel to an RV battery.
When to stop and get help
Say it plainly: some of this is not a beginner job. Stop and bring in a licensed electrician or a solar installer if the array is going on a roof, if it connects to anything wired into a house circuit, if you cannot find Voc and the temperature coefficient for your panels, if the battery is lithium without a published charge profile, or if the total array is beyond a few hundred watts. DC arcs do not self-extinguish the way AC arcs do, and a 40 V string in full sun is live the moment the sun hits it — there is no off switch on a solar panel. Our overview of the real hazards of solar panels is worth reading before you start rather than after.
Sources
- Victron Energy - Which solar charge controller: PWM or MPPT? victronenergy.com The 19% PWM shortfall at 25C into a 13 V battery; PWM landing within 10% of MPPT between 45 and 75C cell temperature; the 40C cell rise over ambient on a still day; and that doubling array voltage cuts cable cross-section by four
- Morningstar - Traditional PWM vs TrakStar MPPT Technology s3.amazonaws.com MPPT delivering 10-15% more than PWM; the array being pulled down to battery voltage; the Vmp above 15 V rule for 12 V PWM systems; the controller rating at 125% of array Isc; and the worked temperature-compensated Voc example using the site record low
- Victron Energy - SmartSolar/BlueSolar MPPT 75/10 to 100/20 technical specifications victronenergy.com Maximum PV open-circuit voltage of 75 V and 100 V by model - the ceiling the cold-weather calculation has to clear
- Renogy 100 W 16BB N-type panel specifications offgridsource.com Every worked number in the article: Vmp 19.97 V, Imp 5.01 A, Voc 22.79 V, Isc 5.31 A, a Voc temperature coefficient of -0.26%/C, and a 15 A maximum series fuse rating
- IAEI Magazine - Into the Forest: details of PV systems that inspectors and installers need to know iaeimagazine.org NEC 690.8(A) at 125% of Isc and its justification at 1,250 W/m2 irradiance, 690.8(B) conductor sizing, and how the two compound to roughly 156%
- SunWize - Combiner box requirements sunwize.com The NEC 690.9(A) exception language and the competing method that applies the full 1.56 factor to the whole array - the disagreement this article reports rather than resolves
- PVEducation (UNSW) - Mismatch effects pveducation.org That a series string's output is set by its lowest-performing member, that the good panels' power is dissipated in the weak one as localised heating and irreversible damage, and that parallel mismatch is materially less severe
- Trojan Battery - Battery maintenance trojanbattery.com That overcharging causes excessive gassing as water breaks down, heat build-up and accelerated aging, and that sealed batteries dry out and are permanently damaged
Questions people ask
Can you put two solar panels on one battery?
Yes, and it is a standard setup. Both panels feed one charge controller, and the controller feeds the battery. You join the panels either in series, which doubles voltage, or in parallel, which doubles current, then run a single pair of leads to the controller's PV terminals. The battery never sees the panels directly. The only real constraints are that the controller must accept the array's cold-weather open-circuit voltage and be rated for at least 125% of the array's short-circuit current.
Can you hook a solar panel directly to a 12 volt battery?
No, not for anything above a few watts. A panel keeps pushing current after the battery is full, driving voltage past the charging limit. Trojan Battery lists the consequences for lead-acid as excessive gassing, heat build-up and accelerated aging, with sealed batteries drying out permanently. Lithium packs fail faster and rely on the BMS disconnecting, which is emergency protection rather than a charging method. A small 2-5 watt maintainer on a large battery is the one exception.
Can I mix 100W and 200W solar panels?
In parallel, usually yes. In series, usually not worth it. Series strings run at the lowest current in the string, so a 100 W panel pulling about 5 amps will hold a 200 W panel to 5 amps as well - roughly 200 watts out of 300 watts of nameplate. Parallel is limited by voltage instead, and panels vary far less in operating voltage than in current, so the loss is typically a few percent. Check that the two Vmp figures are within about a volt before wiring them in parallel.
How many batteries do I need for 400 watts of solar?
Size the bank to the charge current, not the panel wattage. A 400 W array on a 12 V system delivers roughly 30 amps of charging current through an MPPT controller. Flooded lead-acid batteries want a charge rate between C/10 and C/8, which puts the minimum bank at around 240-300 Ah. LiFePO4 accepts much higher rates, so 200 Ah is comfortable. Undersizing the bank means the controller throttles charging and you waste array capacity.
How long will a 400W solar panel take to charge a 200Ah battery?
For a 200 Ah lithium battery, figure about 2,560 watt-hours of capacity. A 400 W array realistically delivers around 300 W in good conditions, so a full charge from empty takes roughly 8.5 hours of strong sun - more than one winter day provides. A 200 Ah lead-acid battery should only be drawn down to 50%, so replacing 1,280 watt-hours takes about 4.5 hours. Both figures stretch considerably under cloud or at low sun angles.
What happens to a solar panel when the battery is full?
Nothing harmful. The charge controller stops drawing current and the panel drifts up toward its open-circuit voltage, producing almost nothing. The energy is not stored, converted to heat in the controller, or forced anywhere - it simply is not harvested. The panel itself runs a few degrees warmer than it would while working, because none of the absorbed sunlight is leaving as electricity. No damage occurs to the panel, the controller or the battery.
Do I need a fuse between two solar panels wired in parallel?
Usually no, with only two panels. NEC 690.9(A) waives overcurrent protection where the fault current available from the other strings stays within the module's ratings, and with two strings the worst case is one panel back-feeding the other at 1.25 times its short-circuit current - well under the maximum series fuse rating printed on a typical panel label. Three or more parallel strings normally do need fuses. The battery-to-controller cable always needs one, regardless of the array.
