How to Match Solar Panels to a Charge Controller: Voc, Isc, Series and Parallel

To match solar panels to a charge controller, do two checks. First, the string’s total open-circuit voltage (Voc) must stay at or below 90% of the controller’s maximum PV voltage. Second, the total panel watts must stay at or below the controller’s maximum PV input at your battery voltage. For example, three PowMr 100W flexible panels in series (64.8 V, 300 W) fit a PowMr Keeper 30A MPPT on a 12V battery, but would over-volt a Keeper 20A.

This guide explains the numbers on a panel’s label, how series and parallel wiring change them, why we keep a 10% safety margin, and which pairings from our product list pass every check.

The five numbers on a panel label

Every proper panel datasheet lists these. If a panel doesn’t publish them, you can’t safely match it.

• Pmax (W): rated power under standard test conditions.
• Voc (open-circuit voltage): the highest voltage the panel makes, with nothing connected. This is the number that can damage a controller.
• Isc (short-circuit current): the highest current the panel makes, with its terminals shorted. Used for cable, fuse and input-current checks.
• Vmp (voltage at maximum power): the voltage when the panel is delivering its rated power. Used to check the string is high enough to charge the battery or reach an inverter’s minimum.
• Imp (current at maximum power): the matching current at Vmp.

Verified figures for panels in our product list

*BLUETTI’s Japanese and Australian stores and the Lazada listing give three different electrical sets for the PV350S, so we use the worst case: Voc 47 V and Isc 11.4 A.

A few notes you should know before buying:

• BLUETTI PV200 listing sells two panels. The Lazada listing offers the PV200 and the Sora 220 as variants. Their figures differ, so check which one you’re ordering. The listing’s FAQ gives a PV200 Voc of 24.6 V, but BLUETTI Philippines gives 26.1 V; we use 26.1 V. BLUETTI also says not to expose the PV200 to rain, so treat it as a portable panel, not a permanent outdoor one.
• PowMr 150W listing conflict. The Lazada variant reads “150W 20.4V 9.5A”. PowMr’s own page gives Voc 21.6 V and Isc 9.16 A. We use PowMr’s figures.
• Bosca figures are listing-only. No official Bosca datasheet was found. The same Voc (22.32 V) appears on all three sizes, which suggests a template. The connector type isn’t stated either, so check whether the panel has MC4 leads or a bare junction box.
• PowMr flexible connectors. PowMr doesn’t publish the connector type. Check before buying.

Panels we can’t match

The PowMr 100W and 200W rigid panels, the PowMr 200W flexible panel, the Flowatt 200W rigid and 120W flexible panels and the Greenest 290W panel have no verified Voc or Isc. The numbers printed in some of their titles (such as “21.7V” or “25.2V”) aren’t labelled, so we can’t tell if they’re Voc or Vmp. Until you have the real datasheet or the label on the panel, don’t wire these to a controller with a tight voltage limit.

Series wiring: voltages add

In series, the positive of one panel connects to the negative of the next. The string’s voltage is the sum of the panels’ voltages. Current stays the same as one panel.

3 × PowMr 100W flexible in series
Voc  = 3 × 21.6 V = 64.8 V
Vmp  = 3 × 18 V   = 54 V
Isc  = 6.11 A (unchanged)
Power = 3 × 100 W = 300 W

Series wiring is how you reach a 24V battery’s charging voltage, or an inverter’s minimum MPPT voltage. It also keeps the current low, which suits long cable runs from the roof.

The catch: one shaded panel in a series string drags down the whole string’s current. Plan the layout so all panels in a string get the same sun.

Parallel wiring: currents add

In parallel, all positives join and all negatives join. The voltage stays the same as one panel (or one string). Currents add.

2 strings of (2 × PowMr 100W in series), in parallel
Voc  = 2 × 21.6 V = 43.2 V (per string, unchanged in parallel)
Isc  = 2 × 6.11 A = 12.22 A
Power = 4 × 100 W = 400 W

Parallel strings let you add power without going over the controller’s voltage limit. They raise the current instead, so your electrician sizes the cable and decides whether each string needs its own fuse.

Keep each string to identical panels, and make parallel strings the same length. Mixing panel types makes the maths unreliable.

The safety margin: why 90%

A panel’s Voc rises as the panel gets cooler. The label value is measured under standard test conditions (STC). On a cool, clear morning, or the first bright minutes after a rain shower, the panel can briefly produce more than its label Voc.

Manufacturers handle this with a temperature coefficient. None of the panels above publish one. On top of that, listings sometimes disagree with the manufacturer. So we apply one simple rule:

Allowed string Voc ≤ 0.9 × controller's maximum PV Voc
Panels in series = round down (0.9 × max PV Voc ÷ panel Voc)

For the PowMr Keeper controllers, whose manual gives maximums of below 60 V, 75 V and 100 V:

*Voltage allows 2 PV200s on the Keeper 20A and 30A, but on a 12V battery their wattage limits (260 W and 390 W) cap it at 1 panel. Always do the wattage check below as well.

Over-voltage is the mistake that can damage a controller, and the margin is cheap insurance. We also keep every pairing within the published wattage limit.

Minimum voltage: enough to charge

The string also has to be high enough. An MPPT controller can lower voltage, not raise it.

• 12V battery: one 12V-class panel (Vmp about 18 V) is enough.
• 24V battery: one 12V-class panel is below the battery’s charging voltage. Use at least 2 in series.
• Hybrid inverters: PowMr’s 4.5kW and 6.5kW hybrid inverters have an MPPT range starting at 55 V. With 18 V-Vmp panels, that means at least 4 in series (4 × 18 V = 72 V). Our hybrid inverter guide covers string sizing up to their 450 V limit.

Wattage: the controller’s max PV input

PowMr publishes the maximum panel watts each Keeper accepts at each battery voltage:

The Keeper manual doesn’t publish a maximum PV input current, so the wattage figure is the limit we use for parallel strings.

Worked examples

Example 1: Keeper 20A, 12V battery, PowMr 100W flexible

• Voltage: 2 × 21.6 V = 43.2 V (under 54 V). 3 in series would be 64.8 V, over the controller’s 60 V maximum. Max 2 in series.
• Wattage: 2 × 100 W = 200 W (under 260 W). A second string would make 400 W. Max 1 string.
• Result: 2 in series, 1 string, 200 W.

Example 2: Keeper 20A, 24V battery, PowMr 150W flexible

• Minimum: at least 2 in series for a 24V battery.
• Voltage: 2 × 21.6 V = 43.2 V (under 54 V).
• Wattage: 2 × 150 W = 300 W (under 520 W). A second string would make 600 W, over 520 W.
• Result: 2 in series, 1 string, 300 W.

Example 3: Keeper 40A, 24V battery, BLUETTI PV200

• Voltage: 3 × 26.1 V = 78.3 V (under 90 V). 4 would be 104.4 V, over the controller’s 100 V maximum.
• Wattage: 3 × 200 W = 600 W (under 1,040 W). A second string of 3 would make 1,200 W, too much.
• Result: 3 in series, 1 string, 600 W.

Example 4: Keeper 20A, 24V battery, BLUETTI PV200 (a tight one)

• Voltage: 2 × 26.1 V = 52.2 V. That’s under 54 V, but only just. It passes.
• Wattage: 400 W (under 520 W).
• Result: 2 in series, 1 string, 400 W. If you ever swap in a panel with a higher Voc, recheck.

Example 5: BLUETTI PV350S on a Keeper 20A, 12V battery

• Voltage: 47 V (worst case) is under 54 V, so voltage is fine.
• Wattage: one 350 W panel is already over the 260 W limit at 12V.
• Result: not recommended. On a 24V battery the same controller takes one PV350S (350 W ≤ 520 W). A Keeper 30A or 40A on 24V takes two in parallel (700 W).

Example 6: why PWM controllers fail these checks

PowMr’s Pstar PWM controller manual caps panel voltage at 23 V on a 12V battery. With our 10% margin that’s 20.7 V, below every panel in the table. On a 24V battery its PV input range starts at 30 V, so one 12V-class panel is too low, and two (43.2 V or more) exceed the 41.4 V margin limit. Our compatibility data marks every pairing as failing. See MPPT vs PWM charge controllers for why.

Verified pairings at a glance

Maximum verified configuration for each Keeper, written as panels in series × parallel strings (total watts). Fewer panels is fine, as long as you keep at least 2 in series on a 24V battery.

Bosca rows use marketplace listing figures only. Three Bosca panels in series (66.96 V) sit just under the Keeper 30A’s 67.5 V margin limit, and four (89.28 V) just under the 40A’s 90 V. That’s within the rule, but there’s very little room if the listing figures are off. If you can read the real Voc from the panel’s label, recheck.

Step-by-step: matching your own panels

1. Get the datasheet. You need Voc, Isc, Vmp and Pmax from the manufacturer, or at least from the label on the panel itself.
2. Find the controller’s limits in its manual: max PV Voc and max PV watts at your battery voltage.
3. Series count: round down (0.9 × max PV Voc ÷ panel Voc).
4. Minimum series count: on 24V, at least 2 of the 12V-class panels.
5. Watts per string: series count × panel watts. If it’s over the controller’s max, reduce the series count.
6. Parallel strings: round down (max PV watts ÷ watts per string).
7. Write down the string Voc and total Isc for your electrician, who will size the cables, fuses and isolators.

Safety

Plug-and-play portable panels and power stations can be set up by the owner following the manual. Anything wired to the house, a breaker panel, the roof or the grid needs a licensed electrician or PEE and an LGU electrical permit. Series strings can reach dangerous DC voltages: three or four panels already exceed 60 V Voc. Panels produce voltage whenever light hits them, so cover them before connecting or disconnecting. Our guide to whether DIY solar is legal in the Philippines explains who may do this work. Grid export needs your electric company’s process; see net metering.

What to buy

• Controller: PowMr Keeper 20A on Lazada, Keeper 30A on Lazada or Keeper 40A on Lazada, chosen from the table above
• Panels with manufacturer figures: PowMr 100W flexible on Lazada, PowMr 150W flexible on Lazada, BLUETTI PV200 / Sora 220 on Lazada (portable; pick the variant carefully) or BLUETTI PV350S on Lazada (24V systems)
• Panels with listing-only figures (check the label): Bosca 60W, Bosca 120W, Bosca 160W on Lazada
• Tool for MC4 connectors: MC4 connector spanner pair on Shopee, for unlocking MC4 plugs without damaging them
• Cables, fuses and isolators: sized by your electrician from the string Voc and Isc

Check today’s price on each listing before you buy.

Bottom line

Matching panels to a controller comes down to two numbers from the controller’s manual and two from the panel’s datasheet. Keep the string Voc at or below 90% of the controller’s maximum, keep total watts within its limit for your battery voltage, and put 12V-class panels at least 2 in series on a 24V battery. Use the table above for verified PowMr Keeper pairings, and skip any panel that doesn’t publish its Voc.