Solar panels 101 for power stations
What Voc, Isc and MPPT mean on the label, and why the station cares. Five numbers, one curve, and a calculator that lets you play charge controller.
Every solar panel has a label on the back, and most shop listings copy it. Here is the label of the Jackery SolarSaga 200 (JS-200E). From the manual
Four of those numbers decide whether a panel is safe for your station. The fifth, the watts, decides how fast it charges.
The rated watts
The big number is measured under standard test conditions: strong lab light and a panel at 25°C (77°F). Outdoors, panels usually make less, because the sun is lower, the air is hazy or the panel is hot. Our refill times assume about 70% of the rating.
The two voltages: Voc and Vmp
Voc Open-circuit voltage. The highest voltage a panel makes, with nothing connected. Rises in cold. is the open-circuit voltage: the voltage with nothing connected. It is the highest voltage the panel makes. Vmp Voltage at maximum power. The working voltage when the panel is actually charging. is the working voltage, when the panel is actually charging something. Vmp is always lower than Voc.
Your station has a voltage window for its solar input. The EcoFlow RIVER 3 Plus takes 11 to 55 V. Below the bottom of the window it will not start charging. Above the top, the manuals warn it can be damaged. EcoFlow and BLUETTI set that top limit against Voc. Jackery rates its inputs by working voltage.
Voc also rises when the panel is cold, so we always check it at your coldest morning. More in Why cold weather matters for solar voltage.
The two currents: Isc and Imp
Isc Short-circuit current. The most current a panel can push. Stations cap this. is the short-circuit current, the most current the panel can push. Imp Current at maximum power. The working current when the panel is actually charging. is the working current while charging. Isc is a little higher than Imp.
Stations limit current too. On many inputs, the station simply caps the current at its limit and you lose some watts. Some manuals are stricter. EcoFlow's DELTA 3 Plus manual says to keep the total short-circuit current within 15 A on each input.
MPPT: the part that finds the best point
Watts are volts times amps. A panel can run anywhere between zero volts and its Voc. Near Voc there is almost no current. Near zero volts there is plenty of current but almost no voltage. The most watts come from one point in between, the max power point, close to Vmp and Imp.
Inside the station, the MPPT Maximum power point tracker. The charge controller inside the station that finds the best voltage to pull from the panels. charge controller keeps hunting for that point as the light changes. Try doing its job below.
Be the MPPT: find the most watts
That is the max power point. The MPPT keeps the panel here. Label max power point: 18 V x 11.5 A = 207 W. Rated 200 W. From the manual
The curve is a standard panel model drawn through the four label numbers. A real panel's curve differs a little, and it moves with light and temperature.
The plug
Panels and stations do not always use the same plug. EcoFlow's panels end in MC4 connectors. Jackery's SolarSaga panels use DC8020. EcoFlow stations take XT60 or XT60i, and BLUETTI's AC180 takes DC7909. When the plugs differ you need the right adapter cable. The checker lists the cables for each pairing.
Rules of thumb
- Compare the panel's Voc with the top of the station's voltage window, and leave room for cold mornings.
- Compare the panel's current with the station's current limit. The manual says whether that limit means Isc or working current.
- Do not judge a panel by its watts alone. The Jackery SolarSaga 200 and the Anker SOLIX PS200 Bifacial are both 200 W panels. One has a Voc of 22 V, the other 57.6 V (a shop listing we have not confirmed yet).