A solar panel simulator: find the 1 W point
Mokxi is a free online solar panel simulator that runs in your browser, with nothing to install and no account needed. A 6 V 1 W panel, the small epoxy kind in STEM kits, drives a load resistor with one meter on its current and one on its voltage. Drag the sunlight, change the load, and see why a solar panel does not behave like a battery.
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What you can do here
In the circuit above, or in the editor it opens.
- Drag the sunlight from full sun down to dark
- Change the load and plot the panel’s current against its voltage
- Find the load that takes the most power, the maximum power point
- See why a panel is a current source and not a battery
- Charge an 18650 cell from the panel through a TP4056 charger
- Run a microcontroller from the panel and a cell
What the running circuit shows
The light sets the current; the load sets the voltage.
In full sun, the slider at 1000 W/m², the panel gives about 161.8 mA into 36 ohms at about 5.99 V: about 0.97 W, near its 1 W rating. The amps meter’s 1 ohm shunt is in series, so the panel sees 37 ohms. Take the load away and the panel reads its open-circuit voltage, about 7.18 V. Make the load 1 ohm and the current is about 199.6 mA, almost the whole 0.2 A it gives into a short, at only 0.40 V.
That is the shape of every solar cell: nearly the same current from a short up to about 5.5 V, then the voltage climbs the last stretch to 7.2 V as the cells’ own diode takes the current back. Power is current times voltage, so it is small at both ends. 10 ohms gets almost the full 0.2 A at 2.17 V, about 0.43 W, and 100 ohms gets 6.89 V at 68 mA, about 0.47 W. Between them is the maximum power point, near 33 ohms here, where the panel gives about 0.99 W.
Drag the sunlight to 500 and the current the panel can give halves, to about 0.1 A, while the open-circuit voltage barely moves, to about 6.70 V. Now 36 ohms gets only about 0.34 W, while 56 ohms gets about 0.44 W: in dimmer light the best load is a bigger resistance. That moving target is why solar chargers with maximum power point tracking exist.
How the panel is modeled
A current source and a diode, fitted to the listing.
Like a real cell, the panel is a current source set by the light with a diode across it, here twelve cells in series. The diode is fitted to what these panels are sold with: 7.2 V open, 0.2 A into a short and 1 W at 6 V. That fit comes from a product listing, not a datasheet, since most small panels are sold without one. The meters load the circuit the way real ones do: a 10 megohm input on volts and a 1 ohm shunt on amps.
What it leaves out: temperature, which costs a hot panel about half a percent of its voltage a degree; the angle of the sun, so tilting the panel does nothing here; partial shading; and a blocking diode, which is a separate part you can add. For a whole project, the Solar sensor node example charges an 18650 from the same panel through a TP4056 and runs a Pro Mini from it, and the Solar tracker example steers a panel with four light sensors and two servos.
Questions
Is it free?
Yes. The circuit on this page, every part in it and the editor it opens in are on the free plan, with no card.
Do I need an account?
No. The circuit runs the moment the page does, and the editor opens without one. A free account is only for saving a project to the cloud and sharing it by link.
Why does my solar panel read 7 V but give less under load?
Because 7 V is its open-circuit voltage, with no current flowing. A panel is a current source: under load it gives what the light makes, about 0.2 A for this one in full sun, at whatever voltage the load sets. Into 36 ohms that is about 6 V.
What is the maximum power point of a solar panel?
The load at which the panel gives the most power, where current times voltage is largest. For this 6 V 1 W panel in full sun it is near 33 ohms, about 0.99 W. At half sun it moves to a larger resistance, around 60 ohms.
Can I charge a battery from a solar panel?
Yes, through a charger. Put a TP4056 between the panel and an 18650 cell; the charger stops at 4.2 V. Wiring a lithium cell straight to a panel is how cells get overcharged.
More simulators
Each one opens on a circuit that is already running.
Put a panel in the sun
Open the circuit, drag the sunlight and change the load until the power peaks. Nothing to install, and no account needed.