The product label of the Renogy 100 Watt 12 Volt Polycrystalline Solar Panel gives us the electrical characteristics of the panel, according to the manufacture Renogy when it is exposed to an irradiance of 1000 W/m^{2}. But what does it all mean.

The electrical data label states that the panels Open Circuit Voltage, (V_{OC}) is 22.4 volts and that its Short Circuit Current, (I_{SC}) is 5.92 amps. So we can use Ohm’s Law then to find the output power of the solar panel, right!. But V times I gives us 22.4*5.92 = 132.6 watts, which is a lot more than the 100 watts quoted by Renogy, so what’s going on?.

The open-circuited voltage, V_{OC} means that the PV panel is not connected to any load, so its terminals are therefore open (infinite resistance) resulting in maximum voltage, in this case 22.4 volts, at its terminals. As its terminals are open there will be no current flow (I = 0) because there is no electrical circuit or load for the current to circulate through. Then the output power of the solar panel in this instance is P = V*I = 22.4*0 = 0 watts. In other words no generated electrical power.

Likewise, the short-circuited current, I_{SC} means that the PV panels terminals are shorted or connected together (zero resistance) creating a fully closed electrical circuit allowing maximum panel current, in this case 5.92 amps, to flow. However, as the terminals are shorted together there will be no output voltage drop (V = 0), so the output power of the solar panel will be P = V*I = 0*5.92 = 0 watts. Again no generated electrical power.

Then we may think that this Renogy solar panel can generate 132.6 watts of solar electricity, in reality it can not. Photovoltaic panels provide useable electricity when connected to an electric load and by measuring the output of a solar panel, we can use Ohm’s Law to determine the maximum output power point, or MPP.

We said previously that the output power of a solar panel depends on the electrical load connected to it. This load can vary from an infinite resistance to a zero resistance value producing an open-circuit voltage, V_{OC} and a short-circuit current, I_{SC} respectively. Then we want to find a resistive value somewhere inbetween. As the theoretical maximum power is 132 watts and the maximum open-circuit voltage is 22.4 volts, if we assumed a wattage of 150 watts and a voltage of 25 volts, this would give us a resistive value of 6 Ohms, (150/25).

There are many ways to achieve this 6 Ohm value using fixed value resistors, to variable resistors to rheostats, the option used would depend on what’s available and budget, as power resistors can be expensive. In the example given the load will be a Rheostat giving us a fully variable resistance between a minmum value of zero Ohms (0Ω) and a maximum value of 6Ω. We know that our solar panel has a manufacturers rating of 100 watts, so this would be the minimum power rating of our rheostat.

To measure the voltage across the terminals of the PV panel we would require a voltmeter. This could be a digital or analogue multimeter or a simple voltmeter but must have a scale high enough to read the panels open-circuit voltage (V_{OC}). The type used would depend on what’s available and budget.

To measure the current generated by the PV panel would require an ammeter to measure the short circuit current (I_{SC}) into a dead short. Again this second meter could be digital, analogue or a multimeter depending on what’s available and budget.

Let’s assume that we bought a rheostat which is variable between 0 and 10 Ohms, then increasing the load resistance in steps of 1 Ohm while measuring the solar panels output voltage and current, we can complete the following table to determine the electrical power supplied by the PV solar panel for different values of load resistance.

Resistance (in Ohms) | Voltage (in Volts) | Current (in Amperes) | Power (in Watts) |

Open Circuited | 22.4 | 0.00 | 0.00 |

10 | 22.4 | 2.24 | 50.2 |

9 | 22.3 | 2.47 | 55.1 |

8 | 22.1 | 2.76 | 61.0 |

7 | 21.6 | 3.08 | 65.9 |

6 | 20.80 | 3.50 | 72.8 |

5 | 20.05 | 4.01 | 80.4 |

4 | 18.80 | 4.70 | 88.4 |

3 | 16.86 | 5.62 | 94.7 |

2 | 11.60 | 5.80 | 67.3 |

1 | 6.0 | 6.00 | 36.0 |

Short Circuited | 0 | 5.92 | 0.00 |

Having taken our readings and tabulated the results in the above table, we can clearly see that the maximum power occurs when the load resistance R_{L} has a resistive value around the around the 3 Ohms value giving a panel voltage of about 16.9 volts producing 5.6 amperes thus giving a calculated output power of 94.7 watts. This value closely matches the manufactures data label for an operating voltage (V_{mp}) and operating current (I_{mp}) of 17.8V and 5.62A respectively. This then shows that the maximum power is generated by the panel around this point and we could if so wished fine tune our measurements to get closer to the 100 watt target.

Using the data from the table above, we can plot a graph of the measured voltage against the panels current as shown.

Above is the I-V characteristics graph for our PV solar panel example and is typical for all PV solar panels. Measuring the power of other types and ratings of solar panel will produce similar results, just the voltages and current values will be different. Also notice that power is zero for an open-circuit (zero current condition) and also for a short-circuit (zero voltage condition).

The maximum power output of a PV panel can be defined as its peak DC output given by multiplying the voltage and the current. Here the optimum operating point for our solar panel is shown at the mid-point in the bend (or knee) of the characteristics curve. In other words, this is the point at which the solar panel generates its maximum power, known commonly as the *maximum power point* or **MPP**.

The I-V characteristics curve presents an important property of a photovoltaic solar panel, or cell in that it shows it to be a current source device rather than a voltage source device, like a battery. Unlike a battery which has a constant terminal voltage, (12V, 24V, etc.) and provides variable amounts of current to a connected load, the photovoltaic cell or panel provides a constant supply of current over a wide range of voltages for a given amount of solar insolation.

Measuring the power of a solar panel is not too dificult but requires an assortment of multimeters, power resistors, or a single rheostat capable of handling the generated power, remembering that the greater the power output of your panel, 200W, 285W, 330W, etc. the greater will be the voltages and currents.

does the number of watts determine the panel size

Quick answer, kind of.

Long answer, it will affect it along with an other parameter, the efficiency rate. The efficiency rate is determined by the type of cells (mono vs polycristaline), the technology used to create ce cells and the quality of the materials.

So a low quality 100w polycristaline cheap solar panel at 17% efficiency will be bigger than a 100W quality monocrystaline solar panel at 22% efficiency (i believe the surface would need to be about 5% smaller, but there’s more elements to consider…)

So theres more than the watts.