The voltage a solar panel produces can vary for a few reasons. Some of the reasons are positive, some are not. The voltage produced by a panel is really only part of a more important question: How many watts should the panel produce?
There are three factors that impact this question Every panel on the market is designed to produce a certain voltage and current under various conditions. These specifications are generally printed on the back of the panel. Knowing how to assess the specifications of a panel will help you determine if it will provide the power you need. The voltage of a solar panel is the result of individual solar cell voltage, the number of those cells, and how the cells are connected within the panel. Every cell and panel has two voltage ratings. The Voc is the amount of voltage the device can produce with no load at 25º C. This value is a little like the maximum horsepower a car’s engine can put out. It is a lab-produced value that has little value in the field. The value will vary due to atmospheric conditions and temperature. See also: Calculate Solar Panel kWp & KWh (KWh Vs. KWp + Meanings) The Vmp is the voltage the device will produce a maximum power output. This is essentially the working voltage of the device. It is the voltage the panel will supply to a battery or charge controller. Solar panels can be designed to produce just about any voltage. A panel is a collection of individual solar cells. Individual cells produce between 0.45 and 0.6 volts (Vmp) at 25º C. The voltage output of the individual cells can vary due to the type and quality of the cell used. Groups of cells are wired together in a panel to produce various voltages. This is where we find part of the answer to, “How many volts should my panel put out?” Most 32 cell panels are wired in series to produce voltage for a 12-volt system. Most 72 cell panels are wired in series to produce 24 volts, but could also have pairs of strings wired in parallel to produce more current at 12 volts. When looking at a panel of a given nominal voltage, a good rule of thumb for estimating the Vmp is to add about 20% to the nominal voltage. To estimate the Voc value, add about 80% to the nominal value. These will almost never be exactly right but are a good estimate. The certificate on the back of the panel or other manufacturer documentation is the only place to find the exact voltage ratings of a panel. If you measure the voltage of a panel that is not connected to any load and is in full sun you should measure the Voc value. As soon as you connect the leads to a load, the voltage will drop to something near the Vmp value. It will vary based on the load applied. Measuring volts is a fairly simple procedure. A simple Voltmeter or Multi-meter from your local hardware store is all you need. Set the meter to DC Volt in the appropriate range. Touch the probes of the meter to bare wire at the end of the cables and you can measure the voltage of the panel. Be careful not to let wires touch each other. To calculate the power (watts) provided by a solar panel we need to know the size of the electrical wave (volts) and the force of the current (amps) behind the wave. Most solar panels list two current values: Maximum Current (Ipm) and Short Circuit Current (Isc). A typical solar cell produces around 30 milliamps per square centimeter or about 187 milliamps per square inch. At that rate, a 4-inch square cell will produce approximately 3 amps. Different cell materials and cell sizes will produce various current outputs. Various sized cell output at 187 Milliamps per square inch. Higher amp ratings are achieved by wiring groups of cells in parallel. This will lower the volt rating of the panel but may increase the overall power (watt) output. Measuring current is not as simple as measuring volts. The Current at Maximum Power (Imp) can only be measured while there is power running through the wire attached to the panel. DC Amp Meters are a little pricey but are available if you have the urge to measure your current. The Short Circuit Current (Isc) requires shorting the Positive and Negative leads through an appropriate Amp Meter. Short-Circuiting the leads of the panel could damage it, so this practice should be avoided unless you know what you are doing. I have lived off-grid for 20 years and have never found it necessary to measure the Isc of a panel. Take the manufacturer’s word for the rated current. It is safer and easier. Watts is a measure of work. It is the amount of energy the panel can provide to your system at maximum solar exposure at 25º C. It is calculated by multiplying Volts at Maximum Power (Vmp) and the Current at Maximum Power (Ipm). This calculation expresses the maximum potential power the panel could provide. Load, atmospheric conditions, and temperature, can all impact this value. The likelihood that you will experience this output very often is pretty slim. As is often the case, a simple question does not have a simple answer. “How many volts should my solar panel put out?” is not as straightforward as one might expect. There are a lot of variables at play. Sources 
Solar Panel Voltage
Open Circuit Voltage
Voltage at Maximum Power
How Various Panel Voltages Are Produced
Number of Cells for Typical Voltage Panels
Vmp to Voc Ratio
Estimating Voc and Vmp Value For a Panel
Measuring Panel Voltage
Panel Current: Watt – Volts – Amps – Ipm
How Various Amp Ratings Are Achieved.
Measuring Amps of a Panel
Watts
