Six ways to check DC-side voltage when power output is low

When you feel power generation is low, the first things you may want to check are the amount of electricity sold, solar irradiance, and the operating status of the power conditioner. However, in solar power systems, misreading the DC-side voltage can make isolating the cause take a longer, more roundabout route. Because DC-side voltage is affected by the solar modules, string configuration, junction boxes, DC wiring, input circuits, and irradiation and temperature conditions, it cannot be judged simply as high or low.

This article organizes six perspectives on DC-side voltage that practitioners will want to check when power generation is low. It focuses on the approach for safely inspecting on-site, points to note when viewing recorded data, and perspectives for distinguishing abnormal conditions from normal fluctuations. Note that measuring or switching DC circuits carries risks of electric shock, arcing, and fire. On-site measurements and work inside panels must always be carried out under the direction of qualified personnel or the person in charge, and in accordance with the equipment’s procedures, applicable laws and regulations, and the safety precautions indicated by the manufacturer.

Table of Contents

• Consider the DC-side voltage as the entry point for a reduction in power generation.

• Perspective 1: Distinguish whether the voltage is close to the open-circuit voltage or the operating voltage

• Method 2: Check for possible poor connections or open circuits from voltage differences between strings

• Method 3: Read the voltage taking into account changes in solar radiation and temperature

• View 4: Examine the relationship with the power conditioner input range

• Perspective 5 Separate instantaneous anomalies and sustained anomalies from time-series data

• Interpretation 6: Assess in combination with current, generated power, and alarms

• Judgment errors to avoid when checking DC-side voltage

• Summary Do not determine the DC-side voltage on its own when power output is low

View DC-side voltage as the entry point for power output decline

When power output is low, the DC-side voltage is the starting point for investigating the cause. In photovoltaic systems, the DC power generated by the solar cell modules is fed into the power conditioner through strings and junction boxes. Therefore, if there are abnormal changes in the DC-side voltage, there may be issues to check hidden on the module side, the wiring side, the connection side, or the input-circuit side.

However, it is dangerous to conclude that it is "faulty" or "no problem" based solely on the DC-side voltage. DC voltage varies with factors such as weak solar irradiance at dawn and dusk or on cloudy days, high module temperatures in summer, periods when shadows fall, and differences in string configuration. When investigating the cause of low power output, you should consider not only the voltage value itself but also the operating status at the time, the weather, the time of day, temperature, current, power output, and alarm history.

In practice, you first check the DC input voltage on the monitoring device or power conditioner display, then compare it with other circuits within the same installation and with historical data. By narrowing down the range of anomalies from recorded data before performing on-site measurements inside the panel, you can more easily reduce hazardous work and unnecessary shutdowns. Especially in installations with multiple strings, small abnormalities that are difficult to detect from total power generation alone can appear as imbalances in DC voltage or DC current.

The purpose of checking the DC-side voltage is not to determine the cause in a single step. It is to narrow down, when generation is low, which system is suspect, when the change began, whether it is a natural fluctuation due to the weather, or a persistent equipment fault. Simply adopting this premise makes it easier to prioritize inspections and organize the documents to be reviewed.

Approach 1: Distinguish between a state close to the open-circuit voltage and an operating-voltage state

When examining the DC-side voltage, the first thing to distinguish is whether the value is close to the open-circuit voltage or the voltage during power-generating operation. The meaning of voltage for a solar cell module changes between the state where no load is connected and the state where a power conditioner is taking power. If you make a judgement without confirming whether the DC voltage displayed when generation is low is the input voltage during operation or a voltage close to that when stopped or on standby, it can lead to misunderstandings.

When the system is near open-circuit voltage, the voltage can appear high even though almost no current is flowing. For example, if the power conditioner is stopped, waiting due to grid-side conditions, or not accepting input because of some protective action, a DC-side voltage may be present but no generated power is being extracted. If you conclude “there is voltage, so the module side is completely normal” in this situation, you may overlook connection faults, input stoppage, control-side waiting, or conditions in which no current is produced.

On the other hand, the operating voltage is the value measured while the power conditioner is controlling the system as it draws power from the solar panels. Normally, during operation the input voltage fluctuates with solar irradiance and temperature. If, when generation is low, the operating voltage is significantly lower than usual, or if only a particular input is abnormally high or low, this should prompt checking the string configuration, shading, connection status, input circuits, and control state.

When checking on site, always look at whether the power conditioner is generating, on standby, or stopped at the same time as the DC voltage value. In addition, check the DC current and AC output recorded at the same moment. The range of suspected causes differs when there is voltage but almost no current versus when both voltage and current are low. To narrow down the cause of low power output, it is important not to read the DC-side voltage number in isolation, but first determine which operating state that voltage was recorded in.

Also, immediately after startup in the morning and just before shutdown in the evening are periods when, even if DC voltage is present, it is difficult to obtain sufficient power generation. Rather than judging an anomaly based only on values during periods of unstable solar irradiance, comparing data from periods when irradiance is more stable makes it easier to interpret the equipment condition. When keeping inspection records, record not only the measured values but also the time, weather, operating status, and whether any alarms were present, as this makes it easier to assess things when reviewing them later.

Method 2: Check for possible poor connections or open circuits from voltage differences between strings

When power generation is low, a useful approach is to compare the DC voltages between strings. If strings are configured with the same azimuth, the same tilt, the same number of modules, and under the same conditions, their voltages at the same time of day tend not to diverge significantly. Therefore, if the voltage of a particular string is unnaturally low or higher than the others, it can be considered an indicator that there are inspection points to check.

In a string with an extremely low voltage, depending on the measurement location and operating conditions, this can prompt suspicion of poor connections, open circuits or broken wires, connector faults, blown fuses, switch status, loose terminals, module damage, and so on. However, in conditions close to a disconnection or open circuit, the voltage may appear low at some measurement points and may appear to remain high at others. Do not pinpoint the location of a break or fault based on voltage alone; it is important to judge by considering the circuit configuration, measurement points, current, alarms, and visual inspection.

Shading patterns and differences in string module counts can cause voltage differences. If you simply compare strings with different module counts, you may mistake a normal difference for an abnormal one. Before comparing, check the design drawings and the string table to confirm they have the same number of modules, the same orientation, the same tilt, and the same connection destination.

Conversely, caution is required when only the voltage appears higher than others. A higher voltage does not necessarily mean that power generation is in good condition. If no current is flowing, the input is not taking the load, or the circuit is nearly open, only the voltage may remain high. If power generation is low but a particular circuit's voltage is high and its current is low, you should suspect that the power is not being extracted.

In string comparisons, it is important to select circuits with similar conditions within the same installation. In installations where orientations differ east-west, where roof planes have different tilts, or where shading varies, the behavior of DC voltage and current can differ even at the same time. If you choose the wrong comparison target, you may incorrectly judge a difference caused by solar irradiance conditions to be a fault.

In practice, first check the design drawings, as-built drawings, string table, and the circuit numbers of the junction boxes, and identify the number of modules per string, their connection destinations, orientation, and tilt. Then check voltage differences within groups under the same conditions. If past inspection records exist, compare them with data from the same season and the same time of day. If only a particular string has been gradually diverging over time, it may indicate ongoing deterioration or changes in contact conditions. On the other hand, if a sudden difference appears for only a single day, temporary shading, weather, switch operations, or failure to restore systems after work should also be checked.

Voltage differences between strings can help narrow down the causes of low power generation, but rather than starting on-site work based only on voltage differences, it is safer and more reliable to verify current differences, alarms, visual appearance, connection status, and historical records together.

Method 3: Read the voltage by accounting for changes in solar irradiance and temperature

The DC-side voltage is affected by solar irradiance and temperature. When checking the DC voltage during periods of low power generation, judging without taking the weather and ambient temperature into account can cause normal fluctuations to be mistaken for abnormalities. In particular, photovoltaic (PV) modules tend to have lower voltage as their temperature rises, so on days with high ambient or module temperatures the voltage on the same installation may read lower than in spring or autumn.

On clear summer days when generation does not increase as much as expected, you may notice that DC voltage is low despite strong solar irradiance. In such cases, before simply concluding there is equipment failure, consider the voltage drop caused by a rise in module temperature. Solar power generation does not necessarily continue to increase proportionally with stronger irradiance. At high temperatures, a module’s output characteristics change and this can affect the voltage.

On the other hand, on sunny winter days or when ambient temperatures are low, DC voltage can be higher than usual. The voltage rise at low temperatures is also an important item to check in equipment design. Among inquiries reporting low power generation, there are cases where the apparently high winter voltage is perceived as abnormal, but in some instances this can be explained by changes due to temperature conditions. However, if the condition exceeds the input range or is accompanied by alarms, verification against the design conditions and input specifications is necessary.

Fluctuations in solar irradiance also affect how DC-side voltages should be read. Thin clouds, passing clouds, post-rain conditions, yellow dust or dirt, and partial shading can cause voltage and current to become unstable. If you only look at instantaneous values during periods when irradiance changes rapidly, they may appear abnormal. When checking the causes of low power output, it is important to look at data from periods when irradiance is as stable as possible and to compare the same time periods across multiple days.

Also, when solar irradiance is low, you may see voltage but not get sufficient current. Low power generation on cloudy days can be natural, but if it is extremely low compared with adjacent equipment or strings under the same cloudy conditions, the equipment should be checked. In other words, irradiance and temperature are used not to "explain away all low generation as natural variability," but to determine whether the low generation falls within the range that can be explained by natural variability.

In practice, it is easier to make judgments when DC voltage data are examined alongside solar irradiance, ambient temperature, module temperature if available, and weather notes. Even when monitoring data are insufficient, recording the inspection day's weather, time of day, shading conditions, whether there was wind, and the presence of dirt or snow around the site will help with later comparisons. The lower the power output, the more important it is not to look at voltage numbers in isolation but to consider them together with environmental conditions.

Perspective 4: Examine the relationship with the power conditioner input range

An indispensable consideration when checking the DC-side voltage is its relationship with the power conditioner’s input range. If the DC voltage generated by the solar cells does not fall within the device’s operable input range, it may not be possible to extract generated power stably. When generation is low, you should not just look at the string voltage; you should also verify that the voltage is remaining within the appropriate range of the input circuit.

If the input voltage is too low, the power conditioner may have difficulty starting, may stop soon after starting, or may be unable to extract sufficient power. It may naturally be lower in the morning and evening or on cloudy days, but if the low condition continues even during stable daytime sunlight, you should check the number of modules per string, connection status, shading, open circuits, module faults, and the condition of the input circuit.

Be cautious when the input voltage is too high. Voltage can increase at low temperatures or under near-open-circuit conditions, so it is important to ensure that the design or connections do not exceed the equipment’s allowable range. If high voltage is related to alarms or shutdowns, a drop in power generation may be caused by the input protection operating. In particular, for cold climates or high-voltage string configurations, you must verify the temperature conditions assumed in the design against the actual operating conditions.

Also, the input range is not just the minimum and maximum values; it includes an operational range for extracting power via MPPT control and similar mechanisms. If the DC voltage during operation tends to fall outside that range, there may be cases where power generation does not increase as expected despite solar irradiance. This can be caused not only by problems in equipment design or string configuration, but also by errors made later during module replacement, system expansion, circuit changes, or restoration work.

When checking the causes of low power generation, verify whether the string configuration listed in the equipment documentation matches the actual connection state. If, after completion, the number of modules has changed, some modules have been disconnected for fault handling, circuits have been rearranged inside junction boxes, or the input destinations have been changed, the documented assumptions can differ from the on-site reality. If such a discrepancy exists, judgments about DC-side voltages are also more likely to be incorrect.

When verifying against the input range, it is important to check not only displayed values and monitoring data but also equipment specifications, single-line wiring diagrams, string tables, inspection records, and past refurbishment history. The lower the power generation, the more effective it is—rather than taking measurements on site immediately—to sequentially compare design values, operating values, measured values, and historical records to more easily narrow down the cause.

Perspective 5: Separate instantaneous anomalies and persistent anomalies from time series data

When looking at DC-side voltage, it is important to check not only a single instantaneous reading but also its behavior over time. On a day when you feel power generation is low, if you only look at the voltage at the moment a cloud happens to pass, it may appear abnormal. Conversely, even if the instantaneous value does not seem to show a major problem, examining the trend over days, weeks, or months may reveal a gradual decline.

Momentary anomalies can be related to the passage of clouds, temporary shadows caused by birds or flying objects, brief pauses during work, waiting due to grid-side conditions, the timing of monitoring data acquisition, and similar factors. In such cases, if voltage or current is disturbed only for a short time but recovers immediately afterward, it does not necessarily indicate a sustained equipment fault. However, if the same pattern repeats every day at the same time, it is necessary to check shading from buildings or trees, the effects of nearby equipment, control settings, and startup and shutdown conditions.

In cases of persistent anomalies, you may see patterns such as a particular string’s voltage being low every day, only a specific input taking longer to start in the morning, voltages dipping unnaturally during the daytime, or conditions remaining lower than in the past even on sunny days. In these situations, it is easier to judge by selecting and comparing multiple days with the same irradiance conditions rather than by looking at single days. When investigating the cause of low power generation, it is important to determine whether the anomaly was a one-time occurrence or is recurring.

When examining time-series data, it is easier to organize your analysis by using three perspectives: daily, time-of-day, and seasonal. At the daily level, look at how the DC voltage on days with low power generation differs from normal days. At the time-of-day level, determine whether it is low only in the morning, also during midday, or if it drops sharply in the evening. At the seasonal level, check whether the differences can be explained by changes in temperature and solar altitude, or whether they are clearly different from the same period in the previous year or from days with the same conditions.

Also, when power generation is low, it is useful to check whether changes in voltage appear before, at the same time as, or after the drop in generated power. For example, if the trends in DC voltage and current change around a certain day, check the work carried out before and after that time, power outages, thunderstorms, strong winds, snowfall, cleaning, inspections, equipment changes, and so on. By linking historical records and time-series data, it becomes easier to concretize candidate causes.

Pay attention to the granularity of monitoring data. Data recorded at short intervals and data averaged over long intervals will reveal different anomalies. Momentary voltage dips can be hard to see in averaged values, while instantaneous values can overstate temporary fluctuations. When investigating the cause of low power output, it is important to also check the data recording intervals and aggregation methods.

Interpretation 6: Assess by combining current, generated power, and alarms

DC-side voltage is an important item to check, but voltage alone cannot determine the cause of reduced power generation. Because generation depends on both voltage and current, even if the DC voltage appears normal, if current is not flowing the generated power will be low. Conversely, a slightly low voltage may not indicate a significant anomaly if it is within a natural range given solar irradiance and temperature conditions, and if the current and generated power are comparable to those of other systems.

When generation is low, the combination to check first is DC voltage, DC current, DC power, and AC output. If there is DC voltage but the DC current is extremely small, suspect that the circuit is not producing sufficient power. If both DC voltage and DC current are low, broadly check for insufficient irradiance, shading, poor connections, string abnormalities, etc. If the DC side is producing power but the AC output is low, you need to check the conversion side, the grid side, and the control side.

Alarm history is also important. Check whether alarms such as input voltage abnormalities, insulation-related issues, overvoltage, undervoltage, grid abnormalities, temperature abnormalities, and communication abnormalities were issued in the period before and after days with low power generation. Even if no alarms were issued, that does not necessarily mean there are no abnormalities, but if the alarm occurrence time matches a change in DC voltage, it can help narrow down the cause. In particular, if the system is repeatedly undergoing temporary shutdowns or restarts, the daily cumulative energy generation can drop significantly.

In systems with multiple inputs, it is important to check the balance of voltage and current for each input. Even when overall power generation is low, the diagnosis changes depending on whether all inputs are similarly low or only specific inputs are low. If all are similarly low, consider the weather, grid-side control, a system-wide shutdown, soiling, or widespread shading. If only specific inputs are low, focus on the strings in that circuit, the junction box, wiring, modules, and the input terminals.

On-site, voltage anomalies and current anomalies do not necessarily present in the same way. With partial shading, the current side may be strongly affected. When a connection is nearly open, current may not flow easily even if voltage remains. If module temperature is high, this may more easily appear as a voltage drop. In other words, DC-side voltage is an entry point, and the final judgment requires combining multiple pieces of data.

When reviewing maintenance inspection reports or monthly reports, we check not only voltage records but also current, power output, downtime, alarm counts, weather notes, and work history together. To explain the cause of low power generation, saying "the voltage was low" is insufficient. Organizing which circuit, when, to what extent, compared with what it was low, and how it related to current and alarms will make the report lead to follow-up inspections and corrective actions.

Judgment errors to avoid when checking DC-side voltage

When checking the DC-side voltage when power generation is low, there are several judgment errors to avoid. The most common is assuming there is no problem because voltage is present. Even if DC voltage can be confirmed, if no current is flowing the generated power will not be sufficient. Because in near-open-circuit conditions only the voltage may be visible, it is important not to judge the system as normal based solely on the presence of voltage.

Next, this is another judgement where you should avoid immediately concluding a fault just because the voltage is low. DC voltage varies with temperature, solar irradiance, time of day, shading, and operating conditions. Especially during high temperatures or periods of unstable solar irradiance, it can appear lower than usual. To determine whether low power generation is occurring, you need to compare with past data under the same conditions and with adjacent circuits, and verify whether the observation can be explained by natural variation.

Also, be careful not to confuse design values with operating values. The values listed in the specifications and the design voltages are guidelines based on certain conditions. On-site operating voltages change depending on actual solar irradiance, ambient temperature, and control conditions. A discrepancy between the design values and the displayed values does not necessarily indicate an immediate fault. However, if a condition that deviates significantly from the design values persists, it is necessary to check the configuration and connections.

Care must also be taken when selecting comparison targets. Simply comparing strings with different orientations or slopes, strings with different numbers of panels, or surfaces with different shading can lead to incorrect conclusions. When investigating installations with low power output, compare systems with similar conditions, and when conditions differ, interpret the results in light of those differences. In particular, on rooftop installations or sites with complex terrain, solar irradiance conditions can vary greatly even within the same power plant.

Mistakes in safety-related judgment must also be avoided. DC circuits can retain voltage even after the AC side has been shut down. Because photovoltaic panels generate power when exposed to light during the day, actions such as casually disconnecting connectors, touching terminals, or switching under no-load conditions are dangerous. Even when investigating the cause of low power generation, on-site work must be carried out only after confirming safety procedures, personal protective equipment, the ratings of measuring instruments, and the work arrangements.

Finally, failing to keep records is also a major mistake. In investigations when power generation is low, you may not be able to determine the cause on the spot. If you record voltage, current, time, weather, operating conditions, alarms, work performed, and the circuit numbers checked, it will be easier to compare later. If records are insufficient, when the same anomaly recurs decision-making will be set back to square one and responses are likely to be delayed.

Summary: Do not determine the DC-side voltage on its own when power generation is low

Checking the DC-side voltage when power generation is low is useful for isolating the cause. However, DC-side voltage is not a number on which to draw a conclusion by itself. You should check in order whether the voltage is close to the open-circuit voltage or an operating voltage, whether there are abnormal differences between strings, whether the readings can be explained by irradiance or temperature, whether they are within the input range, whether the condition persists over time, and whether the values are consistent with current and alarms.

In practice, always prioritize safety first and grasp the overall picture from monitoring data and displayed values. From there, by comparing strings under the same conditions, comparing with past data, checking solar irradiance and temperature, verifying against the power conditioner input range, and reviewing alarm history, you can more easily narrow down potential causes. If on-site measurements or work inside panels are necessary, it is important to carry them out according to procedures under the judgment of qualified personnel or the person responsible for management.

By learning how to read DC-side voltages, you can move beyond vague, intuitive judgments like “it seems low” or “it’s probably faulty” when power generation is low. If you can clarify which circuit, at what time of day, under what conditions, and compared to what it is low, requesting inspections, preparing reports, and deciding on corrective actions becomes much easier.

When checking for decreased power generation, it is important to keep daily data in a visible form and to detect signs of abnormalities early. By preparing an environment where DC-side voltage, current, power generation, alarms, and inspection history can be checked together, you can improve the accuracy of on-site judgments and make it easier to reduce delays in response. If you want to continuously identify the causes of low power generation and streamline the operation and management of solar power generation equipment, it is important to centralize monitoring data, inspection records, and on-site verification results and to establish a system that allows stakeholders to share the same information.