6 items for factories to avoid overlooking lunch-break operations when calculating power generation

When calculating solar power generation for a factory, attention tends to be paid to installed capacity, roof area, solar irradiance, orientation, tilt angle, and generation losses. These are basic considerations for evaluating an installation, but in practical decision-making, unless you also look at when and how much of the generated electricity can actually be used, it becomes difficult to accurately assess the benefits of the installation. What is particularly easy to overlook is the operating status during lunch breaks.

Lunchtime is often thought of as a period when production lines stop, but in actual factories not all power use ceases. Many loads continue to operate during the lunch break, such as air conditioning, ventilation, lighting, compressors, cooling equipment, conveyors, standby power, office areas, quality control rooms, warehouses, and charging equipment. On the other hand, in factories where major processing machines and assembly lines temporarily stop, midday power demand can fall sharply.

Solar power generation tends to produce greater output around midday on sunny days, although this depends on weather, season, and installation conditions. Therefore, if you estimate power demand during lunch breaks higher than it actually is, assumptions about how much can be self-consumed will be excessive. Conversely, if you underestimate demand because auxiliary equipment and air conditioning are operating during lunch breaks, you will undervalue the potential to utilize solar power. This article explains six items for factory operational staff who are searching for "solar power generation output calculation" to ensure lunch-break operations are not overlooked and are reflected in generation calculations.

Table of Contents

• Why operation during the lunch break is important in factory power generation calculations

• Do not assume that the load during the lunch break will stop

• Break down daytime electricity usage by line and by equipment.

• Confirm overlap between power generation peaks and break periods.

• View self-consumption rate and surplus power including the lunch break

• Improve calculation accuracy through operational improvements and measurement data

• Summary

Why Lunchtime Operation Becomes Important in Factory Power Generation Calculations

When calculating solar power generation for a factory, the first things to check are the installable solar panel capacity, the orientation of the mounting surface, obstructions on the roof or site, local solar irradiance conditions, and generation losses for each piece of equipment. Estimating annual and monthly generation based on these factors is the starting point for evaluating an installation. However, in actual factory operations, what matters is not just the amount generated but how much of that generation can be used on-site.

Midday operation becomes an issue because the period when solar power output tends to be high overlaps with the period when factory operating patterns are likely to change. In a typical daytime-operated factory, generation tends to increase from the morning through early afternoon. On the other hand, during lunch breaks workers take a break and production lines or some equipment may be stopped. How demand during this period is handled affects how the generated electricity can be used.

For example, in factories where air conditioning, ventilation, dust collection, cooling, refrigeration, water supply and drainage, compressed air, inspection equipment, and IT equipment continue operating even during the lunch break, a certain level of power demand remains even if the main production lines stop. If you calculate lunch-break demand as nearly zero, you will overlook power that could actually be self-consumed on-site. Conversely, in factories where many systems shut down during lunch and standby power is low, surplus power is more likely to appear during daytime peak generation. Looking only at annual generation without reflecting these differences makes it difficult to correctly assess post-installation electricity savings and the operational considerations.

In calculating a factory's solar power generation, it is essential not only to consider "how many kWh can be generated annually" but also "how many kWh the factory consumes during that time period." Especially during lunch breaks, operations and shutdowns tend to be mixed, and the perceptions of staff can easily differ from actual measurements. Even when the site describes it as "stopped at noon," power data may show that a certain load remains. Conversely, even when they say "machines run during the lunch break," the main loads may drop, reducing the capacity available to absorb peak generation.

Therefore, not overlooking operation during lunch breaks is not a mere minor adjustment. It is an important item to confirm, affecting the capacity design of self-consumption solar PV, the need for batteries, measures against reverse power flow, operation during holidays and long vacations, impacts on contracted power, and alignment with equipment renewal plans. Treating the lunch break not as a uniform downtime but as a separate verification point within generation calculations leads to an installation decision that is appropriate for the factory.

Do not assume the load will be stopped during the lunch break

The first thing to avoid when calculating power generation is oversimplifying the lunch break as a time when the factory stops. Even if on-site operations halt, the factory’s overall electricity use often continues. Even during periods when production equipment is shut down, power is used for building environmental control, quality control, storage, office work, safety management, equipment standby states, and the like.

A clear example of a remaining load during the lunch break is the air conditioning. In factories, temperature and humidity control may be required not only for worker comfort but also for product quality and equipment protection. Even when workers leave for lunch, completely stopping the air conditioning can cause a large load when restarting in the afternoon or affect the condition of products and materials. Therefore, some factories keep the system running and merely relax the set temperature.

The same applies to ventilation and exhaust. Even during periods when work is halted, ventilation equipment may be kept running to maintain the indoor environment and for safety reasons. Dust collection systems and local exhaust systems are divided into those that can be shut down according to the process and those that need to continue operating for a certain period. Just because a process stops for a lunch break does not mean that all related equipment will stop at the same time.

In factories that use compressed air, the behavior of compressors during the lunch break is also important. Even when production equipment is stopped, power may be consumed to maintain pressure in the piping, due to air leaks, idle equipment, cleaning work, and so on. If compressors are starting periodically during the lunch break, that means there is a certain load available as a self-consumption destination for on-site solar power generation. On the other hand, if the lunch-break load remains solely because of large air leaks, there is room for energy-saving improvements separately from the solar power calculation.

Utilities such as refrigeration, freezing, temperature maintenance, drying, heating, circulation, pure water supply, and wastewater treatment should not be overlooked. These systems do not always operate in complete sync with the production line and may continue running during the lunch break. In factories where maintaining quality and environmental conditions is necessary—such as food, chemical, electronic components, precision machining, printing, and resin molding—lunchtime power demand can remain higher than expected.

It is not always possible to turn all the lights off during the lunch break. Even if work areas are darkened, lighting for aisles, warehouses, offices, break rooms, inspection rooms, and locations required for security may remain on. Even when lighting efficiency has improved, in factories with large floor areas it can still represent a non-negligible load.

Thus, the load during the lunch break cannot be judged solely by whether production stops. To reflect it in power generation calculations, it is necessary to check the loads remaining during the lunch break for each piece of equipment and clarify which loads continue to operate, which stop, and which temporarily increase or decrease. In particular, when determining the scale of a solar PV installation, it is important to check not only the average demand during the lunch break but also the minimum demand. By understanding how far demand falls during periods of high generation, you can anticipate the potential for surplus power earlier.

Break down daytime power usage by line and by equipment

When using a factory’s power usage for power generation calculations, looking only at the total energy consumption makes it difficult to grasp the reality of the lunch break. Factory-wide incoming power data is important, but by itself it cannot determine which equipment stops during the lunch break and which remains running. To improve the accuracy of solar power generation calculations, it is necessary to categorize the characteristics of power consumption by production line, process, and equipment.

The first thing to confirm is the operating patterns of the main production lines. Daytime demand varies greatly depending on whether the entire line stops during the lunch break, only certain processes stop, or unmanned or automated operation continues. Equipment such as machining machines, molding machines, drying ovens, painting equipment, inspection devices, and conveyors differ in how easily they can be stopped depending on the process. Even equipment that can be stopped may continue to use some power during the lunch break if restarting takes time or if temperatures need to be maintained.

Next, separate and examine the utility-related loads. Compressors, pumps, cooling towers, chillers, boiler auxiliary equipment, wastewater treatment equipment, pure water systems, dust collection equipment, and ventilation systems form the plant’s fundamental power demand. These loads can be divided into portions that vary with production volume and portions that occur to a certain extent regardless of operating status. Whether solar power generated during the lunch break can be self-consumed is strongly influenced by the size of this base load.

Office and administrative loads are often overlooked in a factory's power generation calculations. During lunch breaks, office air conditioning, lighting, IT equipment, hot water supply, and break room facilities may be in use. While these loads can be smaller in scale compared with a shutdown of the manufacturing area, in factories spread across multiple buildings or those with large administrative departments, they are worth considering as a certain level of lunchtime demand.

Warehouses and logistics areas also need to be checked. Even if the production line stops for the lunch break, shipping and receiving, goods intake, conveying, sorting, refrigerated storage, charging, lighting, shutters, and conveying equipment may still be operating. Especially in factories where logistics hours do not align with production-line break times, power demand can remain during the lunch break. If these loads are present at times close to the peak of solar power generation, they can be evaluated as a sink for self-consumption.

In this breakdown work, it is not necessary to classify equipment too finely. In the initial stage of power generation calculations, it is effective enough to organize them by broad characteristics, such as "main loads that stop during the lunch break", "base loads that continue through the lunch break", "auxiliary loads that fluctuate during the lunch break", and "loads that increase during ramp-up after the break". The important thing is not to treat the entire plant as a single average value.

Also, the ramp-up load before and after the lunch break should not be overlooked. Even if demand falls during the lunch break, when operations resume in the afternoon equipment may start up all at once, causing electricity use to increase sharply over a short period. If solar power generation is sufficient during this time, part of the electricity needed for the restart could potentially be supplied by generation. On the other hand, if the startup timing coincides with generation fluctuations caused by clouds, the expected reduction effect may be difficult to achieve.

Line-level and equipment-level breakdowns are useful not only for solar power generation but also for energy conservation and reviewing equipment operation. They provide an opportunity to check whether equipment that could be stopped during lunch breaks is still running, whether there is waste in compressed air or air conditioning, and whether standby power is excessively high. Not leaving generation calculations as mere feasibility estimates for installation but linking them to overall factory energy management is a significant benefit for operational staff.

Check for overlap between power generation peaks and break periods

In calculating solar power generation, it is important to look not only at total annual or monthly generation but also at generation by time of day. To avoid overlooking operations during the lunch break, you need to check how much the break period overlaps with the peak generation. Factory lunch breaks are often scheduled during times when solar output tends to be high, and this overlap affects self-consumption rates and surplus power.

Solar power generation output varies depending on weather, season, installation orientation, tilt angle, and shading. South-facing installations tend to produce more power around midday, east-facing ones tend to favor the morning, and west-facing ones tend to favor the afternoon. If a factory's lunch break is scheduled around noon, solar panels installed on south-facing roofs or mounts are likely to have their high-output period overlap with the break time.

If there is still sufficient demand during the lunch break, it becomes easier to use the power generated at peak times on-site. However, if major loads drop significantly during the lunch break, the more electricity is generated during those periods the more likely surpluses will occur. A high annual generation may seem good at first glance, but if it cannot be consumed during those periods, the effectiveness of introducing a self-consumption system may be smaller than expected.

When checking by time of day, at a minimum separate representative patterns for sunny days, cloudy days, high-demand summer days, low-demand winter days, and holidays or non‑operating days to better reflect actual conditions. While surpluses tend to occur during lunchtime on sunny days, on cloudy days generation falls and surpluses may not appear. In summer the air‑conditioning load is large, and some factories tend to have residual power demand even during lunch breaks. In winter demand varies depending on air‑conditioning and heating conditions. If you judge only by annual averages without looking at seasonal differences, you cannot properly evaluate the impact of lunch breaks.

It is also important whether lunchtime is fixed. In some factories, break times are staggered by department. In shift-based factories, break times may differ by line. If break times are dispersed, the overall drop in factory demand will be gradual, making it easier to self-consume solar power generation. On the other hand, if the entire company takes a break at the same time, demand tends to drop quickly, and if this coincides with a generation peak, surpluses are more likely to occur.

Check the length of the break period as well. Whether the lunch break is about 45 minutes or one hour, or whether the equipment is stopped longer when including cleaning and inspections before and after, will affect generation calculations. Because solar power output changes continuously, even a slight difference in the break start and end times can change the amount of generation available for self-consumption.

There is nothing inherently wrong with a generation peak coinciding with the lunch break. What is important is to understand in advance whether there will be load available at that time, and how any surplus will be handled. When calculating generation, rather than simply assuming “it generates during the day so the factory can use it,” you need to overlay which times during the day generation occurs and which times the factory is using power.

Viewing self-consumption rate and surplus power including the lunch break

In solar power systems for factories, how you view the self-consumption rate is important. The self-consumption rate refers to the proportion of generated electricity that was consumed on-site within the factory. The higher the self-consumption rate, the easier it is to leverage generated power to reduce on-site electricity use. If operation during lunch breaks is overlooked, the calculation of the self-consumption rate may deviate from actual conditions.

In factories where power demand drops during lunch breaks, there can be periods when generation exceeds demand. The handling of surplus electricity depends on contract terms and equipment configuration, but at the planning stage it is important to understand how much surplus might occur. If a significant surplus is expected, measures should be considered such as reducing installed capacity, considering battery storage, shifting loads that can be run during the lunch break, or adjusting equipment operating times.

On the other hand, at factories where a certain base load remains during the lunch break, solar power generation may be easier to self-consume. For example, if cooling, air conditioning, ventilation, storage, wastewater treatment, or compressed air continue to operate, the demand during the lunch break can serve as a sink for the generation. In such cases, treating the lunch break simply as downtime can lead to underestimating the self-consumption rate. When calculating generation, it is important to reflect the actual measured demand during the lunch break and to accumulate, by time period, the amount of electricity that can be self-consumed.

When checking surplus power, we consider multiple conditions, not just times of maximum generation. On clear spring or autumn days, air-conditioning loads are relatively small and photovoltaic generation tends to be high. On such days, surplus during the lunch break can become noticeable. In summer, generation also increases, but because demand for air conditioning and cooling rises, surplus may be suppressed. In winter, solar irradiance, snowfall, and hours of sunlight affect generation, and the overlap with demand changes depending on the factory’s heating and warming conditions.

Factory operating days and non-operating days should be considered separately. Even if there is little surplus during weekday lunch breaks, large surpluses may occur on weekends or during long vacations. Solar power will generate electricity when the weather is good even if the factory is closed. Checking the lunch break is a matter within weekday operations, but when considering surplus electricity it is important to look at weekends, public holidays, equipment inspection days, inventory days, and long vacations together.

When calculating the self-consumption rate, using finer time-interval data in addition to coarse hourly data, if possible, improves accuracy. During periods when demand changes on the order of an hour—such as lunch breaks—daily or monthly totals alone do not reveal the actual situation. Overlay generation and consumption using the same time intervals, and separate the portions where generation exceeds demand from the portions that can be consumed within demand.

Also confirm the impact on contracted power and demand management. Solar power generation can reduce purchased power during the daytime, but even if you only look at the period when demand falls during the lunch break, the impact on contracted power can be limited in factories where peak demand occurs at another time. Conversely, if the afternoon restart or ramp-up and the peak of air-conditioning load coincide with solar generation, it may contribute to demand reduction. By considering the lunch-time power decline and the afternoon rebound together, generation output calculations can be tied to more practical decision-making.

Improving Calculation Accuracy with Operational Improvements and Measurement Data

To reflect lunch-break operations in power generation calculations, it is important to use measurement data as well as on-site interviews. Even if staff perceive that "operations are almost stopped during lunch," actual power data may show that a certain load remains. Conversely, even when it is recognized that "operations continue during lunch," major loads may stop, reducing the site's ability to absorb solar power.

First, check the power data at the power receiving point. By examining how power consumption changes before, during, and after the lunch break, you can understand the overall load fluctuations of the plant. If possible, review multiple days of data on a day-by-day basis and classify them into sunny days, cloudy days, busy days, quiet days, and normal operating days. Data from only a single day may be affected by special operating or weather conditions.

Next, overlay power data by major equipment and by major production line, if available. Data from the service entrance alone does not reveal what the load consists of during lunch breaks. By separating the load that drops when lines stop, the load that remains as utilities, and the load that varies seasonally for air conditioning and ventilation, you can concretely identify self-consumption targets for solar power generation. Even if metering at the equipment level is difficult, having data at the distribution-board, building, or process level will improve calculation accuracy.

When using measurement data, it is also important to record the exact start and end times of the lunch break. In practice, work stoppages can begin before the break starts. Cleaning, setup changes, inspections, material replenishment, and worker movements can cause the load on production equipment to change gradually before and after the break. Rather than delimiting the lunch break by time alone, checking the sequence of equipment shutdowns and restarts makes it easier to reflect that in power generation calculations.

An operational-improvement perspective is also useful. If surplus power is expected during the lunch break, don’t just immediately reduce equipment capacity; also examine whether there are loads that can be shifted to the lunch period. For example, check whether, within a range that does not affect processes or quality, power use can be moved to daytime generation hours for activities such as thermal storage, cooling, charging, wastewater treatment, pre-cooling or pre-heating of HVAC, operation of cleaning equipment, and work in warehouse areas. However, forced operational changes can increase on-site burden and quality risks, so any measures need to be considered in light of actual process constraints.

Even if the load during the lunch break is large, if that load is caused by wasted standby power or leaks, it may be better to consider energy-saving improvements before covering it with solar power. In particular, compressed air, air conditioning, ventilation, lighting, and pumps are types of equipment where it is easy to separate what needs to run during the lunch break from what does not. The purpose of the generation calculation is not to make solar power look larger, but to develop an equipment plan that matches the actual conditions of the factory. If unnecessary loads are assumed as-is, when energy-saving improvements are made after installation the self-consumption rate and the projected surplus power will change.

To improve calculation accuracy, carefully check the conditions on the power generation side as well. Consider the orientation and tilt of the installation surface, the timing of shadow occurrence, the placement of rooftop equipment, soiling, temperature rise, losses from wiring and conversion, and shutdowns during inspections. Even if demand data for the lunch break is accurate, if the generation-side estimates are significantly off, the actual on-site self-consumption will also be inaccurate. Aligning both generation output and electricity consumption by time of day is the basis of calculations for factories.

It’s a good idea to consider not only pre-installation estimates but also post-installation verification. After installing a solar power system, being able to check how much is generated and how much is self-consumed during lunch breaks can lead to operational improvements. By analyzing days with large midday surpluses, days when self-consumption increases due to air-conditioning loads, and days when there is a noticeable benefit to the post-break ramp-up, you can inform decisions on further equipment additions, battery storage evaluation, and improvements to energy management.

Summary

When calculating solar power generation for factories, it is important to check not only the total generation but also the operational status during the lunch break. The lunch break often coincides with periods when solar output tends to be high. Therefore, how you handle power demand during the lunch break will affect decisions on self-consumption rate, surplus power, equipment capacity, and operational improvements.

Even if production lines stop during lunch breaks, loads such as air conditioning, ventilation, compressors, cooling, storage, lighting, offices, logistics, and wastewater treatment may remain. Conversely, in factories where major loads decline significantly, surplus electricity tends to occur during generation peaks. In either case, it is important not to judge based only on intuition or averages, but to overlay and check electricity consumption and generation by time of day.

In practice, rather than treating the lunch break uniformly as downtime, we organize loads into those that stop, those that remain, those that fluctuate, and those that restart after the break. On top of that, checking differences for sunny days and by season, operating and non-operating days, by production line, and by building allows for calculations that more closely reflect actual conditions. If a surplus is expected during the lunch break, it is effective to consider a combination of measures such as adjusting equipment capacity, revising operating hours, considering battery storage, shifting loads, and implementing energy efficiency improvements.

Calculating solar power generation is not simply the task of determining "how much can be generated." By overlaying it with a factory's electricity usage patterns, it becomes the process of identifying "at which times, for which loads, and to what extent it can be utilized." If operations during the lunch break are accounted for carefully, you can reduce discrepancies between expected and actual performance after installation and make it easier to design a self-consumption system plan suited to the site.

When considering solar power generation that utilizes a factory's roof and premises, it is important to review not only the expected generation but also operating patterns (including lunch breaks), equipment-specific loads, surplus power, and opportunities for operational improvement together. By calculating generation based on site conditions, you can avoid overly optimistic or pessimistic assumptions and make adoption decisions that reflect the factory's actual circumstances.