Six methods to calculate solar power generation for orientations other than south

Table of Contents

• Clarify the assumptions first when calculating for orientations other than south

• Method 1: Calculate by adjusting the generation for east-facing roofs

• Method 2: Calculate by adjusting the generation for west-facing roofs

• Method 3: Calculate by summing both east- and west-facing surfaces

• Method 4: Carefully calculate roofs that are skewed toward the north

• Method 5: Calculate by combining orientation and roof pitch

• Method 6: Calculate by adjusting for each month and time of day

• Situations where it remains effective even for non-south-facing orientations

• Common calculation mistakes in practice

• Summary

Clarify the assumptions first when calculating for orientations other than south-facing

When calculating solar power generation, discussions often proceed on the assumption of a south-facing roof. Indeed, in terms of the ease of assessing sun exposure conditions, south-facing is an easy baseline and convenient for initial estimates. However, in practice it is not always possible to secure sufficient system capacity using only south-facing surfaces. Whether for homes or commercial facilities, there are many cases where east-facing, west-facing, both east-and-west surfaces, and in some instances even surfaces oriented toward the north must be considered.

The important point here is not to lump all non-south-facing orientations together as disadvantaged. Indeed, because non-south-facing orientations receive different solar irradiation, their annual power generation will differ from the reference case. However, that difference is not simply a matter of better or worse; the evaluation depends on which times of day they tend to generate, how much area can be secured, and how well they match daytime demand. In other words, when calculating for non-south-facing orientations, it is important to consider not only the annual total but also the time-of-day generation profile and the ease of self-consumption.

When calculating solar power generation, the first thing to understand is the difference between system capacity and energy generation. System capacity is expressed in kW, while energy generation is expressed in kWh. For example, a 10 kW system indicates the size of the installation, and how much it will generate in a year is another matter. Actual annual energy production depends on where the system is located, which direction it faces, the tilt angle at which it is installed, how much shading it receives, and how much loss occurs. When calculating for orientations other than south-facing, direction and tilt have a particularly large impact among these factors.

Another common misconception when considering orientations other than south-facing is the idea that if they generate less power than south-facing panels, there is no point in considering them. In reality, however, if the south-facing area is too small to accommodate sufficient capacity, distributing panels between east- and west-facing surfaces can be advantageous in terms of total energy generation. Moreover, east-facing panels tend to contribute to morning generation and west-facing panels to afternoon generation, so when viewed in terms of daytime demand profiles they can actually be more useful. Therefore, calculating orientations other than south-facing is not merely to provide an alternative, but to optimize the entire installation.

This article organizes six approaches to calculating solar power generation on roofs and installation surfaces that are not south-facing. We will look in order at east-facing, west-facing, east-and-west (both sides), north-leaning, combinations with tilt, and month-by-month and time-of-day reinterpretations. By the time you finish reading, it should be fairly clear how to calculate and evaluate solar power generation even for orientations other than south-facing.

Method 1: Calculate by adjusting the energy output of an east-facing roof

The basic approach to calculating the power generation of an east-facing roof is to take the annual generation based on a south-facing orientation and apply a correction for the east-facing orientation. The idea is to first derive a baseline annual generation value from the system capacity, then apply an azimuth correction for the east-facing orientation and any necessary loss corrections. In formula terms, the annual generation is: system capacity × region-specific baseline generation × east-facing correction × loss correction.

For example, if you assume a 10 kW system yields 1,050 kWh per kW per year in a given area, the baseline annual generation for a south-facing orientation is 10,500 kWh. Applying an east-facing correction to this will reduce the annual generation slightly. How much to reduce depends on the region and the tilt, but in practice it's common to assume it will be slightly weaker than south-facing. The important thing is not to apply a blanket large reduction simply because it is east-facing, but to determine how far toward the east it is, what the tilt is, and what the surrounding conditions are.

A characteristic of east-facing roofs is that they tend to generate relatively more power in the morning. In other words, for buildings or facilities with high electricity demand from morning through late morning, east-facing roofs can be advantageous in terms of usability, even if their annual total is slightly lower than south-facing roofs. For example, in facilities that operate equipment from the morning, or in homes that use a lot of electricity for morning chores and water heating, east-facing generation is often practically meaningful.

However, because east-facing roofs tend to produce less power in the afternoon and later, facilities with high afternoon demand may see a weaker overlap with on-site consumption. Therefore, when evaluating east-facing roofs, you should consider not only the annual generation but also the time of day when that electricity is produced. Judging the value of a system solely by the simple annual kWh difference can easily overlook its actual usability.

Also, east-facing roofs tend to be more strongly affected by surrounding obstructions on the morning side. For example, if there are nearby buildings or trees to the east, the morning advantage can be reduced. Therefore, when applying an east-facing correction, it is important to check not only the orientation but also the obstruction conditions on the east side. In other words, calculations for east-facing roofs should not end with a correction from a south-facing baseline; in practice, it is better to consider the meaning of the time of day and the surrounding conditions as well.

Method 2 Calculate by adjusting the power generation of west-facing roofs

The calculation for west-facing roofs follows the same basic concept as for east-facing ones. First, you derive an initial annual generation value based on a south-facing reference, then apply a correction for west-facing orientation. Starting from the system capacity × the region-specific reference generation, you multiply by the west-facing correction and the loss correction. The calculation framework itself is simple, but it is important to understand the significance of the generation time periods specific to west-facing systems.

West-facing roofs tend to generate electricity from the afternoon through the evening. In other words, facilities with high afternoon demand, businesses that operate until the evening, and homes where usage increases after people return home offer a usability that cannot be measured by annual totals alone. Even if the annual total is slightly lower than that of south-facing roofs, generating during peak demand hours can be quite advantageous from the perspective of self-consumption.

On the other hand, west-facing roofs can be more affected by high afternoon temperatures. In summer, panel temperatures tend to rise in the afternoon, and even with strong solar irradiance the output may not increase as much as expected. Therefore, when calculating for west-facing roofs, taking seasonal temperature conditions into account as well as orientation corrections will improve accuracy. This point is especially easy to overlook when assessing overlap with summer afternoon demand.

Also, the effects of obstructions to the west should not be overlooked for west-facing roofs. For example, if there are tall buildings or trees on the west side, the afternoon generation period that would normally be advantageous can be reduced. In this case, not only the total annual generation but also the time-of-day value may decline. In other words, when applying a west-facing adjustment, it is better to consider the surrounding conditions on the west side together rather than applying a uniform correction based only on orientation.

In practice, east- and west-facing orientations are sometimes treated as the same under identical conditions. However, even when they are not south-facing, there is a difference: east-facing tends to favor the morning, while west-facing tends to favor the afternoon. Even if the total annual generation is similar, the compatibility with usage patterns differs. Therefore, when calculating for west-facing roofs, it is important to consider both annual kWh and the time-of-day value.

Method 3 Calculate by summing both the east and west sides

For roofs or layouts with both east- and west-facing surfaces, it is practical to calculate the east and west sides separately and then combine them at the end. This is particularly effective for projects where a single favorable south-facing surface cannot secure sufficient capacity. If east and west faces are treated together as a single condition, the differences in generation characteristics between morning and afternoon become obscured, making it harder to see the true usability of the system.

For example, suppose you have a system with 5kW facing east and 5kW facing west. In this case, rather than treating the entire system as 10kW and applying a single correction for orientations other than south, it is closer to reality to calculate the east-facing 5kW and the west-facing 5kW separately. Be mindful that the east side will need a morning-weighted correction and the west side an afternoon-weighted correction, and by also examining each side’s shading and tilt conditions, you can more easily interpret not only the total annual generation but also the time distribution of generation.

The advantage of using both east and west faces is that it is easier to increase the total installed capacity than with only the south side. It is not uncommon for a project to be limited to only 6 kW on the south alone but able to reach up to 10 kW by using east and west. In this case, even if the power generation efficiency per 1 kW is more favorable for the south-facing side, the east-west dual-sided setup can potentially produce a larger total generation. In other words, it is important to consider efficiency per unit capacity separately from the total capacity of the entire installation.

Also, having both east- and west-facing sides tends to contribute broadly to periods of demand. Because the east side generates in the morning and the west side in the afternoon, this can be advantageous for self-consumption in facilities with long daytime operating hours or in households with a wide range of usage times. Unlike south-facing orientations, generation is not overly concentrated around noon, and the resulting dispersion across time periods leads to greater ease of operation.

However, east–west orientations are often slightly less favorable than south-facing, so simply increasing the number of panels is not necessarily the solution. You should assess how much east–west dispersion is effective by considering the annual total, time-of-day value, shading conditions, and tilt conditions together. For east–west configurations, it is important to consider not only the total installed capacity but also the distribution of generation.

Method 4 Carefully calculate a roof oriented slightly toward the north

How to treat north-leaning roofs is the part that requires the greatest caution in calculations for orientations other than south. North-leaning roofs generally tend to receive less favorable solar radiation conditions, so expected power generation should be estimated conservatively. However, it would be premature to automatically rule them out from consideration. Decisions should be made after assessing on-site conditions, roof pitch, regional differences, and their role within the overall system configuration.

For example, there are cases where the south-facing and east/west-facing surfaces alone do not reach the required capacity, and by including surfaces that are somewhat north-facing you can secure a much larger overall system capacity. In this situation, adding north-facing surfaces may reduce the generation efficiency per panel, but you cannot tell whether it is truly disadvantageous without looking at how the annual total for the entire installation changes. In other words, a north-facing surface may be disadvantageous when viewed on its own, but can be meaningful in the context of overall optimization.

However, when using a surface that is somewhat north-facing, not only the orientation (azimuth) but also the tilt is extremely important. If the tilt is shallow, it may not be an extreme disadvantage, but if the tilt is steep the irradiance conditions become even more severe. Also, when you include the winter decrease in solar altitude and the effects of surrounding obstructions, power generation may be lower than expected. Therefore, when using a north-facing surface, it is safer to set the input coefficient quite conservatively.

In practice, rather than deciding across the board not to use north-facing roofs, it's better to first calculate the generation for each surface and assess how much those kWh matter to the overall system. If the generation is small, it's easier to decide to exclude them, and if they unexpectedly provide a measurable contribution you can explain why to keep them. The important thing is not to cut by intuition, but to actually run the numbers.

Also, north-leaning surfaces are evaluated differently depending on the timing of self-consumption and seasonal variations. Rather than looking only at the annual total, checking how much generation occurs in each month makes it easier to decide whether to use that surface. In other words, slightly north-facing roofs should be treated cautiously, but if you carefully quantify the numbers, they can be sufficiently useful as a basis for decision-making.

Method 5: Calculate by combining azimuth and roof pitch

To calculate orientations other than south-facing correctly, you need to consider not only the azimuth but also roof pitch and installation angle together. This is because even if two roofs face the same east, differences in pitch change the solar radiation they receive, and even for the same west-facing orientation, differences in installation angle change how the power output behaves. In other words, it is more practical to treat orientation and angle as a set.

For example, the annual generation adjustment for an east-facing roof with a gentle slope is not the same as for an east-facing roof with a steep slope. The same applies to west-facing roofs. Furthermore, when the surface is more north-facing, the influence of slope tends to become greater. In other words, when calculating systems that are not south-facing, you need to consider not only "which direction" but also "how much it is tilted," otherwise the input adjustment will be coarse.

In practice, these are often combined and treated as a single azimuth correction factor. The idea is to use south-facing as the baseline, treat east- and west-facing slightly more conservatively, and be even more cautious for north-leaning orientations. However, rather than applying this uniformly, it is better to reflect differences in roof pitch. In other words, if you want to improve accuracy for orientations other than south-facing, it is clearer to assess direction and pitch separately and then reconcile them at the end.

Also, this combination is linked to shadows. With a steep slope, the way shadows are received in winter tends to change more easily, and orientation alters the shadow conditions between morning and afternoon. In other words, orientation and roof pitch are factors that change not only how much solar radiation is received but also how shadows are cast. If you deal with this by intuition alone, explanations of seasonal and time-of-day differences become imprecise.

For beginners, it's helpful to remember that when calculating anything other than south-facing, you should always check not only the orientation but also how much it is tilted. For practitioners, simply knowing this makes it much easier to compare projects with different roof shapes.

Method 6: Recalculate by month and time of day

For installations that are not south-facing, it is very effective to reinterpret and assess not only the annual total but also monthly and time-of-day breakdowns. This is because non-south-facing systems tend to have skewed generation periods, and that skew directly affects self-consumption and operational value. East-facing systems tend to be stronger in the morning, west-facing systems in the afternoon, and east–west dual-facing systems tend to have more dispersed generation periods. Looking only at annual kWh can easily overlook the true value of the installation.

For example, east-facing installations may be slightly disadvantaged in annual total compared with south-facing ones, but they can be more suitable for facilities with large morning demand. Similarly, west-facing installations can be advantageous for facilities with large afternoon demand. If both east and west faces are used, they are likely to contribute to daytime demand over a wider range of hours, not just to the annual total. In other words, differences in orientation and tilt should be interpreted not only as differences in annual generation but also as differences in time-of-day value.

Also, when viewed on a monthly basis, winter power output and the effects of shading become clearer. North-facing or east- and west-facing surfaces can be strongly affected by the winter sun angle and shorter hours of sunlight. Conversely, in spring and autumn the differences are often not as pronounced. In other words, installations that are not south-facing tend to reflect actual performance more closely when examined month by month.

In practice, it becomes easier to make judgments if you break generation down into monthly and daily outputs and then overlay the demand time periods. For example, even if the annual totals for a south-facing 6kW and an east–west 10kW are similar, adding time-of-day value can make the east–west 10kW more advantageous. Conversely, if the business is primarily selling electricity, there are cases where you should place more emphasis on the annual total. In other words, translating results into monthly and time-of-day perspectives is not merely adding detail but a way to increase the evaluation axes for the equipment.

Using this method, instead of simply dismissing non-south-facing surfaces as "disadvantageous," you can evaluate them by "when and how much they will be useful." This is a crucial perspective for practitioners. Because orientation and tilt affect not only energy production but also usability, analyzing by month and time of day is the final step in properly evaluating non-south-facing surfaces.

Cases Where It Is Effective Even If Not South-Facing

We've looked at six methods so far, but in practice I think the main concern is whether there's any point in using orientations other than south-facing. To answer briefly: there definitely is. That's because evaluating power generation should not be decided solely by efficiency per unit capacity; it should also take into account total capacity, time of day, and compatibility with self-consumption.

For example, in a project where you can only install 6 kW on the good south-facing surface alone but can install 10 kW if you include the east and west surfaces, the annual total power generation may be more favorable with the east-west distribution. Furthermore, at facilities with long daytime demand, having east-facing panels effective in the morning and west-facing ones in the afternoon can raise the self-consumption rate. In other words, even if orientations other than south are somewhat disadvantageous when viewed individually, they can be worthwhile in the context of overall optimization.

Also, for residences, the value of east- or west-facing orientations changes depending on whether a household spends more time at home in the morning, does most of its housework and water heating during the daytime, or has heavier usage from the afternoon into the evening. For commercial properties, this difference becomes even clearer because operating hours often overlap significantly with air-conditioning loads. If you judge solely by a south-facing orientation, you can easily overlook the value of these usage patterns.

On the other hand, there are cases where it is unnecessary to force the use of non-south-facing orientations. If the site leans toward the north with a steep slope, has a lot of shading, and does not align with peak demand hours, it can reduce the overall efficiency of the installation. That is precisely why, instead of relying on intuition, it is important to quantify non-south-facing options and make decisions using the method shown here.

There are more situations in which orientations other than south-facing can be effective than you might think. However, their value is not easily seen from annual totals alone; you only understand it after looking at time of day, seasonal variations, and how they align with demand. That is precisely why it makes sense to calculate non-south-facing orientations.

Common calculation mistakes in practical work

One common mistake in practice when calculating for orientations other than south-facing is to use the same coefficients as for south-facing systems. If you assume that generation won't change much because the system capacity is the same, you overlook differences in azimuth and tilt. That might be acceptable for an initial rough estimate, but if you use it directly for proposals or decisions, discrepancies with actual site conditions are likely to appear later.

The next most common mistake is treating east-facing and west-facing systems the same. Although their annual totals can be similar, their value differs between morning and afternoon. Considering overlap with demand periods, east- and west-facing systems imply different usage patterns. Processing them together with the same adjustment tends to produce coarse evaluations of self-consumption and operational value.

Also, evaluating east–west dual-facing installations solely by total capacity is risky. If you treat a 10 kW east–west system the same as a 10 kW south-facing one, the characteristic of time-of-day dispersion disappears. Conversely, if you judge it too unfavorably just because it seems worse than south-facing, you will overlook advantages in total capacity and self-consumption. In other words, installations that are not south-facing must be assessed not only by total capacity but also by their configuration conditions.

Furthermore, treating the effects of shading uniformly is a common mistake. Even though there are differences such as morning shading on east-facing surfaces, afternoon shading on west-facing surfaces, and winter shading on north-leaning surfaces, if you group the entire installation under a single shading condition it becomes difficult to tell where and by how much it is being lost. The more you deal with surfaces other than south-facing ones, the greater the importance of evaluating each surface separately.

Ultimately, calculation errors for orientations other than south-facing often arise from aggregating differences in conditions too broadly. Simply separating and examining the differences in orientation, tilt, shading, and time of day a little will considerably increase confidence in the numbers. In practice, the thoroughness with which these distinctions are made determines the accuracy of the estimates.

Summary

As methods for calculating solar power generation for orientations other than south-facing, six approaches that are practical in the field are: adjusting calculations for east-facing roofs, adjusting calculations for west-facing roofs, summing the east- and west-facing sides, carefully calculating roofs that are more north-facing, combining orientation and roof pitch in the calculations, and calculating by month and time of day. What these all have in common is using the south-facing orientation as the baseline while applying corrections according to site conditions.

Using orientations other than south-facing is not simply about creating alternatives. It can lead to overall system optimization by making it easier to secure total installed capacity, by spreading generation across different times of day, and by better aligning with self-consumption. Conversely, forcing the use of areas with poor conditions can reduce efficiency, so it is important not to rely on intuition but to compare the numbers carefully.

Also, to properly evaluate orientations other than south, the annual total alone is not enough. It is far more useful in practice to look at monthly generation patterns, the contribution by time of day, and how shading occurs. In other words, the reason for including orientation and tilt in generation calculations is not simply to nudge kWh up or down a little, but because they change the very nature of the installation.

For that, it is indispensable to grasp the on-site positional relationships with high accuracy. If the roof surface orientation, the positions of surrounding obstacles, or elevation differences are ambiguous, corrections for orientations other than south-facing inevitably become coarse. As a means to capture on-site positional relationships with high precision, LRTK, an iPhone-mounted GNSS high-precision positioning device, is extremely effective. Because it makes it easier to accurately record candidate equipment locations and obstacle positions on site, it naturally facilitates generation estimates that take into account orientation, tilt, and shadows. If you want solar power generation figures for orientations other than south-facing to be truly usable numbers, accurately capturing on-site conditions with a method like LRTK is a major advantage.