Using logged energy consumption data to plan solar and backup power system

Designing solar and backup systems for commercial clients isn’t just about kW and kWh—it’s about understanding the nuances of load profiles, phase distribution, and power factor. Too often, I see EPCs rely on utility bills or rough estimates, leading to systems that fail to deliver. With over a decade of hands-on experience, I’ll show you how to use granular energy logging data to plan solar and backup power systems, to size inverters, batteries, and arrays with precision—ensuring your projects are both technically sound and financially compelling.

This is Part 2 of our energy measurement series. If you haven’t read Part 1 (logging setup), click here to catch up.

Ok, so now that we have done the logging and we have downloaded and analysed the data, lets apply it in designing backup and solar power systems.

Maximum Power

Ok, let us start of by determining the maximum power required. From the maximum power column, you can extract the peak value. This value can be used to size the backup inverter you need to be able to meet the peak power demand. Use the Excel MAX function. From the sample data we can get the following value:

Max Power Per Phase

In the previous step we got the max power for all three phases together. But when selecting a grid-tied inverter or backup-power inverter one needs to consider the max power of the individual phases.

Grid-tied Systems

For grid-tied inverters, this is particularly important in a situation where energy cannot be exported to the grid, in other words, where energy exported to the grid is curtailed. This is done by installing a smart meter at the grid connection point. As soon as the meter picks up that any energy is exported to the grid, it limits the output of the grid-tied inverter. The problem with unbalanced loads is that all three phases are limited to the phase with the lowest power consumption. This results in energy consumption that could have been covered by solar consumption not being covered and being lost. See the FAQ section below for a more detailed explanation.

Backup Power Systems

For backup-power inverters, the inverter needs to be able to supply the maximum power required on each individual phase. If the inverter and battery are unable to supply the maximum power required, then it will trip / switch off.

Getting the Max Power Per Phase

The following image shows how to determine the maximum power per phase. First find the date and time where the peak load occurred. From the previous image we can see that it was 2024/03/18. By expanding that day into hours, we can see exactly at what time this occurred and get the actual values.

Interesting to note is that the peak occurred right after a power failure, where all the machines started up at once after the power failure causing the spike. Staggering the startup of machines could be an opportunity to reduce this peak load.

In a graph format it will look like this:

From this image we can see that the peak power consumption occurred at 10. And the power consumption of each phase was:

1. 77,218 W
2. 68,344 W
3. 44,683 W
4. Total: 178,212 W or 178.2 kW

Power Factor

One also needs to consider the power factor. Inverters need to be sized according to kVA requirements and not kW requirements.

See FAQ section below for detailed explanation of how kVA relates to kW and power factor

Looking at the power factor for this installation, it looks like the image below. From this we can see that the power factor is close to 1 and will not have a big impact on the Apparent power.

Peak Apparent Power in kVA

If we have a look at the Reactive power, we see the table below.

From here we can see the maximum apparent power per phase and in total. You can see there is not a big difference between Active power and Apparent power because the power factor is close to one.

Active Power	Apparent Power
1. 77,218 W 2. 68,344 W 3. 44,683 W 4. Total: 178,212 W or 178.2 kW	1. 77,409 VA 2. 68,454 VA 3. 44,831 VA 4. Total: 178,212 VA or 178.2 kVA

Phase Imbalances

From this example once can see that phase one uses almost double the power of phase three. There is an opportunity to balance the phases and then the peak demand per phase could be reduced to 70 kVA instead of 80 kVA. If planning a backup power system, one would need at least three 80 kVA inverters to meet the maximum demand.

Average Consumption Per Day

Let’s get the average consumption of energy per day. Once we have this, we will be able to more accurately size the amount of energy required to be generated from the solar panels and the amount of energy to be stored by the batteries.

From the following table we can see the total energy used per day over a week.

Using the Excel Average function for the total energy consumption per day for a week we get the value of 1,206,167 Wh converted to kilo Watt hours it is 1206 kWh.

Sizing the Solar Array

Using this figure calculated above you can size your solar system to generate all of this energy or a certain percentage of it.

All the factors that affect solar energy production should be taken into account, such as orientation, inclination, solar radiation and losses.

All these factors can be entered into one of the various solar simulation software tools available to calculate the size of solar array needed to generate the energy required.

Average Consumption Per Day and Night

One can go into even more detail and get the average day time and nighttime energy consumption. This will enable you to more accurately size the solar array and battery bank.

To get the day and nighttime energy consumption for a single day one can group the hours by daytime and nighttime. In this example daytime is from 8 am to 5 pm.

We can also get the average day and nighttime consumption over a period of time for example one week. We can have a look at the follow table showing a week’s worth of data and showing the average energy consumption per hour and per day. One can manually add the average energy consumption column as in Excel that is not possible using the pivot table as far as I know.

Then one can manually calculate the total average daytime and night-time consumption.

We get the following figures in this example:

Total average daytime consumption: 824 kWh

Total average night-time consumption: 1,286 kWh

Sizing the Battery

There are two factors that consider when sizing the battery. One is how much power should be able to provide and the second is how much energy would you like to be able to store.

From our maximum Apparent power calculations, we determined that the power requirements is 70 kVA.

From our average daytime and nighttime calculations we determined that if we would like to cover all energy consumption during the night we would need at least: 824 kWh

Conclusion

Designing solar and backup systems for commercial clients is a technical challenge—but with the right data and tools, you can deliver projects that truly perform. By leveraging granular energy logging, phase analysis, and power factor insights, you’ll avoid costly mistakes and engineer solutions that meet your clients’ real-world needs.

FAQ Section

How does a grid-tied inverter on a three-phase load work if it is not allowed to export any energy to the grid?

A grid-tied inverter on a three-phase load with zero export operates by continuously monitoring power consumption and dynamically adjusting its output to match the load demand without feeding excess power back to the grid.

Here’s how it works:

Core Operating Principle The inverter uses real-time power monitoring to track the instantaneous power consumption of each phase. It then adjusts its AC output to supply exactly what the load requires, keeping the net power flow at the grid connection point at zero or slightly importing.

Key Components and Control

• Current transformers (CTs) measure the current flow at the grid connection point on each phase
• Power monitoring system calculates real-time power consumption
• Control algorithm continuously adjusts inverter output based on load demand
• Phase-locked loop (PLL) synchronizes the inverter output with grid frequency and phase

Three-Phase Operation Each phase operates independently with its own control loop. Some inverters can supply different amounts of power to each phase based on the individual phase loads. This is crucial since three-phase loads are often unbalanced in residential and commercial applications.

Dynamic Response When load increases suddenly (like a motor starting), the inverter ramps up output. When load decreases, it reduces output proportionally. The response time is typically within milliseconds to prevent any reverse power flow.

Safety and Compliance The system includes multiple safeguards:

• Anti-islanding protection disconnects the inverter if grid power fails
• Overvoltage/undervoltage protection
• Frequency monitoring
• Ground fault protection

This configuration maximizes solar self-consumption while maintaining grid compliance and eliminating export penalties or restrictions that some utilities impose.

In a three-phase grid tied system with export limitation, what happens if the consumption on the phases is not balanced and one phase uses a lot less power than other phases?

In a three-phase system with unbalanced loads, the inverter’s zero-export control becomes more complex because each phase must be managed independently to prevent any phase from exporting power to the grid.

Per-Phase Control The inverter monitors and controls each phase separately. If Phase A has high consumption, Phase B has medium consumption, and Phase C has very low consumption, the inverter will:

• Output maximum available power to Phase A (up to load demand)
• Output moderate power to Phase B (matching its load)
• Output minimal power to Phase C (matching its low load)

Underutilized Solar Capacity When one phase has very low consumption, the solar energy that could have been used on that phase cannot simply be shifted to the other phases in a standard grid-tied system. This creates several scenarios:

1. Wasted Solar Potential: The inverter may not be able to use all available DC power from the solar array if the total three-phase load is significantly lower than solar generation capacity.
2. Power Limiting: The inverter will limit its overall output to prevent any phase from exporting, even if the other phases could use more power.

Mitigation Strategies Several approaches can help balance the system:

• Load balancing: Redistribute electrical loads across phases during installation
• Smart switching: Use contactors to automatically switch single-phase loads between phases based on consumption patterns
• Three-phase inverters with phase balancing: Some advanced inverters can internally balance phases to some degree
• Consumption monitoring: Install smart switches or timers to activate loads on underutilized phases when solar production is high

Net Metering Considerations In systems with net metering, the utility typically measures net consumption across all three phases combined, which helps utilize more solar production. However, with zero-export requirements, this flexibility is lost, making load balancing more critical for maximizing solar utilization.

The key is designing the system with balanced loads from the start and implementing smart load management where possible.

What is the relationship between kVA, kW and power factor?

In electrical engineering, kVA, kW, and power factor are fundamentally related through the power triangle concept.

kVA (Kilovolt-Amperes) represents apparent power – the total power supplied to a circuit. It’s calculated as voltage × current and represents the maximum power that could theoretically be delivered.

kW (Kilowatts) represents real power – the actual power consumed by the load and converted to useful work (like mechanical energy, heat, or light).

Power Factor is the ratio of real power to apparent power, expressing how efficiently the electrical power is being used.

The mathematical relationship is:

• kW = kVA × Power Factor
• Power Factor = kW / kVA

Power factor ranges from 0 to 1 (or 0% to 100%). A power factor of 1 means all the apparent power is being used as real power – this is ideal but rarely achieved in practice.

The third component of this relationship is kVAR (Kilovolt-Amperes Reactive), which represents reactive power. These three form a right triangle where:

• kVA² = kW² + kVAR²

Reactive power doesn’t perform useful work but is necessary for magnetizing inductors and capacitors in AC circuits. Motors, transformers, and fluorescent lights typically have power factors less than 1, meaning they draw more apparent power than they convert to useful work.

This relationship is crucial for electrical system design, as utilities must size their equipment based on kVA (apparent power) even though customers primarily pay for kW (real power).

• Power Factor = Active power (kW) / Apparent power (kVA)
• Example Theta angles:
  • Θ = 45º → Cos(Θ) = Power Factor = 0.71
  • Θ = 30º → Cos(Θ) = Power Factor = 0.87
  • Θ = 20º → Cos(Θ) = Power Factor = 0.94
  • Θ = 10º → Cos(Θ) = Power Factor = 0.98
  • Θ = 0º → Cos(Θ) = Power Factor = 1.0
• Please note: Active power = Real power = True power
  • They are synonyms and used interchangeably.
• Pythagoras
  • Q = √(S² – P²)

Glossary of Key Terms

• Load Profile: A representation of the variation in electrical load over time. It shows how much electricity is used at different times of the day or week.
• Phase Distribution: The allocation of electrical load across different phases in a multi-phase power system. Proper phase distribution is crucial for balanced power delivery.
• Power Factor: A measure of how effectively electrical power is being used. It is the ratio of real power (kW) to apparent power (kVA) and indicates the efficiency of power usage.
• Inverter: A device that converts direct current (DC) from solar panels or batteries into alternating current (AC) for use in homes and businesses.
• Battery Storage: Systems used to store energy for later use, typically to provide backup power or to store excess solar energy for use during non-sunny periods.
• Energy Logging: The process of recording energy consumption data over time using devices like energy loggers. This data is used for analysis and system design.
• kW (Kilowatt): A unit of power equal to 1,000 watts. It measures the rate of energy consumption or production.
• kVA (Kilovolt-Amperes): A unit of apparent power in an electrical circuit, equal to 1,000 volt-amperes. It represents the total power used by a system, including both real and reactive power.
• Reactive Power: The portion of electricity that establishes and sustains the electric and magnetic fields of AC equipment. It is measured in kVAR (kilovolt-amperes reactive).
• Apparent Power: The combination of real power and reactive power, measured in kVA. It represents the total power flow in an electrical system.
• Grid-Tied System: A solar power system that is connected to the electrical grid. It allows for the export of excess solar energy to the grid and import of electricity when solar production is insufficient.
• Zero Export: A configuration in grid-tied systems where no excess solar energy is exported to the grid. The system is designed to match the load demand exactly.
• Phase Imbalance: A condition where the electrical load is not evenly distributed across all phases in a multi-phase system, leading to inefficiencies and potential equipment issues.
• Net Metering: A billing mechanism that credits solar energy system owners for the electricity they add to the grid. It allows for the offsetting of electricity costs with solar production.
• EU CSRD (Corporate Sustainability Reporting Directive): A European Union directive that requires companies to report on their sustainability practices, including energy consumption and carbon emissions.