MODULE 5 — Energy System Design & Sizing

What is a 'Load'?

A load is anything that consumes electrical power.

Examples include:

• lights
• TVs
• Wi-Fi routers
• fridges
• computers
• kettles
• pumps
• air conditioners
• machinery

Every load uses a certain amount of power, usually measured in:

• watts (W)
• or kilowatts (kW)

Why Load Assessment is So Important

The battery and inverter can only supply a limited amount of power and energy.

If the system is not designed around the actual loads:

• the inverter may overload
• the batteries may drain too quickly
• runtime may be much shorter than expected
• equipment may trip or shut down

A proper load assessment ensures the system is designed realistically for the customer's needs.

Understanding Two Important Things

A load assessment looks at:

These are two different things.

1. How much power is needed at one time
2. How long the loads must run

1. Power Requirement (Instant Demand)

This is the total power the system must supply at a specific moment.

2. Energy Requirement (Runtime)

This determines how long the system must supply power.

Think of it like a fuel tank:

• power = how hard the engine works
• energy capacity = how long the fuel lasts

Critical Loads vs Non-Essential Loads

Not everything needs backup power. A good load assessment helps identify:

• critical loads
• non-essential loads

Why Accurate Information Matters

Customers often underestimate their usage.

For example:

• a kettle may use 2000W+
• a hairdryer may use 1800W+
• an air fryer may use 1500W+

Just one of these appliances can overload a small backup system.

This is why installers must properly assess the real expected loads.

Future Expansion Must Also Be Considered

Good system design also considers:

• future appliances
• additional batteries
• solar expansion
• changing customer needs

A system designed too tightly may become limiting later.

Understanding the Difference Between kW and kWh

This is one of the most important concepts in battery system design.

KW(Kilowatts) =Power

This refers to:

• how much power is being used at a specific moment

Think of it like:

• the speed of a vehicle
• or how hard the engine is working

kWh (Kilowatt-hours) = Energy

This refers to:

• how much energy is used over time

Think of it like:

• how much fuel the vehicle uses during the trip

Simple Formula

Backup energy requirement is calculated as:

Power (kW) × Time (Hours) = Energy Required (kWh)

Why Backup Time Requirements Are So Important

Two customers may have exactly the same loads but completely different battery requirements depending on how long they want backup power.

Customer A – Required backup time of 2 hours for a 1kW load

Customer B – Required backup time of 10 hours for a 1kW load

Even though the load is identical:

• Customer B needs a much larger battery bank

If the battery is too small:

• runtime will be shorter than expected
• the battery may discharge too quickly
• the system may shut down prematurely
• battery lifespan may reduce due to excessive cycling

If the battery is too large:

• system cost increases unnecessarily
• charging times may become longer
• the customer may pay for unused capacity

The goal is therefore to select a battery size that is practical, efficient, reliable, and suitable for the customer’s actual needs.

Battery Capacity is Measured in kWh

Battery size is usually measured in:

• kilowatt-hours (kWh)

This represents:

• how much energy the battery can store

The larger the kWh rating:

• the longer the system can run the loads

Why Additional Capacity is Important

Real-world systems must account for:

• inverter losses
• reserve capacity
• surge loads
• battery aging
• temperature effects
• future expansion
• depth of discharge limits

This means installers usually recommend a slightly larger battery than the minimum calculated requirement.

Although the calculation shows:

• approximately 2kWh required

An installer may recommend:

• a 5kWh battery

Why?

Because the larger battery:

• reduces battery stress
• improves runtime stability
• allows for future load growth
• provides reserve capacity
• improves battery lifespan

Matching Battery Size to Inverter Size

The battery must also be suitable for the inverter’s power requirements.

A very large inverter connected to a very small battery may:

• overload the battery
• exceed discharge limits
• trigger BMS protection events

The battery and inverter must therefore be correctly matched.

Understanding Inverter Size

Inverter size is usually measured in:

• watts (W)
• or kilowatts (kW)

This refers to:

• how much power the inverter can supply at a specific moment

Unlike battery sizing, which is mainly based on runtime and energy storage (kWh), inverter sizing is mainly based on instantaneous power demand.

Continuous Power vs Surge Power

Most inverters have:

• a continuous power rating
• and a surge rating

Matching the Inverter to the Battery

The inverter and battery must work together correctly.

A very large inverter connected to a very small battery may:

• exceed the battery's discharge limits
• trigger BMS protection events
• cause voltage drops
• reduce battery lifespan

The battery must be capable of supplying the current the inverter requires.

Matching the Inverter to the System Voltage

The inverter must also match the battery bank voltage.

For example:

• a 48V inverter requires a 48V battery bank
• a 24V inverter requires a 24V battery bank

Incorrect voltage matching can damage equipment or prevent operation entirely.

Communication Compatibility

Modern lithium systems also rely heavily on communication between:

• the inverter
• and the battery BMS

Compatible communication improves:

• charging accuracy
• protection
• SOC accuracy
• system stability

Proper inverter selection therefore includes:

• electrical compatibility
• communication compatibility
• manufacturer support compatibility

Installer Consideration

Customers often focus only on: (How many appliances can the inverter run?)

But installers must also consider:

• surge loads
• battery capability
• runtime expectations
• future growth
• charging requirements
• system efficiency

Understanding Solar Panel Power

Solar panels are usually rated in:

• watts (W)
• or kilowatts peak (kWp)

This rating indicates the maximum power the panel can produce under ideal test conditions.

Solar Array Sizing Starts with Energy Usage

The first step in PV sizing is understanding:

• how much energy the system uses per day

This is usually measured in:

• kWh per day

Imagine the system uses 5kWh per day.

The solar array must generate enough energy to:

• run the daytime loads
• recharge the battery for nighttime use
• compensate for system losses

In practice, the solar array must therefore generate more than the exact daily usage figure.

Understanding Peak Sun Hours

Solar panels do not produce full power all day long.

The amount of usable sunlight is often referred to as:

• Peak Sun Hours (PSH)

This represents the average number of hours per day during which the solar panels produce near-rated output.

Losses Must Be Considered

Solar systems experience losses from:

• inverter efficiency
• temperature
• cable losses
• panel mismatch
• dirt and dust
• charging losses
• shading
• weather variation

Because of this, installers usually oversize the array slightly to ensure reliable performance.

Panel Orientation and Tilt Matter

Solar production is also heavily affected by:

• roof direction
• tilt angle
• shading

Poor panel placement can significantly reduce performance.

For example: Partial shading on one panel may affect the output of an entire string.

This is why proper site assessment is important.

Series and Parallel PV Design

Solar panels can also be connected in:

• series
• parallel

Similar to batteries.

Series PV Connection = Increases voltage

Parallel PV Connection = Increases current

The PV array must be designed within:

• inverter limits
• MPPT voltage ranges
• current limits
• safety requirements

Matching the PV Array to the Inverter

The solar array must remain within the inverter's:

• maximum PV voltage
• maximum PV current
• MPPT operating range

Incorrect PV sizing can:

• damage equipment
• prevent startup
• reduce efficiency
• trigger faults

This is why solar design calculations are extremely important.

Why Compatibility Matters

Modern lithium batteries are intelligent systems controlled by a BMS (Battery Management System). The inverter and BMS constantly exchange information to ensure the battery operates safely and efficiently.

The inverter relies on the battery BMS to provide information such as:

• battery state of charge (SOC)
• charge and discharge limits
• battery temperature
• alarms and warnings
• battery protection status

If the inverter cannot properly communicate with the BMS:

• charging may become inaccurate
• battery protection may reduce
• runtime estimates may be incorrect
• nuisance faults may occur
• battery lifespan may shorten

Voltage Compatibility

The first requirement is ensuring the inverter and battery operate at the same system voltage.

Communication Compatibility

Modern lithium systems rely heavily on communication between:

• the inverter
• and the battery BMS

This communication usually occurs through:

• CAN Bus
• RS485
• Modbus

The inverter and battery must support compatible communication protocols and correct communication settings.

Charge and Discharge Current Compatibility

The battery must also be capable of safely supplying the current the inverter requires.

A large inverter connected to a very small battery may demand excessive current.

This can result in:

• BMS overcurrent protection events
• voltage drops
• system shutdowns
• excessive battery stress

The inverter and battery must therefore be sized appropriately together.

Charging Parameter Compatibility

The inverter charging settings must match the battery manufacturer's specifications, including:

• charge voltage
• float voltage
• charge current
• low-voltage cut-off
• temperature behaviour

Incorrect charging settings can:

• reduce battery lifespan
• cause poor SOC accuracy
• trigger BMS faults
• prevent proper balancing

Parallel Battery Compatibility

If multiple batteries are installed in parallel:

• battery firmware
• communication settings
• battery versions
• addressing

must also be compatible.

Incorrect parallel configuration may lead to:

• communication faults
• uneven load sharing
• unstable operation
• protection events

What is Voltage Drop?

Voltage drop is the reduction in voltage that occurs as electricity travels through a cable.

As current flows through a cable:

• the cable creates resistance
• resistance causes energy loss
• some voltage is lost along the cable

The longer the cable or the higher the current:

• the greater the voltage drop becomes.

Effects of Excessive Voltage Drop

Excessive voltage drop can cause:

• poor inverter performance
• incorrect charging behaviour
• low battery voltage alarms
• reduced efficiency
• excessive heat
• unstable system operation

Special Considerations for Battery Systems

In battery systems, this becomes especially important because battery systems often operate at:

These conditions make proper cable sizing critical for reliable operation.

• high current
• relatively low voltage

Batteries Perform Best Within Safe Operating Conditions

Lithium batteries are designed to operate within specific limits.

The BMS continuously protects the battery against:

• overcharging
• deep discharge
• excessive current
• overheating
• unsafe operating conditions

However, good system design aims to avoid these conditions altogether rather than relying on the BMS to constantly intervene.

Avoid Constant High Stress

Batteries last longer when they are not constantly pushed to their limits.

A system designed too tightly may cause:

• deep daily discharges
• high charging current
• excessive discharge current
• frequent protection events
• increased operating temperatures

Over time, this increases battery wear and reduces lifespan.

Depth of Discharge (DoD) Affects Lifespan

Although lithium batteries support deep discharge, constantly operating at maximum depth of discharge increases stress on the cells.

For example:

• regularly cycling a battery from 100% to 0% creates more stress than:
• cycling between 80% and 30%

Good system design considers:

• realistic usable capacity
• reserve margins
• reduced battery stress

to improve long-term performance.

Temperature Has a Major Impact

Battery temperature significantly affects:

• performance
• charging behaviour
• lifespan

Excessive heat is one of the biggest causes of accelerated battery aging.

Poor ventilation or incorrect installation locations can lead to:

• overheating
• reduced efficiency
• shortened battery life
• increased protection events

This is why batteries should always be installed in:

• well-ventilated areas
• clean environments
• suitable operating temperatures

Proper Charging is Critical

Correct charging settings are essential for battery health.

Incorrect:

• charge voltage
• charge current
• float settings
• inverter configuration

can:

• reduce battery lifespan
• prevent balancing
• cause inaccurate SOC
• trigger BMS alarms

Good system design always follows manufacturer charging specifications.

Correct Solar Sizing Improves Battery Health

A solar array that is too small may:

• leave batteries partially charged for long periods
• prevent balancing
• reduce SOC accuracy
• increase cycling stress

Proper solar sizing helps ensure:

• healthy charging behaviour
• regular full charge opportunities
• stable system operation

Cable Quality and Voltage Stability Matter

Undersized cables and poor terminations can cause:

• voltage drops
• unstable charging
• excessive heat
• inverter faults

Stable voltage and good power delivery improve:

• battery performance
• charging accuracy
• system efficiency

Communication Improves Long-Term Performance

Modern lithium systems rely heavily on proper communication between:

• the inverter
• and the battery BMS

Good communication allows:

• smarter charging
• improved protection
• better balancing
• accurate SOC calculations
• stable system control

Poor communication can reduce overall system performance and battery lifespan.

Example 1 – Small Home Backup System

This example illustrates a basic residential backup system designed to keep essential household appliances operating during power outages. The emphasis is on simplicity, reliability and cost-

effective backup for everyday loads.

As you study this design, notice how the battery capacity, inverter size and solar array are matched to relatively small continuous loads and longer runtime requirements.

Example 2 – Medium Home Backup System

This example demonstrates a larger residential installation supporting additional household appliances and higher daily energy demand. Compared with the previous example, larger battery

storage and inverter capacity are required while maintaining efficient system operation.

Observe how increasing the number of appliances affects every major design decision.

Example 3 – Large Home Backup System

This example shows a whole-home backup solution designed for larger properties with higher energy consumption. Multiple high-power appliances and larger surge loads require careful sizing

of the inverter, battery bank and solar array.

Notice how the design balances high performance with future scalability and long-term system reliability.

Example 4 – Office Backup System

Business continuity is the primary objective of this design. Critical office equipment such as computers, servers, networking equipment, lighting and communication systems are prioritised to

minimise downtime during power outages.

Consider how commercial installations often focus on protecting productivity rather than powering every electrical load.

Example 5 – Restaurant Backup System

Restaurants place greater demands on backup systems because refrigeration, food preparation equipment, lighting and point-of-sale systems are business-critical. Larger continuous and surge

loads require careful component selection and appropriate battery capacity.

Observe how maintaining operations and protecting refrigerated stock become key design priorities.

Example 6 – Commercial BESS Solution

This example introduces a commercial Battery Energy Storage System (BESS) designed for industrial and utility-scale applications. Unlike residential backup systems, these installations may

provide peak shaving, demand management, renewable energy integration, grid support and energy arbitrage in addition to backup power.

Notice how the scale of the system changes dramatically, but the same fundamental design principles still apply—correct sizing, system protection, efficiency, reliability and future scalability.

Key Design Lessons

Although these systems vary greatly in size and application, they all follow the same design process:

1.Understand the customer's operational requirements.

2.Identify the critical loads.

3.Calculate continuous and surge power demand.

4.Determine the required backup duration.

5.Size the battery bank to provide sufficient usable energy.

6.Select an appropriately rated inverter.

7.Size the solar array to recharge the batteries and support ongoing energy needs.

8.Design for safety, efficiency, reliability and future expansion.

The difference between a small residential backup system and a large commercial BESS is not the design methodology—it is the scale of the application. Professional installers apply the same

engineering principles to every installation, regardless of size.

Remember: Successful system design is not about installing the biggest battery or the largest inverter. It is about designing a balanced system where every component is correctly matched to

the customer's energy requirements, operational goals and future needs.