Correct solar sizing is one of the most important parts of designing a reliable solar power system. Installing solar panels without calculating the electrical loads can result in a system that produces too little energy, while installing an unnecessarily large system can increase the project cost without providing proportional benefits.

A complete solar calculation should consider electricity consumption, appliance power, operating hours, solar availability, system losses, inverter efficiency, battery capacity, battery chemistry, required backup duration and future electricity demand.

For solar installation, system sizing and professional electrical solutions in Kenya, contact 0723763173.

This guide explains how to calculate solar panel and battery requirements step by step.

START WITH THE LOADS

The first step is identifying what the solar system needs to power.

Typical residential loads include:

• Television
• Refrigerator
• Freezer
• Lights
• Wi-Fi router
• Computers
• Phones
• Washing machine
• Microwave
• Blender
• Water pump
• Cooker
• Electric kettle
• Iron
• Air conditioner
• Water heater

Commercial loads can include:

• Computers
• Printers
• Lighting
• Refrigeration
• Pumps
• Motors
• Compressors
• POS systems
• Servers
• CCTV
• Air conditioning
• Production machinery

The solar system should be calculated around the actual loads rather than an assumed number of panels.

POWER VS ENERGY

One of the most important concepts in solar sizing is the difference between power and energy.

Power is normally measured in:

WATTS (W) or KILOWATTS (kW).

Energy is normally measured in:

WATT-HOURS (Wh) or KILOWATT-HOURS (kWh).

A 1,000-watt appliance operating for one hour consumes approximately:

1 kWh

The basic calculation is:

ENERGY = POWER × TIME

SIMPLE EXAMPLE

Suppose a television consumes 100 watts and operates for five hours per day.

The calculation is:

100 W × 5 hours = 500 Wh

Therefore:

500 Wh = 0.5 kWh per day.

If the television operates seven hours:

100 W × 7 hours = 700 Wh

That equals:

0.7 kWh per day.

This calculation is repeated for every important appliance.

CREATE A LOAD TABLE

A load table makes solar sizing easier.

For example:

Appliance	Power	Hours/day	Daily Energy
TV	100 W	5	500 Wh
Refrigerator	150 W average	10	1,500 Wh
Lighting	200 W	5	1,000 Wh
Router	15 W	12	180 Wh
Laptop	70 W	6	420 Wh
Water pump	750 W	2	1,500 Wh

The total daily energy is obtained by adding the individual consumption values.

ACTUAL APPLIANCE POWER

Do not always rely on estimates.

Whenever possible, check the appliance rating label.

The label may show:

• Voltage
• Frequency
• Current
• Power
• Rated input
• Energy consumption

For some appliances, the rated power is not equal to average daily consumption.

Refrigerators, air conditioners and pumps can cycle.

REFRIGERATOR CALCULATION

A refrigerator may have a compressor rated at approximately 150 watts, but it does not necessarily consume 150 watts continuously for 24 hours.

The compressor switches on and off according to temperature.

Therefore, actual daily energy consumption may be lower than:

150 W × 24 hours.

Using actual measured energy consumption can produce a more accurate system design.

MEASURING ENERGY CONSUMPTION

For an existing property, energy monitoring can improve accuracy.

Possible methods include:

• Smart energy meters
• Plug-in energy meters
• Clamp meters
• Inverter monitoring
• Utility bills
• Dedicated energy monitoring equipment

A professional installer can use measured data to understand the property's load profile.

USING THE ELECTRICITY BILL

Electricity bills can provide useful information about total consumption.

However, the bill usually shows total energy purchased from the grid rather than precisely identifying which appliance consumed the energy.

It is therefore useful for establishing an overall consumption baseline.

If a property consumes 300 kWh per month:

300 ÷ 30 = approximately 10 kWh per day.

That gives a starting point for solar sizing.

DAILY ENERGY REQUIREMENT

Suppose a household uses:

10 kWh per day

The solar system must generate enough energy to cover approximately 10 kWh of useful daily consumption, plus losses.

It is therefore not enough to install panels whose theoretical output is exactly 10 kWh.

The system experiences losses.

SYSTEM LOSSES

Solar systems have several types of losses.

These can include:

• Panel temperature losses
• Dust
• Cable losses
• Inverter losses
• Battery losses
• Connector losses
• Mismatch losses
• Shading
• Conversion losses

A professional design therefore includes an appropriate system-loss allowance.

PEAK SUN HOURS

Solar panel output depends on available solar radiation.

Solar designers often use the concept of equivalent peak sun hours.

A location might be estimated using a particular number of useful peak-sun hours for preliminary calculations.

The actual result changes according to:

• Location
• Weather
• Season
• Orientation
• Tilt
• Shading
• Panel temperature

For Kenya, solar resource is generally favorable, but the exact value should be determined for the installation location and design assumptions.

BASIC PANEL CALCULATION

A simplified calculation is:

Required Solar Power = Daily Energy ÷ Peak Sun Hours

Then system losses are incorporated.

For example, if the property needs 10 kWh per day and the design uses 5 equivalent peak-sun hours:

10 kWh ÷ 5 = 2 kW

That is the theoretical panel capacity before allowing for losses and design margins.

A practical design may therefore require more than 2 kW of panels.

WHY PANEL CAPACITY IS NOT THE SAME AS DAILY OUTPUT

A 2 kW solar array does not necessarily produce exactly 2 kWh per day.

The 2 kW figure represents the rated instantaneous capacity under specified test conditions.

Daily energy production depends on sunlight.

For example, the same 2 kW array may produce different amounts of energy on:

• Clear days
• Cloudy days
• Rainy days
• Hot days
• Dusty days

Therefore, panel sizing must be based on expected energy production rather than simply panel nameplate capacity.

PANEL COUNT

Once the required panel capacity is estimated, the number of panels can be calculated.

For example, if the design requires approximately 5 kW of panels and each panel is rated at 500 W:

5,000 W ÷ 500 W = 10 panels.

Therefore, approximately ten 500-watt panels would provide 5 kW of nominal panel capacity.

The actual final configuration must also satisfy inverter voltage and current limits.

PANEL STRING CALCULATION

The number of panels cannot be determined only from total wattage.

The installer must also determine how many panels should be connected in series.

For example, suppose a panel has an operating voltage of approximately 40 V.

Five panels in series would have an approximate operating voltage of:

40 × 5 = 200 V.

The actual open-circuit voltage must also be considered.

MAXIMUM VOLTAGE

Solar panel voltage increases under certain conditions and must remain within the inverter's maximum DC input voltage.

The designer should consider the panel's open-circuit voltage and temperature effects.

This is especially important for systems using high-voltage MPPT inputs.

MPPT RANGE

The inverter has an MPPT operating range.

The solar string should operate within that range under expected conditions.

If the string voltage is too low, the inverter may not track the array effectively.

If the voltage is too high, the equipment may be damaged or shut down.

PARALLEL STRINGS

If more solar capacity is needed, multiple strings can sometimes be connected in parallel.

This increases current.

The inverter's maximum input current must be considered.

The designer must therefore check:

• Maximum DC voltage
• Maximum DC current
• MPPT voltage range
• Number of MPPT inputs
• Maximum recommended PV power

BATTERY SIZING

Battery sizing is different from panel sizing.

The battery is primarily sized according to how much energy must be stored and delivered when solar production is insufficient.

Important factors include:

• Daily energy requirement
• Nighttime energy consumption
• Required backup time
• Battery voltage
• Battery chemistry
• Depth of discharge
• Battery efficiency
• Inverter efficiency
• Maximum discharge current

BASIC BATTERY CALCULATION

A simplified calculation is:

Battery Energy Required = Load Energy × Backup Hours

But the final battery size must also consider usable capacity and losses.

For example, suppose a customer wants a 2 kW load to operate for four hours.

The theoretical energy requirement is:

2 kW × 4 hours = 8 kWh.

The battery therefore needs more than 8 kWh of nominal capacity if only part of its capacity is intended to be routinely used.

DEPTH OF DISCHARGE

Suppose a battery has a nominal capacity of 10 kWh.

If the design allows 80% usable capacity:

10 kWh × 0.8 = 8 kWh usable energy.

Additional system losses may further reduce the energy delivered to the AC loads.

Therefore, a nominal 10 kWh battery should not automatically be treated as delivering 10 kWh to appliances.

BATTERY EFFICIENCY

Batteries are not perfectly efficient.

Some energy is lost during:

• Charging
• Storage
• Discharging
• Conversion

The system designer should therefore include appropriate efficiency assumptions.

INVERTER EFFICIENCY

The inverter also has conversion losses.

If battery energy is converted from DC to AC, not all of the stored DC energy reaches the appliance.

For example, if a battery provides 10 kWh of usable DC energy, the amount available to AC loads will depend on inverter efficiency and operating conditions.

NIGHTTIME LOAD

Nighttime consumption is particularly important for battery sizing.

A household might use:

• Lighting
• Television
• Refrigerator
• Wi-Fi
• Security systems
• Computers
• Cooking appliances

If the customer wants these loads to continue operating after sunset, the battery must provide sufficient energy.

EXAMPLE: SMALL HOME

Suppose a home has the following estimated daily consumption:

• Lighting: 1 kWh
• Television: 0.5 kWh
• Refrigerator: 1.5 kWh
• Internet: 0.2 kWh
• Computers: 0.8 kWh
• Water pump: 1 kWh
• Small appliances: 1 kWh

Total:

6 kWh/day

The solar system must generate approximately this amount plus system losses and design allowance.

EXAMPLE: MEDIUM HOME

Suppose another home consumes:

• Lighting: 1.5 kWh
• Refrigeration: 2 kWh
• Television: 1 kWh
• Computers: 1 kWh
• Water pumping: 1.5 kWh
• Washing machine: 1 kWh
• Kitchen appliances: 2 kWh
• Miscellaneous: 1 kWh

Total:

11 kWh/day

The solar array and battery should be designed around this actual energy requirement.

EXAMPLE: LARGE HOME

A larger home may have:

• Multiple refrigerators
• Freezers
• Air conditioners
• Electric cooking
• Water heating
• Pumps
• Entertainment systems
• Computers
• Security equipment

Daily energy consumption can therefore become much higher.

The system may require a large inverter, substantial solar generation and significant battery storage.

BATTERY BACKUP EXAMPLE

Suppose essential household loads average 1.5 kW.

The customer wants four hours of backup.

Energy requirement:

1.5 kW × 4 hours = 6 kWh.

If the design allows approximately 80% usable battery capacity and includes conversion losses, the nominal battery requirement would need to be greater than 6 kWh.

A professional system designer would then select a suitable commercially available battery configuration.

INVERTER SIZING

Battery and solar sizing are not enough.

The inverter must handle the instantaneous load.

Suppose the connected appliances can draw 5 kW simultaneously.

A 3 kW inverter would not be appropriate for full simultaneous operation.

The designer must consider:

• Continuous power
• Surge power
• Motor startup
• Phase requirements
• Battery voltage
• Solar input capacity

PEAK LOAD

Peak load is the maximum electrical demand occurring at a particular moment.

For example, a house may have:

• Cooker operating
• Microwave operating
• Refrigerator compressor starting
• Water pump starting
• Lighting operating

The combined demand may be significantly higher than the average daily load.

The inverter must be sized appropriately.

MOTOR STARTING LOAD

Motor loads can create significant startup demand.

Examples include:

• Borehole pumps
• Water pumps
• Compressors
• Refrigerators
• Air conditioners
• Workshop machines

A system can have sufficient daily energy but still fail to start a motor if the inverter cannot handle the starting requirement.

SOLAR FOR WATER PUMPS

Pump sizing requires more than calculating motor wattage.

The installer should consider:

• Pump power
• Flow rate
• Head
• Borehole depth
• Static water level
• Dynamic water level
• Pipe size
• Required daily volume
• Pump efficiency

The electrical solar system must be matched to the hydraulic design.

SOLAR PUMP EXAMPLE

Suppose a pump is rated at 1.5 kW and operates for four hours per day.

The theoretical energy consumption is:

1.5 kW × 4 hours = 6 kWh.

The solar system must generate enough energy to supply the pump while accounting for system losses.

If the pump starts under load, the inverter or pump controller must also accommodate the startup characteristics.

COMMERCIAL LOAD CALCULATION

Businesses often have different operating schedules.

For example, a shop might operate from 8 a.m. to 7 p.m.

An office might operate from 7 a.m. to 6 p.m.

A hotel may operate 24 hours.

A factory may operate multiple shifts.

The load profile therefore matters.

DAYTIME CONSUMPTION

Solar energy is most directly useful when appliances are operating while the sun is available.

For a business with significant daytime consumption, solar generation can be consumed directly.

This can reduce the amount of energy drawn from the grid.

NIGHTTIME CONSUMPTION

If the business operates at night, battery storage may be required.

The more nighttime energy required, the larger the battery system may need to be.

For a 24-hour facility, solar and battery design becomes more complex.

OFFICE EXAMPLE

Suppose an office consumes:

• Lighting: 2 kWh
• Computers: 5 kWh
• Internet equipment: 0.5 kWh
• Printers: 1 kWh
• Air conditioning: 8 kWh
• Security: 0.5 kWh

Total:

17 kWh/day

The designer can then determine the approximate solar generation required.

SHOP EXAMPLE

A small shop might consume:

• Lighting: 1 kWh
• Refrigerator: 2 kWh
• Freezer: 2 kWh
• Television: 0.7 kWh
• Security: 0.2 kWh
• Charging: 0.3 kWh

Total:

6.2 kWh/day

The actual system should be adjusted using measured consumption where possible.

HOTEL EXAMPLE

Hotels may have:

• Lighting
• Refrigeration
• Kitchen equipment
• Water pumps
• Laundry machines
• Water heaters
• Air conditioners
• Entertainment
• Computers

A complete energy audit is usually more appropriate than a simple appliance estimate.

FACTORY EXAMPLE

Factories can have highly variable loads.

A factory may operate:

• Motors
• Compressors
• Pumps
• Conveyors
• Welding machines
• Heating equipment
• Refrigeration
• Lighting

The designer may need interval load measurements rather than relying on a utility bill alone.

BATTERY VOLTAGE

Battery systems can operate at different DC voltage levels.

Common configurations include:

• 12 V
• 24 V
• 48 V
• Higher-voltage battery systems

Large systems often use higher-voltage battery architectures to reduce current for a given power level.

Battery voltage must be compatible with the inverter.

CURRENT CALCULATION

Power can be related to voltage and current using:

POWER = VOLTAGE × CURRENT

For a simplified DC example:

A 5,000 W load supplied at 48 V would theoretically require:

5,000 ÷ 48 ≈ 104 A

Actual current requirements can be higher depending on conversion efficiency and operating conditions.

This illustrates why large systems require appropriately sized battery cables and protection.

WHY HIGH CURRENT MATTERS

High current creates greater cable and connection requirements.

If cables are too small, they can experience:

• Excessive heating
• Voltage drop
• Energy losses
• Insulation damage

Therefore, battery cabling is a major part of system design.

BATTERY BANK CONFIGURATION

Multiple batteries may be combined to increase capacity.

They may be connected in parallel or, where specifically supported, in series.

However, the battery manufacturer's instructions must be followed.

Important factors include:

• Maximum parallel units
• Communication
• Cable length
• Current sharing
• Protection
• Battery balancing

FUTURE LOADS

Solar systems should consider future electricity consumption.

Possible future additions include:

• Electric vehicle
• Electric cooker
• Water heater
• Air conditioner
• Additional refrigeration
• Borehole pump
• Workshop machinery

A system that is perfectly sized today may become inadequate after major electrical upgrades.

SOLAR PANEL AREA

Panel sizing must also consider available roof area.

Suppose a system requires 20 panels.

The installer needs enough usable roof area for all 20 panels while maintaining safe access and appropriate mounting spacing.

Theoretical panel capacity is therefore only one part of the design.

SHADING

Shading can reduce production.

A tree may shade part of the array.

A neighboring building may block afternoon sunlight.

A water tank or chimney can cast shadows.

The panel layout should minimize these effects.

ORIENTATION

Panel orientation should be considered together with:

• Location
• Roof direction
• Tilt
• Daily consumption
• Shading
• Available area

The installer should optimize the actual site rather than applying one generic layout to every property.

TEMPERATURE

Solar panel output is affected by temperature.

Panel specifications normally include temperature coefficients.

As panel temperature rises, voltage generally decreases.

This is one reason system designers consider operating conditions rather than using only nominal panel ratings.

SOLAR ARRAY OVERSIZING

Some inverter manufacturers permit a solar array to have a DC capacity greater than the inverter's nominal AC output.

This can improve energy harvesting during lower-light periods.

However, the inverter's manufacturer limits must be respected.

The designer must check:

• Maximum PV power
• Maximum DC voltage
• Maximum input current
• MPPT limits

BATTERY OVERSIZING

A larger battery is not automatically better.

A battery should be sized around the required backup and energy-management strategy.

An unnecessarily large battery increases cost.

An undersized battery may produce inadequate backup.

The objective is to find the appropriate balance.

SYSTEM DESIGN MARGIN

Solar designs often include an appropriate margin to accommodate:

• Weather variation
• Equipment losses
• Future loads
• Degradation
• Uncertainty in consumption

The margin should be based on engineering judgment rather than arbitrary oversizing.

SOLAR PANEL DEGRADATION

Solar modules gradually lose some output over time.

The expected degradation rate depends on the technology and manufacturer.

A long-term solar design should account for the fact that a panel will not necessarily produce exactly its initial rated output throughout its entire service life.

BATTERY AGING

Battery capacity also changes over time.

Battery performance depends on:

• Cycle count
• Temperature
• Depth of discharge
• Charging practices
• Discharge rate
• Age

This is another reason system sizing should not operate continuously at the absolute minimum capacity.

LOAD PRIORITIZATION

A hybrid system can prioritize important loads.

For example:

PRIORITY 1

Security, communication and essential lighting.

PRIORITY 2

Refrigeration and selected appliances.

PRIORITY 3

Comfort loads.

PRIORITY 4

Heavy discretionary loads.

Load prioritization can improve battery utilization.

USING AN ENERGY AUDIT

For larger projects, an energy audit can provide much better data.

An energy audit may determine:

• Daily consumption
• Peak demand
• Load profile
• Operating hours
• Equipment efficiency
• Power factor
• Seasonal changes

This information can then be used to design the solar system.

WHY POWER FACTOR MATTERS

AC equipment such as motors can have a power factor below unity.

This means apparent power in kVA can be greater than real power in kW.

In commercial and industrial systems, inverter sizing may therefore need to consider:

• kW
• kVA
• Power factor
• Motor starting
• Phase balance

SOLAR FOR THREE-PHASE LOADS

Three-phase systems require phase-aware calculations.

The installer may need to determine:

• Load on each phase
• Three-phase motor requirements
• Phase balance
• Inverter configuration
• Grid connection

Large three-phase solar installations should be professionally engineered.

USING SOLAR MONITORING

After installation, monitoring data can improve future calculations.

The installer can observe:

• Solar production
• Battery charging
• Battery discharge
• Grid consumption
• Load consumption
• Peak demand

If actual consumption differs significantly from the original assumptions, the system can be evaluated.

COMMON SIZING ERRORS

Common errors include:

• Counting appliance wattage but ignoring operating hours
• Ignoring nighttime consumption
• Ignoring motor startup
• Ignoring battery losses
• Ignoring inverter efficiency
• Ignoring shading
• Ignoring future loads
• Using average consumption without considering peak demand
• Selecting batteries without checking compatibility
• Selecting panels without checking inverter voltage
• Ignoring cable losses

WHY A PROFESSIONAL CALCULATION IS IMPORTANT

A solar system is an integrated electrical system.

The panel capacity must match the inverter.

The inverter must match the battery.

The battery must match the load.

The mounting system must match the roof.

The cables must match the current and distance.

The protection must match the electrical architecture.

All these components must work together.

SIMPLE SIZING FORMULA

A simplified planning process can be summarized as:

STEP 1: CALCULATE DAILY ENERGY

Add the energy consumption of the loads.

STEP 2: DETERMINE PEAK LOAD

Identify the maximum simultaneous demand.

STEP 3: ESTIMATE SOLAR RESOURCE

Determine appropriate peak-sun assumptions for the site.

STEP 4: SIZE THE SOLAR ARRAY

Account for daily energy requirements and system losses.

STEP 5: SIZE THE INVERTER

Consider continuous power and surge requirements.

STEP 6: SIZE THE BATTERY

Consider backup duration, usable capacity and efficiency.

STEP 7: CHECK ELECTRICAL COMPATIBILITY

Verify voltages, currents, protection and communication.

STEP 8: CONSIDER FUTURE LOADS

Allow for realistic expansion.

FINAL EXAMPLE

Suppose a home uses approximately:

10 kWh/day

The homeowner wants:

5 hours of essential backup

The average backup load is:

1.5 kW

Backup energy required:

1.5 × 5 = 7.5 kWh

The battery therefore needs sufficient nominal capacity to provide at least this amount after accounting for usable depth of discharge and system losses.

The solar array then needs to generate the home's daily energy consumption plus appropriate losses and design allowance.

The inverter must also be capable of handling the highest simultaneous appliance demand.

This example demonstrates why a solar system cannot be sized using battery capacity or panel wattage alone.

PROFESSIONAL SOLAR SIZING IN KENYA

Solar panel and battery sizing should begin with the customer's real electricity requirements.

The correct calculation considers daily energy, peak demand, solar availability, battery storage, inverter capacity, system losses, cable sizing, appliance characteristics and future expansion.

For a small home, a simple load assessment may be sufficient.

For a large home, business, farm, hotel, school, workshop, borehole or factory, a detailed energy assessment may be required.

A correctly sized solar system can provide better energy production, more reliable backup, improved battery utilization and better long-term value.

For solar panel sizing, battery sizing, hybrid solar systems, off-grid installations, commercial solar, solar water pumping and professional solar installation in Kenya, contact 0723763173.

The next topic in the series is the most common solar installation problems, why they happen and how they can be prevented before they become expensive failures.