One of the most common mistakes people make when buying solar in Bangladesh is choosing the system size before calculating how much electricity they actually use.
Some people install too many panels and spend more money than necessary.
Others install a system that is too small and later discover that it cannot cover enough of their electricity consumption.
The good news is that solar system sizing is not as complicated as it sounds.
You mainly need:
- Your average daily electricity consumption
- Your location’s peak sun hours
- A reasonable system-loss factor
- Your panel wattage
- Your available roof space
From there, you can calculate a realistic starting system size.
The Quick Solar Sizing Formula
For a basic grid-connected solar system, you can start with:
(Daily Electricity Usage ÷ Peak Sun Hours) × 1.15 = Solar System Size in kW
Then:
System Size in kW × 1,000 = System Size in watts
And:
System Size in watts ÷ Panel Wattage = Approximate Number of Panels
The 1.15 factor is a practical allowance for real-world losses such as inverter conversion, wiring, temperature and other system inefficiencies. The attached source uses the same 1.15 sizing factor.
This is a starting formula—not a final engineering design.
What Actually Determines Your Solar System Size?
Before calculating anything, understand what you are trying to achieve.
Most solar customers generally fall into three categories:
1. Bill Reduction
You want solar to reduce your monthly electricity bill.
2. Maximum Solar Generation
You have enough roof space and want to generate as much electricity as reasonably possible.
3. Electricity Bill Offset
You want solar to cover a specific percentage of your annual electricity consumption.
For example:
- 50% offset
- 80% offset
- 100% offset
For most homeowners and businesses, starting with actual electricity consumption is the most reliable approach.
Step 1: Find Your Electricity Consumption
Start by collecting your electricity bills.
Ideally, use 12 months of electricity bills rather than looking at only one month.
Why?
Because your electricity consumption can change significantly throughout the year.
For example, your electricity use may increase during hotter months because of:
- Air conditioners
- Fans
- Refrigerators
- Water pumps
It may also change depending on business activity, occupancy or production levels.
The attached source recommends using 12 months of bills to calculate a more reliable average.
Calculate Average Monthly Consumption
Add your total electricity consumption for the last 12 months.
Then:
Annual kWh ÷ 12 = Average Monthly kWh
For example:
Annual consumption:
12,000 kWh
Average monthly consumption:
12,000 ÷ 12 = 1,000 kWh/month
Calculate Average Daily Consumption
Now divide the average monthly consumption by approximately 30 days.
1,000 ÷ 30 = 33.3 kWh/day
So this example property uses approximately:
33 kWh/day
That is the number we need for the solar sizing formula.
What If You Don’t Have 12 Months of Bills?
If you have only a few months of electricity bills, you can still make an initial estimate.
You can also calculate your approximate consumption by listing your major appliances.
For example:
- Air conditioner
- Refrigerator
- Fans
- Lights
- Water pump
- Television
- Computer
- Washing machine
- Microwave
- Office equipment
- Factory machinery
This can help identify which appliances are responsible for most of your electricity use.
However, for a serious solar investment, actual electricity bills are much more useful than guessing based on appliance wattage alone.
Step 2: Find Your Peak Sun Hours
This is one of the most important parts of solar sizing.
Peak sun hours are not the same as daylight hours.
For example, a location may have many hours of daylight but only several hours equivalent to full-strength solar radiation.
Peak sun hours represent the equivalent amount of solar energy received at standard peak intensity.
Your peak sun hours depend on:
- Location
- Season
- Weather
- Atmospheric conditions
- Solar radiation
- Panel orientation
- Shading
For Bangladesh, the exact value should ideally be based on the project’s location and a reliable solar resource assessment rather than using one number for the entire country.
Dhaka, Chattogram, Rajshahi, Sylhet, Khulna and other regions can have different solar conditions and weather patterns.
Why Peak Sun Hours Matter
Imagine two properties both use:
30 kWh/day
Property A receives stronger usable solar radiation.
Property B receives lower solar radiation.
Property B may require a larger solar array to produce the same amount of electricity.
That’s why simply saying:
“My neighbour has a 5 kW system, so I also need 5 kW.”
is not a reliable sizing method.
Your electricity consumption and solar resource need to be considered together.
Step 3: Apply the Solar Sizing Formula
Now let’s calculate a practical Bangladesh example.
Suppose a home uses:
30 kWh/day
And the design assumption for the site is:
4.5 peak sun hours/day
Use:
(30 ÷ 4.5) × 1.15
First:
30 ÷ 4.5 = 6.67
Then:
6.67 × 1.15 = 7.67 kW
So the starting solar system size would be approximately:
7.7 kW DC
You could then select a practical panel configuration close to that capacity.
Step 4: Convert kW Into Watts
Solar panels are normally specified in watts.
So:
7.67 kW × 1,000 = 7,670 W
Your target solar array is therefore approximately:
7,670 watts
Step 5: Calculate the Number of Solar Panels
Now suppose you choose:
550 W solar panels
Calculate:
7,670 ÷ 550 = 13.95
So you would need approximately:
14 panels
Fourteen 550 W panels provide:
14 × 550 = 7,700 W
or:
7.7 kW
That is very close to our calculated target.
A Second Example for a Typical Bangladesh Home
Suppose another home uses:
600 kWh/month
Average daily consumption:
600 ÷ 30 = 20 kWh/day
Assume:
4.5 peak sun hours
Then:
(20 ÷ 4.5) × 1.15
= 5.11 kW
So the starting system size would be approximately:
5.1 kW
With 550 W panels:
5,110 ÷ 550 ≈ 9.3
So you would need approximately:
10 panels
10 × 550 W:
5,500 W
or:
5.5 kW
The final design would then be checked against the inverter, roof space, shading and other technical requirements.
Quick Solar Sizing Table
The following table provides a rough starting point using 4.5 peak sun hours and the 1.15 loss factor.
| Daily Consumption | Approx. Solar Size |
|---|---|
| 5 kWh/day | 1.3 kW |
| 10 kWh/day | 2.6 kW |
| 15 kWh/day | 3.8 kW |
| 20 kWh/day | 5.1 kW |
| 25 kWh/day | 6.4 kW |
| 30 kWh/day | 7.7 kW |
| 40 kWh/day | 10.2 kW |
| 50 kWh/day | 12.8 kW |
These are planning estimates only.
Actual system sizing should use the project’s solar resource, roof conditions, electricity consumption pattern and equipment specifications.
Don’t Size Solar Based on House Size
A common question is:
“How many solar panels does a 2,000 sq ft house need?”
The problem is that floor area doesn’t directly determine electricity consumption.
Two houses of the same size can have completely different electricity bills.
For example:
House A
- Efficient appliances
- No air conditioning
- Low occupancy
- Low electricity consumption
House B
- Multiple AC units
- Large refrigerator/freezer
- Water pump
- More occupants
- High electricity consumption
The second house may require a much larger solar system despite having exactly the same floor area.
So:
Your electricity bill is more important than your house size.
Air Conditioning Can Change Your Solar Requirement
Air conditioners can be one of the biggest electricity loads in a Bangladesh home.
If you are planning to install solar but also plan to add:
- 1 AC
- 2 ACs
- 3 ACs
in the near future, include that expected electricity consumption in your sizing calculation.
Otherwise, you may install a system today and discover that it is too small after adding the new AC units.
The same applies to:
- Electric water pumps
- Freezers
- Electric ovens
- Large refrigerators
- EV chargers
- Industrial machinery
Step 6: Consider Future Electricity Usage
Don’t only design for today’s electricity consumption.
Think about what your electricity consumption will look like in the next few years.
For example, you may currently use:
700 kWh/month
but plan to add:
- Another AC
- Home office equipment
- Electric vehicle
- Larger refrigerator
- Additional floor
- New machinery
If you know those loads are coming, include them in your planning.
The attached source similarly recommends accounting for future load growth rather than designing a system that only meets today’s usage.
Step 7: Check Your Roof Space
Once you calculate the required solar capacity, check whether your roof can physically accommodate it.
The available area depends on:
- Panel dimensions
- Panel wattage
- Roof layout
- Walkways
- Water tanks
- Stair rooms
- Existing equipment
- Roof edges
- Shading
For example, a 10 kW system using higher-wattage panels may require fewer panels than a 10 kW system using lower-wattage panels.
So:
Same kW ≠ Same number of panels
Higher-Wattage Panels Can Reduce Panel Count
Suppose you need:
10 kW
Using 400 W panels
10,000 ÷ 400 =
25 panels
Using 550 W panels
10,000 ÷ 550 ≈
19 panels
Both systems have approximately the same DC capacity.
But the second system requires fewer panels.
This can be useful when roof space is limited.
Panel Efficiency Also Matters
If roof space is limited, higher-efficiency panels can help you generate more electricity from a smaller area.
However, don’t choose panels based only on efficiency.
Also consider:
- Price
- Warranty
- Manufacturer
- Performance warranty
- Temperature characteristics
- Local availability
- Installer support
The cheapest panel per watt isn’t always the best long-term choice.
Step 8: Consider Roof Direction and Tilt
Your roof orientation affects solar production.
In Bangladesh, the solar array should generally be designed to receive strong solar exposure throughout the year.
The final orientation and tilt should be determined based on the property’s location and the project’s energy-production objectives.
Also consider nearby shading from:
- Buildings
- Trees
- Water tanks
- Stair structures
- Antennas
- Other rooftop equipment
A small amount of shading can sometimes have a larger impact than homeowners expect.
Roof-Mounted vs Ground-Mounted Solar
Rooftop Solar
Usually makes sense when:
- You have enough roof space.
- The roof is structurally suitable.
- Shading is manageable.
- You want to use existing space.
Ground-Mounted Solar
Can make sense when:
- Roof space is limited.
- You have suitable land.
- You want greater flexibility in panel positioning.
- The roof is not suitable for solar.
Ground-mounted systems may require additional:
- Steel structure
- Foundations
- Civil work
- Land
- Security
- Cabling
So don’t assume ground-mounted solar will automatically be cheaper.
Step 9: Check the Inverter Size
After calculating the DC solar capacity, you need to choose an appropriate inverter.
For example, suppose your solar array is:
10 kW DC
That doesn’t automatically mean you must install exactly:
10 kW AC inverter
The appropriate DC-to-AC ratio depends on:
- Inverter specifications
- Solar resource
- System design
- Expected clipping
- Roof orientation
- Project objectives
Your installer or engineer should verify:
- Maximum PV input
- MPPT voltage range
- Maximum current
- Number of MPPTs
- AC output
- Protection requirements
Step 10: Check Net Metering
If you are installing a grid-connected solar system in Bangladesh, check whether your property is eligible for the applicable net-metering arrangement.
This matters because your system may generate electricity when your property does not need all of it.
Under applicable arrangements, eligible surplus generation may be exported to the grid.
That can affect how aggressively you size the system.
Don’t simply assume:
“More solar is always better.”
Your ideal system size depends on:
- Electricity consumption
- Solar generation
- Self-consumption
- Net-metering rules
- Roof space
- Project budget
Should You Size for 100% of Your Electricity Consumption?
Not necessarily.
Suppose your annual electricity consumption is:
12,000 kWh
You could design a system targeting approximately:
- 60% offset
- 80% offset
- 90% offset
- 100% offset
The best target depends on your electricity tariff, net-metering arrangement, available roof space and investment goals.
For some properties, a slightly smaller system can provide better economics than trying to cover every unit of annual consumption.
What About Off-Grid Solar?
This formula is mainly a starting point for grid-connected solar sizing.
If you are designing an off-grid or battery-backed system, the calculation becomes more complicated.
You also need to calculate:
- Battery capacity
- Daily energy demand
- Peak load
- Backup duration
- Depth of discharge
- Battery efficiency
- Inverter capacity
- Worst-case solar generation
The attached source specifically notes that off-grid sizing requires a separate battery calculation in addition to panel sizing.
How to Size an Off-Grid System
Suppose your home uses:
10 kWh/day
and you want:
2 days of battery autonomy
You need approximately:
10 × 2 = 20 kWh
of usable energy storage before accounting for additional battery and system considerations.
You then need to size the solar array large enough to:
- Run your daily loads
- Recharge the battery
- Compensate for system losses
- Handle seasonal changes in solar production
Therefore, don’t use the basic grid-tied formula alone for a fully off-grid system.
What About Hybrid Solar?
Hybrid solar sits between grid-connected and off-grid systems.
A hybrid system may include:
- Solar panels
- Grid connection
- Battery storage
- Hybrid inverter
This allows you to use solar for normal electricity consumption while keeping batteries available for backup.
For a Bangladesh home that already has grid electricity but wants backup during power outages, a hybrid system can sometimes be more practical than a completely off-grid design.
Common Solar Sizing Mistakes
Mistake 1: Using One Month’s Electricity Bill
One month is not enough to understand your annual consumption.
Use 12 months whenever possible.
Mistake 2: Using House Size Instead of Electricity Usage
Square feet doesn’t tell you how much electricity your household consumes.
Use kWh.
Mistake 3: Ignoring Future Loads
If you’re planning to buy an EV or install another AC, include it in your future electricity demand.
Mistake 4: Ignoring Roof Shading
A theoretical 10 kW system may not produce the expected electricity if a significant portion of the roof is shaded.
Mistake 5: Using the Same Peak Sun Hours for Every Location
Solar conditions vary by location and season.
Mistake 6: Forgetting System Losses
Solar panels do not deliver their rated output to your appliances all day, every day.
That’s why a practical loss factor is included in the basic sizing formula.
Mistake 7: Buying More Panels Without Checking the Inverter
Your inverter has specific PV voltage and current limits.
More panels are not automatically better.
Mistake 8: Designing an Off-Grid System Like an On-Grid System
Off-grid systems need battery and backup calculations.
A Complete Worked Example
Let’s bring everything together.
Suppose a home in Bangladesh uses:
900 kWh/month
Step 1: Monthly to Daily
900 ÷ 30 =
30 kWh/day
Step 2: Peak Sun Hours
Assume a planning value of:
4.5 peak sun hours/day
Step 3: Apply the Formula
(30 ÷ 4.5) × 1.15
= 7.67 kW
Step 4: Convert to Watts
7.67 × 1,000
= 7,670 W
Step 5: Choose 550 W Panels
7,670 ÷ 550
= 13.95
So:
14 × 550 W panels
Step 6: Total Panel Capacity
14 × 550
= 7,700 W
or:
7.7 kW DC
This is a good starting point for the system design.
The final system would still need to be checked for:
- Roof space
- Shading
- Panel orientation
- Inverter compatibility
- Electrical connection
- Net-metering requirements
- Structural conditions
How Much Roof Space Does a Solar System Need?
The exact area depends on the panel dimensions.
As a rough planning concept, larger-wattage panels can reduce the number of panels required.
For example:
10 kW system
using 550 W panels:
10,000 ÷ 550 ≈ 18.2
So you need approximately:
19 panels
If each panel occupies roughly 2–2.5 m², the panel surface itself could require approximately:
38–48 m²
Additional space may be required for:
- Maintenance access
- Walkways
- Roof edges
- Equipment
- Safety clearance
Therefore, don’t calculate roof area based only on panel dimensions.
What Does a Solar System Cost After You Know the Size?
Once you’ve calculated the system size, you can request a quotation.
For example, if your calculation indicates approximately:
7.7 kW
you can ask installers to quote for a system around that capacity.
The quotation should clearly specify:
- Panel brand and model
- Panel wattage
- Number of panels
- Total DC capacity
- Inverter brand/model
- Mounting structure
- Cables
- Protection equipment
- Installation
- Warranty
This makes it much easier to compare quotations.
Don’t Let a Solar Sales Quote Replace Your Calculation
If one installer recommends:
5 kW
and another recommends:
10 kW
don’t immediately assume one is trying to overcharge you.
Ask both companies:
“What electricity consumption and solar resource assumptions did you use?”
Then compare their calculations with your own.
The attached source makes this same point: knowing your own kWh usage and peak sun hours allows you to question why an installer has recommended a different system size.
The Simple Formula to Remember
If you remember only one thing from this article, remember this:
Daily kWh ÷ Peak Sun Hours × 1.15 = Approximate Solar Size in kW
Then:
kW × 1,000 ÷ Panel Wattage = Approximate Panel Count
For example:
30 kWh/day
÷
4.5 peak sun hours
×
1.15
=
7.67 kW
Using 550 W panels:
7,670 ÷ 550 ≈ 14 panels
That’s enough to give you a solid starting point before discussing the final design with a professional solar installer.
Frequently Asked Questions
How do I calculate the size of a solar system?
Divide your average daily electricity consumption in kWh by your location’s peak sun hours, then multiply by approximately 1.15 to account for system losses.
Formula:
(Daily kWh ÷ Peak Sun Hours) × 1.15 = Solar System Size in kW
How many solar panels do I need for a 5 kW system?
It depends on the wattage of your panels.
With 550 W panels:
5,000 ÷ 550 ≈ 9.1
So you would need approximately 10 panels, giving you a total of 5.5 kW DC.
How many solar panels does a 2,000 sq ft house need?
There is no fixed answer based on floor area. The number of panels should be calculated from the home’s actual electricity consumption, peak sun hours and panel wattage.
How many solar panels do I need for 1,000 kWh per month?
A home using 1,000 kWh/month consumes approximately:
33 kWh/day
At 4.5 peak sun hours and a 1.15 system-loss factor, the starting system size is approximately:
8.4 kW
The exact design will depend on the property’s solar resource and system conditions.
What is a peak sun hour?
A peak sun hour is a measurement of solar energy equivalent to one hour of sunlight at standard peak intensity. It is not the same as the total number of daylight hours.
Should I use 12 months of electricity bills?
Yes. Twelve months provides a much better picture of your annual electricity consumption and helps account for seasonal changes.
Should I size my solar system for 100% of my electricity usage?
Not necessarily. The ideal offset depends on your electricity consumption, roof space, budget, solar generation and applicable net-metering arrangements.
Does Bangladesh have enough sunlight for solar?
Bangladesh receives significant solar energy throughout the year, making solar a practical renewable-energy option. However, actual system production varies by location, season, weather, shading and system design.
Can solar run an air conditioner?
Yes. But air conditioning can significantly increase electricity consumption. If you plan to run AC regularly, include that load when calculating your required solar capacity.
How do I size an off-grid solar system?
Off-grid sizing requires more than calculating panel capacity. You also need to calculate battery capacity, days of autonomy, peak load, battery depth of discharge and inverter capacity.
Is a 10 kW solar system enough for a house?
It depends on the home’s electricity consumption. A 10 kW system could be appropriate for one property and too large or too small for another.
How much roof space does a 10 kW solar system need?
The exact area depends on panel wattage and dimensions. Higher-wattage panels can reduce the number of panels required, but additional roof space is needed for access, spacing and safety.
Should I install more solar panels than I currently need?
If you expect your electricity consumption to increase soon, it can make sense to plan for future loads. However, oversizing the system without a clear reason can unnecessarily increase the initial investment.
What is the most important number when sizing solar?
For most grid-connected systems, the most important starting point is your actual electricity consumption in kWh. Your location’s solar resource then determines how much solar capacity is required to produce that energy.
Final Verdict
Sizing a solar system doesn’t need to be guesswork.
Start with your actual electricity consumption, not your house size.
Then determine the appropriate peak sun hours for your location.
Apply the sizing formula:
(Daily kWh ÷ Peak Sun Hours) × 1.15
Then convert the result into watts and calculate the approximate number of panels.
After that, check the things the formula cannot fully capture:
- Roof space
- Shading
- Orientation
- Tilt
- Panel efficiency
- Inverter specifications
- Future electricity consumption
- Net metering
- Structural conditions
And if you’re designing an off-grid or battery-backed system, add the separate battery and backup calculations.
At Muspana, we recommend treating the formula as your starting point, not the final engineering design.
The goal isn’t to install the biggest solar system possible.
It’s to install a system that produces the right amount of electricity for your home or business, your roof, your consumption and your budget.
