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How residential, commercial, and industrial solar system sizes are actually calculated - from load list and backup hours to roof area and net-metering limits. The engineering method, explained.
FSP reviews electricity bill, monthly units, load list, backup needs, roof space, inverter capacity, battery requirement, safety margin, and future expansion before recommending solar system size.
Why It Matters
Paying for capacity you never use is a permanent loss.
A system that can’t meet expectation frustrates for years.
Mis-sized batteries cut backup short and die early.
An undersized inverter trips on surge loads.
Motors and compressors need surge headroom.
Capacity that doesn’t fit the roof is a paper number.
A size chosen to hit a price isn’t engineering.
The Method
Each input feeds the next. Skip one, and the final number is a guess.
Inputs
Residential Sizing
Residential sizing depends on home load, backup requirement, battery capacity, inverter size, surge loads, cloudy weather, roof space, and future growth. The figures below are typical illustrative estimates — your final design may differ.
Use case: Small flat essentials.
Load: Lights, fans, router, TV.
Roof: ≈ 80 sq. ft.
Battery: ≈ 2.4 kWh
Backup: ≈ 4 hrs (est.)
Use case: Small family.
Load: Essentials + extra rooms.
Roof: ≈ 150 sq. ft.
Battery: ≈ 5 kWh
Backup: ≈ 5 hrs (est.)
Use case: Most families.
Load: Essentials + refrigerator.
Roof: ≈ 230 sq. ft.
Battery: ≈ 7 kWh
Backup: ≈ 5 hrs (est.)
Use case: Larger homes.
Load: Essentials + partial AC.
Roof: ≈ 380 sq. ft.
Battery: ≈ 10 kWh
Backup: ≈ 5 hrs (est.)
Use case: Big homes / duplex.
Load: Multi-room + AC/motor.
Roof: ≈ 460 sq. ft.
Battery: ≈ 14 kWh
Backup: ≈ 6 hrs (est.)
Commercial Sizing
Commercial sizing depends on daytime load, monthly bill, roof utilization, system type, net metering possibility, and operation hours. The example below is illustrative — your real values give your real design.
| Input | Example (illustrative) |
|---|---|
| Monthly bill | ৳60,000 |
| Monthly unit consumption | ≈ 5,500 kWh |
| Daytime load percentage | ≈ 70 % |
| Proposed system size | 30 kW |
| Estimated generation | ≈ 3,600 kWh/mo (estimate) |
| Estimated saving | ≈ ৳39,000/mo (estimate) |
| ROI / payback | ≈ 5 years (estimate) |
Industrial Sizing
Industrial sizing depends on load profile, monthly units, transformer/MDB/SDB condition, roof area, roof strength, cable route, net metering, and long-term O&M. The example below is illustrative.
| Input | Example (illustrative) |
|---|---|
| Monthly units | ≈ 45,000 kWh |
| Average daily units | ≈ 1,500 kWh |
| Daytime load | ≈ 80 % |
| Proposed capacity | 200 kW |
| Roof area requirement | ≈ 15,000 sq. ft. (≈ 65–80 sq.ft./kW) |
| Inverter location | As per electrical layout |
| Cable route consideration | Routed for minimum voltage drop |
| Net metering feasibility | Subject to utility approval |
| O&M consideration | Covered under AMC scope |
By System Type
Battery & Backup
Battery sizing depends on selected backup load, running hours, inverter efficiency, usable capacity, charging loss, cloudy-weather margin, and future growth. The example below is illustrative.
| Input | Example (illustrative) |
|---|---|
| Selected backup load | 800 W |
| Backup hours required | 4 hours |
| Inverter size | 3 kVA |
| Battery capacity | 5 kWh |
| Usable capacity | ≈ 4 kWh (after depth-of-discharge) |
| Estimated backup duration | ≈ 4 hours (estimate) |
| Safety margin | 20 % (cloudy-day reserve) |
Roof & Panel Capacity
System size is limited by available roof space, shadow, roof type, structure, panel layout, maintenance access, and future expansion. Roughly 65–80 sq. ft. of shadow-free roof per kW.
Safety Margin
A real system loses energy at every stage. Good sizing builds in reserve for all of it.
Conversion is never 100% efficient.
Energy is lost charging and discharging.
Generation drops on overcast days.
Motor startup needs extra headroom.
Loads tend to grow over time.
Voltage drop over distance.
≈ 0.5%/year output decline.
Dust and aging reduce yield.
Avoid These
Letting budget pick capacity, not load.
Guessing instead of measuring real load.
Forgetting motor/compressor startup.
No clarity on required autonomy.
A bill alone hides when energy is used.
Promising capacity that won’t fit.
Counting full capacity, not usable.
Designing with zero reserve.
Sizing without seeing the site.
Ignoring tomorrow’s load growth.
Solar Sizing & Engineering Review Gallery
Add photos and visuals here to show bill review, load analysis, rooftop assessment, inverter/battery planning, and project sizing workflow. Each tile is a ready frame — drop an image in to replace the placeholder.
<img src="…"> inside any tile — the dashed frame disappears automatically and the image fills the frame.Get Sized
Share a few details and our team will review your information and contact you with the next step.
with your Sizing Request form ID. Suggested fields — Client: name, phone/WhatsApp, location · Energy: monthly bill, monthly units, client type (residential/commercial/industrial/agriculture) · Requirement: on-grid/hybrid/off-grid/pump, backup required (yes/no), backup hours, roof size if known · Message: additional notes.Why FSP
The size is engineered before any price.
Real load drives the system, not budget.
The roof’s real ceiling is respected.
Backup hours sized honestly.
Losses and reserves built in.
Checked by engineers, not salespeople.
Designed with room to grow.
A size you understand and can verify.
Keep Exploring
FAQ
Before recommending system size or price, FSP reviews your electricity bill, load list, backup requirement, roof condition, inverter capacity, battery requirement, safety margin, and future expansion plan.