Category: Solar & Energy

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For an apartment building, the solar decision starts with the common-service meter. Lifts, water pumps, corridor lighting, and outdoor lighting operate at different times. A monthly bill can show their combined energy use, but it cannot show how much demand coincides with sunshine.

I revisited a residential solar feasibility case with three towers and six lifts. Its most useful lesson is the need to separate daytime energy savings, grid export, and backup power before comparing quotations.

Audit the existing system first

If panels are already on the roof, record what they deliver before buying additional equipment. Check the meter boundary, inverter generation, import and export registers, shading, and operating history. Log the common services over a representative period, ideally at intervals short enough to see lift and pump demand. Include weekdays, weekends, and expected seasonal changes.

Compare the three architectures

A grid-connected system’s behavior during a grid outage depends on its approved design and protection. A battery system needs a specified backup circuit and operating mode. Verify the duty in the proposal and commissioning demonstration.

Keep kW and kWh separate

Kilowatts describe power. Kilowatt-hours describe energy over time. A 30 kWh battery label does not establish whether an inverter can start a lift motor, nor does it guarantee 30 kWh delivered at the load.

For an illustrative energy check, assume 30 kWh nominal storage, an 80% usable fraction, and 90% conversion efficiency. Delivered energy is approximately 30 × 0.80 × 0.90 = 21.6 kWh. At a steady 5 kW load, that is about 4.3 hours before allowing for further reserve or changing load. A lift system also needs a separate power, starting-current, control, and safety assessment.

Use a seasonal generation budget

Estimate generation by month and compare it with demand in the same intervals. Self-consumed solar, exported solar, battery charging, and grid imports have different roles in the calculation. Use the applicable tariff and approved settlement arrangement for each energy stream.

Make quotations comparable

• Ask for a complete scope covering modules, inverters, batteries where applicable, structures, cabling, protection, metering, installation, approvals, testing, and maintenance.
• State the assumed monthly generation and the share expected to be used within the building.
• Show battery usable capacity, warranted duty, replacement assumptions, and the defined backup loads.
• Check roof structure, access, drainage, electrical protection, and motor compatibility with qualified specialists.
• Calculate payback from verified total cost and annual net savings, with operating and replacement costs visible.

The early feasibility note contains preliminary quotations and savings assumptions. The blog’s worked example explains the calculation method. A current purchase decision needs current scope, tariffs, demand records, and supplier commitments.

My practical view

Meter the building first, then buy the functions it needs. Grid-connected PV can address daytime energy. Storage earns its place when a measured backup or time-shifting requirement justifies it. Agree in advance how the team will demonstrate generation, import/export measurement, and the specified backup duty.

Official guidance

Use SREDA’s Net Metering Guideline 2025 and the Power Division’s net-metering information to check the current approval and metering process with the distribution utility.

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Rooftop Solar Performance in Bangladesh · Diabari Waterlogging and Solid Waste Management · Practice Excel with plant data

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বাংলায় পড়ুন · Home

The roof producing the most electricity may have room to improve its output per installed panel capacity. That distinction is easy to miss in a monthly energy report. My comparison of four industrial rooftop installations in central Bangladesh begins with two questions. How much electricity arrived, and how much arrived per installed kilowatt-peak?

The records cover June 2024–May 2026. For the complete June 2025–May 2026 year, the four installations total 2,753.245 kWp and deliver 2,641.250 MWh. Locations are used to distinguish the systems.

Compare equal reporting boundaries

Fatullah delivers the most electricity, while Birulia has the highest specific yield. Before making this comparison, check that capacity is stated on the same DC basis and that the energy meters cover the intended installation. Shared installations must be counted once.

Try the calculation

Specific yield = recorded energy (kWh) ÷ installed DC capacity (kWp). For Birulia, 283,375.2 ÷ 258.520 = 1,096.14 kWh/kWp for the year. At the same 100 kWp scale, this is about 109.61 MWh. Fatullah’s recorded yield corresponds to about 87.82 MWh per 100 kWp.

The difference between the highest and lowest specific yields is 24.81%. It is a reason to investigate roof geometry, shading, metering, inverter records, interruptions, and maintenance history. The energy figures alone do not identify which factor caused the difference.

Keep season length visible

Pre-monsoon has the highest daily specific yield in this record, 2.867 kWh/kWp/day. Monsoon contributes the largest share of annual electricity, 31.51%, because its 122 days give it more time to accumulate energy. A budget needs both daily productivity and the number of days in the season.

Ask how much sunlight changed

The study pairs monthly energy with ERA5 and ECMWF IFS weather through Open-Meteo. The modeled annual horizontal-irradiation spread between locations is only 1.73% in ERA5 and 2.30% in IFS. Scaling the yields to a common horizontal sunlight basis keeps the same ranking.

This makes regional sunlight an incomplete explanation for the observed gap. The comparison uses gridded horizontal weather data. A standard performance-ratio assessment needs an appropriate plane-of-array irradiation basis and a defined monitoring boundary.

Make the monthly O&M review useful

• Close each month with opening and closing meter readings, installed capacity, and missing-data notes.
• Calculate kWh/kWp and kWh/kWp/day before comparing installations or months.
• Compare matching calendar months and review the corresponding sunlight.
• For a persistent unexplained change, inspect inverter events, shading, soiling, outages, and the meter boundary.
• Record the action, date, and operating conditions, then compare the next suitable period.

The dataset has 95 usable installation-months. One Kaliakair month is unavailable in the earlier year, so matched comparisons use eleven month pairs there. Roof-plane irradiation and module temperature were outside the measurement record. These details define how far the results can be used.

My practical view

Start with dependable metering and a fair comparison. A weather-adjusted difference is a prompt for investigation. Choosing the right maintenance action requires equipment and operating evidence. The same discipline helps when reviewing an energy-saving proposal or a contractor’s performance claim.

References

Open-Meteo Historical Weather API provides the weather-product definitions. NREL’s weather-corrected performance-ratio report explains a more complete monitoring basis. The comparison uses recorded generation and capacity data for the four locations; the figures above are extracted from my analysis.

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Rooftop Solar for Apartment Common Services · Diabari Waterlogging and Solid Waste Management · Practice Excel with plant data

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