How much does a 100 kWp solar system cost, and how fast does it pay back?

Rooftop solar is usually quoted as a capex per kWp. That figure is far more than modules and inverters: electrical materials, mounting structure, installation and permitting take a meaningful share, and most of the spread between quotations sits there.

• PV modules and inverters – typically the two largest equipment items.
• Mounting structure, clamps, corrosion protection and waterproofing at every roof penetration.
• Electrical balance of system: DC/AC cabling, switchboards, protection devices, surge protection, earthing, metering.
• Installation, testing and commissioning, monitoring platform, permitting and grid connection paperwork.

Capex varies with roof type, working height, equipment brands and material prices at purchase time; a firm number only exists after a site survey.

Output follows local irradiation. Based on reference irradiation data (Global Solar Atlas), specific yield for rooftop systems in Vietnam typically falls in the range of about 1,100–1,550 kWh/kWp/year, increasing from the north towards the south-central, Central Highlands and southern regions.

• Northern Vietnam: roughly 1,100–1,200 kWh/kWp/year, i.e. about 110,000–120,000 kWh/year for 100 kWp.
• North-central to central coast: roughly 1,300–1,400 kWh/kWp/year.
• South-central coast and Central Highlands: roughly 1,500–1,600 kWh/kWp/year.
• Southern Vietnam: roughly 1,400–1,500 kWh/kWp/year.

These are reference ranges for a preliminary case. Design yield must be simulated for the actual coordinates, roof orientation, tilt, shading and equipment.

The real value sits in self-consumed energy: every kWh generated and used on site replaces one kWh bought from the grid at the commercial and industrial retail tariff. So the two decisive variables are the self-consumption ratio and the blended tariff you currently pay.

• First-year savings ≈ annual yield × self-consumption ratio × blended tariff from your bills.
• Self-consumption is high (85–100%) when the plant runs in daylight all week, and lower with weekend shutdowns or night shifts.
• Surplus export is only allowed within the limits and at the price defined by Decree 58/2025/ND-CP, so it should not be treated as a primary revenue stream.
• Output declines slightly each year through module degradation (commonly published around 0.4–0.55%/year depending on the product line).

Simple payback is total capex divided by annual savings. At the same capex, a project in a high-irradiation region with near-100% self-consumption pays back considerably faster than one exporting a large share of its output.

• Size the system to the daytime load curve instead of covering the whole roof.
• Prioritise unshaded roof areas with favourable orientation and tilt to raise specific yield.
• If you prefer no capex, use the ESCO model: an investor installs the system and sells power below the grid tariff.
• Run a quick estimate from your own bill using the Orca Energy capacity, yield and payback calculator.