Most underperforming C&I solar plants were mis-sized before anyone broke ground. A system that is too small leaves savings on the table. A system that is too large for the transformer or the load profile creates export losses and grid-connection delays that no module upgrade can fix.
This guide covers the six decisions that set the size of an on-grid system, using the same sequence our engineers follow on live projects.
What sizing actually means in a grid-tied system
Sizing an on-grid system means fixing four numbers together: array capacity in kWp, inverter capacity in kW AC, the DC/AC ratio between them, and the export limit allowed by the local grid operator. These four numbers constrain each other.
Two projects with the same 500 kWp array can behave very differently. One designed at a DC/AC ratio of 1.0 will rarely clip but costs more in inverter capacity. One designed at 1.3 will produce more energy in low-light hours and clip briefly at noon.
Build a load profile, not just an annual bill
An annual consumption figure is not enough to size a system. You need the shape of the load: when the factory runs, how much power it draws at peak, and how much of that load coincides with sunlight.
Collect twelve months of electricity bills, the maximum demand in kVA, the transformer rating, the shift pattern and the roof or land area available. If the customer has 15-minute interval data, use it - it exposes weekend load, night shifts and demand spikes that monthly bills hide.
For a single-shift factory, roughly 55–70% of consumption may fall inside the solar window. For a cold store or a data centre running 24/7, the share is closer to 35–45%. The same array size therefore produces very different savings.
Step 2: Decide what the system is for
On-grid systems create value in three ways: self-consumption (displacing purchased kWh), export (selling to the grid) and peak-demand reduction (lowering kVA charges). The mix determines the target size.
Where export tariffs are low, capped or not permitted, size the array to the daytime load rather than to the roof area. A system designed for 80% self-consumption usually delivers a better return than a larger system that exports most of its output at a fraction of the retail tariff.
Step 3: Estimate yield before committing to a capacity
Annual generation is array capacity multiplied by specific yield, minus system losses. Specific yield is site-specific, but the ranges below are a reasonable first check before a full simulation.
|
Market |
Indicative specific yield (kWh/kWp/year) |
Typical system losses |
|
Germany |
950–1,100 |
10–13% |
|
Romania |
1,150–1,300 |
10–14% |
|
Bulgaria |
1,250–1,400 |
10–14% |
|
Spain |
1,500–1,700 |
11–15% |
|
UAE |
1,600–1,800 |
12–16% |
|
Vietnam |
1,200–1,500 |
11–15% |
These figures are planning ranges, not production guarantees. Final numbers must come from a simulation using site-specific irradiance, tilt, azimuth, soiling and shading. Ask your supplier for the simulation file, not only the result page.
Step 4: Set the DC/AC ratio deliberately
The DC/AC ratio is array capacity divided by inverter AC capacity. A 500 kWp array paired with a 400 kW inverter has a ratio of 1.25.
For commercial and industrial rooftops, 1.15–1.35 is the common working range. A higher ratio increases generation in morning, evening and cloudy conditions, at the cost of clipping around midday. A lower ratio reduces clipping but raises inverter cost per kWp.
The right number depends on the load profile. If the site is export-limited, a ratio above 1.3 is usually wasted capacity. If the load absorbs everything the array produces, 1.25–1.3 is often the better economic choice.
Step 5: Check the grid connection before the array layout
The grid connection, not the roof, frequently sets the ceiling on system size. Confirm the transformer capacity, the approved export limit, the protection settings and the metering arrangement before finalising the array.
A 400 kVA transformer serving a factory may leave only 250–300 kW of export headroom after existing loads. Upgrading that transformer can cost more than the PV system itself. In that case, a zero-export configuration with a controller, or a smaller self-consumption system, is the realistic design.
Step 6: Select modules and inverters as a pair
Module choice drives area efficiency and balance-of-system cost. For C&I roofs in 2026, 580 W TOPCon modules are the mainstream choice; 630 W modules suit area-constrained roofs; 700 W HJT modules are usually reserved for utility-scale ground mounts.
Inverter choice follows the array layout. String inverters in the 100–125 kW class with at least two MPPTs handle most C&I roofs, including partial shading and multiple orientations. Central inverters make sense above roughly 5 MW, where cable runs and maintenance access favour a single point of conversion.
String sizing is a calculation, not a rule of thumb. Check the module's open-circuit voltage at the record low temperature of the site against the inverter's maximum DC input voltage, and confirm the MPPT window still covers the operating range at high temperature.
Reference configurations from our on-grid range
|
System size |
Modules |
Inverters |
Typical application |
|
100 kW |
172 × 580 W TOPCon (99.8 kWp) |
1 × 100 kW string inverter |
Small factory, warehouse |
|
500 kW |
1,000 × 580 W TOPCon |
4 × 125 kW string inverters |
Industrial rooftop, ground mount |
|
1 MW |
1,800 × 580 W TOPCon |
8 × 125 kW string inverters |
Large C&I campus, distributed roof |
These are reference configurations, not fixed kits. Final quantities depend on roof geometry, string design and the destination grid code.
What a usable technical proposal should contain
A quotation is a price. A technical proposal is a design. Ask for the second one before comparing the first.
A usable proposal includes the load analysis, the proposed array layout with row spacing and tilt, a yield simulation with the input assumptions stated, a single-line diagram, the string design with voltage windows at record high and low temperatures, the bill of materials with model numbers, and the grid-connection assumptions.
If two suppliers quote the same capacity at very different prices, compare these documents. The cheaper proposal usually differs in module grade, inverter class, mounting steel thickness or the scope of commissioning support - not in the number of kilowatts.
Comparing quotations on a like-for-like basis
Normalise every quote to the same basis before comparing: price per kWp, DC/AC ratio, module model and efficiency, inverter model and MPPT count, mounting type and steel specification, monitoring scope, warranty terms and delivery terms.
A quote at 0.62 USD/W with a 1.0 DC/AC ratio and a 540 W module is not comparable to a quote at 0.68 USD/W with a 1.25 ratio and a 580 W N-type module. The second may deliver more energy per dollar over twenty years.
Six sizing mistakes that show up after commissioning
|
Mistake |
Consequence |
Fix |
|
Sizing from annual kWh only |
System exports at low tariff while the factory imports at night |
Build an hourly load profile |
|
Ignoring transformer headroom |
Grid operator rejects the application |
Confirm export limit before design |
|
Using STC power as expected output |
Yield shortfall of 10–15% in year one |
Apply temperature and system losses |
|
DC/AC ratio copied from another project |
Excess clipping or unnecessary inverter cost |
Match ratio to load shape |
|
Forgetting degradation |
Financial model overstated from year two |
Use 1% first-year, 0.4%/year thereafter |
|
One MPPT for multiple orientations |
Mismatch losses on L-shaped roofs |
Specify inverter by MPPT count |
A quick financial sanity check
Payback is total installed cost divided by annual savings. Annual savings equal self-consumed kWh multiplied by the retail tariff, plus exported kWh multiplied by the export tariff, plus any demand-charge reduction.
Run the calculation twice: once with the quoted price and once with a 10% cost overrun. If payback moves beyond the customer's threshold in the second case, the design is too tight and should be revisited.
Frequently asked questions
How much roof area does a 500 kW system need?
With 580 W modules at roughly 2.58 m² each, a 500 kW array needs about 1,300–1,600 m² of module area. Practical roof area is typically 30–45% larger after accounting for walkways, fire setbacks and shading spacing.
Can an on-grid system run during a power cut?
No. A standard grid-tied inverter shuts down when the grid fails, as required by anti-islanding protection. If backup power is required, the project needs a hybrid inverter and batteries, which is a different system architecture.
What is a good DC/AC ratio for a factory roof?
1.15–1.35 is the usual range. A factory that consumes most of its output during daylight can support 1.25–1.3. An export-limited site should stay closer to 1.15–1.2.
Do I need to upgrade the transformer?
Only if the existing transformer cannot carry the PV output plus the site load. A 400 kVA transformer serving a 300 kW daytime load can usually host 100–150 kW of PV without reinforcement.
How long does design and delivery take?
A technical proposal typically takes 3–5 working days after receiving load data. Production of a standard C&I system takes 15–25 days, with sea freight added depending on destination.
Get a sizing proposal for your project
Send us twelve months of bills, the transformer rating and a roof drawing. Our engineers will return a capacity recommendation, a simulation-based yield estimate and a reference configuration with pricing.
Request an on-grid system proposal → [link: /contact-us]
