Rooftop PV and battery guarantee for a textile plant
A 250 kWp rooftop PV array is proposed for a textile plant in Pune whose load is 120 kW from 08:00 to 16:00 and 40 kW for the remaining 16 hours. The plant already has a 400 kWh lithium-ion battery with 90% usable depth of discharge, 92% round-trip efficiency and a 95% efficient inverter. The PV vendor quotes a specific yield of 1,500 kWh/kWp/year and a DC/AC ratio of 1.25, with a 200 kW inverter. The vendor guarantees that the system will supply 70% of the plant's annual energy, and the plant manager wants a 4-hour backup for the night load during grid outages. Grid electricity costs INR 8/kWh and export is not permitted under the current net-metering cap. Before signing, the energy manager must check the guarantee, the battery's backup capability and whether inverter clipping will materially reduce yield.
1. Estimate the maximum annual solar fraction the plant can achieve with the proposed array and battery, and judge the 70% guarantee.
Read the worked answer
Daily load = (120 kW × 8 h) + (40 kW × 16 h) = 960 + 640 = 1,600 kWh/day.
Average PV generation = 250 kWp × 1,500 kWh/kWp/year ÷ 365 = 1,026.7 kWh/day.
Even with zero losses, the PV/load ratio is 1,026.7/1,600 = 64.2%. A 70% guarantee would require 1,120 kWh/day, which exceeds the array's entire average output. The guarantee is therefore impossible before any battery or conversion losses are counted.
A realistic check: daytime load is 960 kWh, leaving at most 1,026.7 − 960 = 66.7 kWh of surplus. After battery round-trip and discharge conversion, useful night energy is 66.7 × 0.92 × 0.95 = 58.3 kWh. Useful solar energy ≈ 960 + 58.3 = 1,018.3 kWh/day, or about 63.6% of load. Seasonal variation, soiling and clipping will reduce this further. The vendor's 70% figure is not supportable.
2. Determine whether the 400 kWh battery can provide the requested 4-hour night backup, and state the main caveat.
Read the worked answer
Usable DC energy = 400 × 0.90 = 360 kWh. AC energy delivered at discharge = 360 × 0.95 = 342 kWh.
Night load is 40 kW, so autonomy = 342/40 = 8.55 h. This comfortably exceeds the 4 h requirement, and the discharge rate is only 0.1C, well within normal lithium-ion capability.
Caveat: the same battery is also used to absorb the daytime PV surplus. If 66.7 kWh is charged daily, the energy left for backup is about 360 − 66.7 = 293 kWh DC, or 278 kWh AC, giving 6.95 h. Even at 80% end-of-life capacity, the available energy is roughly 400 × 0.8 × 0.9 × 0.95 = 273.6 kWh, or 6.8 h at 40 kW. The 4 h backup requirement is met, but only if the battery is reserved and not depleted by arbitrage or PV shifting.
3. Assess whether the 1.25 DC/AC ratio and 200 kW inverter are a material risk to the yield guarantee.
Read the worked answer
At a 1.25 DC/AC ratio, clipping begins only when array output exceeds 200 kW, which occurs near midday in high-irradiance months. For a well-oriented array in Pune, the annual clipping loss at this ratio is typically only a few percent, not the roughly 6 percentage-point shortfall between the actual 64% solar fraction and the 70% guarantee.
The guarantee shortfall is about 1,600 × 0.06 = 96 kWh/day, or 35 MWh/year. Clipping at this DC/AC ratio is unlikely to be the dominant cause. The inverter is therefore not the main risk, although an hourly simulation should quantify clipping and check temperature derating. If the grid connection allows, a 210–220 kW inverter would reduce clipping at modest cost, but it cannot rescue an oversized energy guarantee.