DIY solar: free off-grid solar simulator

The starting panel count compares daily use with the least productive average month in your chosen period. It is only a starting point: PVGIS SHScalc then checks panels, battery and demand using historical hourly data. The table shows days when the battery reaches its reserve and average unmet energy.

PVcalc estimates potential production with 14% losses, crystalline silicon and ventilated modules. SHScalc uses its off-grid performance ratio of 0.67 including losses; 14% or 95% is not applied again. These outputs describe different quantities. Daily demand is constant, including occasional use; May–September reports only those months of the full-year simulation. Nearby shade, snow, power limits, ageing and specific backup strategies are not simulated.

PVGIS methods and limitations — European Commission / JRC

Shade and obstacles. For every obstacle you draw (chimney, aerial, skylight, dormer, satellite dish, nearby trees and buildings) the tool works out the sun’s position on the 21st of each month, from sunrise to sunset, every 15 minutes, and follows the shadow on the roof face. A panel touched by shade even partly is counted as switched off at that moment, because its bypass diode cuts it out. For each panel it gives the loss over the year and in the worst month and advises whether to leave it (shade only early in the morning or late in the afternoon, when the sun is low), fit an optimiser or move it. The PVGIS output stays complete; the kWh lost to shade are listed month by month in the project. Carmine suggests optimisers only on the panels shaded during the good hours: when the shade passes a certain share the panel is cut out and a string of 6 works like a string of 5, while the others keep producing.

Strings and inverters. With the Voc and Isc of the panel you choose, the tool works out how many panels to put in series: Voc × 1.12 for cold weather, preferring one panel fewer to one more. The limits come from the Growatt inverter datasheets: 6 kW with 490 V cold, 16 A and 4,000 W per input; 12 kW with 540 V, 27 A and 7,500 W per input; start-up at 120 V. Two roof faces are never mixed on the same string. For single phase it proposes 6 kW, 12 kW or 2 or 3 inverters of 6 kW in parallel; for three phase 3 × 6 kW (18 kW) or 3 × 12 kW (36 kW).

Battery. It works out the capacity needed for the days without sun you choose and compares it with CEG batteries: 16 and 17 kWh counted at 80%, the 34 kWh in full, with the autonomy of each and the estimated charging current.