The inverter's AC output in watts, not the panel wattage. Most balcony kits are 600W or 800W. Check the inverter label or spec sheet.
If you don't know, you're almost certainly single-phase — it's standard for UK domestic properties.
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This tool applies the standard UK EREC G98/G99 thresholds (16A per phase for G98). It's general guidance, not a substitute for confirming with your own DNO. Find your DNO at the Energy Networks Association.