Methodology and sources

• Output G = system size (kW) × 140 kWh/kW/month
• Self-use S = the smaller of G and (monthly use × daytime share)
• Export E = G − S
• Bill saving = Bill(use) − Bill(use − S)
• Export income = E × 2.20
• Payback (years) = (install cost − tax saved) ÷ (12 × (bill saving + export income))

• 140 kWh/kW/month is the midpoint of reports that a 5 kW system makes 600–800 kWh a month.
• The Ft surcharge, monthly service charge and VAT are not included yet.
• Panel degradation is not applied until we find a reliable source.
• Install cost is a range, so payback is a range.

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