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Daily generation = system kW x 4. Self-used kWh = min(daily generation x 0.5, daily usage). Annual savings = (self-used x tariff + exported x feed-in tariff) x 365. Payback = system cost / annual savings.- Apply the formulaDaily generation = system kW x 4. Self-used kWh = min(daily generation x 0.5, daily usage). Annual savings = (self-used x tariff + exported x feed-in tariff) x 365. Payback = system cost / annual savings.4.9 yearsDaily generation 26.4 kWh; self-use 13.2 kWh/day; annual savings $1,638.12; 10-year net $8,381.