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How to Calculate Your Real Solar Payback Period
Solar salespeople often quote payback periods of "3-4 years" to close deals. The honest calculation for most Philippine residential installations is 5-8 years. Here's how to calculate it yourself — and why the salesperson's number is usually wrong.
The Basic Formula
Simple Payback = Total System Cost ÷ Annual Savings
But both numbers have hidden complexity.
Calculating Accurate Annual Savings
Factor | Optimistic (Sales Pitch) | Realistic (Engineering) |
|---|---|---|
System output | 100% of rated Wp × peak sun hours | 75-80% after derating (heat, soiling, wiring losses) |
Self-consumption rate | 100% (all solar used) | 60-80% (excess exported at lower rate) |
Electricity rate used | Current Meralco rate (₱12-14/kWh) | Blended rate after deducting fixed charges |
Annual degradation | Often ignored | 0.5% per year output loss |
Real Example: 5kW System in Metro Manila
System cost: ₱280,000 (installed)
Rated annual output: 5kW × 4.5 PSH × 365 days = 8,213 kWh/year
After derating (80%): 6,570 kWh/year actual production
Self-consumed (70%): 4,599 kWh @ ₱13/kWh = ₱59,787/year savings
Exported to grid (30%): 1,971 kWh @ ₱5.50/kWh credit = ₱10,841/year
Total annual benefit: ₱70,628/year
Simple payback: ₱280,000 ÷ ₱70,628 = 3.96 years
That's actually close to the sales pitch for a well-designed system at today's Meralco rates. But add in:
Inverter replacement at year 10-12: ₱35,000-50,000
Annual maintenance: ₱3,000-5,000/year
Panel degradation: year 10 output is ~95% of year 1
Adjusted 10-year payback including maintenance and inverter replacement: 5.5-6.5 years. Still excellent — the system continues generating savings for 20+ years after payback.
Use the SolarEnergyPH 25-Year Cashflow Calculator for a complete financial analysis tailored to your actual electricity consumption.
Engr. Jason Morales — Founder, SolarEnergyPH