Solar Payback From Your Own Bill
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A solar quote can look attractive while the bill tells a different story. Use your units, your rate and the vendor's capex to screen payback before the sales deck decides for you.
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The formula we use
annualGen = kWp * yield
selfUse = annualGen * share/100 valued at unit rate
export = annualGen * (1 - share/100) * exportRate
annualValue = selfUse + export
simple payback years = (kWp * capexPerKwp) / annualValue
- No degradation, O&M or tariff-change modelling.
- Net-billing treatment, grid-support charges and banking rules change the real answer.
- Monthly units are a sense check only; the formula uses plant size and annual yield.
Typical numbers
Reference figures to sanity-check your inputs. Your own bill and equipment data always win.
BillTrends' current default rooftop-solar yield table totals 1,485 kWh per kWp per year before site-specific edits.
Source: BillTrends default model
BillTrends' default monthly rooftop-solar yield table ranges from 95 to 152 kWh/kWp/month.
Source: BillTrends default model
PM Surya Ghar is a residential rooftop scheme with subsidy capped at 3 kW of system capacity.
Source: PM Surya Ghar (MNRE)
PVWatts estimates grid-connected PV energy output from system size, location, tilt, azimuth, losses and weather data rather than a single national yield number.
Source: NREL PVWatts
Common questions
Is simple payback enough for a solar decision?
No. It is a first screen. Degradation, O&M, taxes, finance cost, net-billing rules and grid charges can change the decision.
What is specific yield?
Specific yield is annual generation per kWp installed. It varies by city, roof, tilt, shadow, module quality and maintenance.
Why does self-use share matter?
Units used inside the site offset your bill rate. Exported units may earn a lower rate or face different rules.