SolarToolsHow This Tool Works
The Solar Savings Calculator projects 25 years of savings from a residential solar system. Unlike simple payback calculators that divide cost by year-1 savings, this tool accounts for three factors that dramatically change the real return: panel degradation (output falls ~0.5% per year), electricity price inflation (rates rise ~3.5% per year on average), and net metering policy (do you get full retail credit for exports, or only wholesale?).
The headline number is 25-year net savings — total savings over the warranty period minus the system cost. For a typical US 7.5 kW system costing $17,500 after incentives, with 5 PSH, $0.16/kWh electricity, and 1:1 net metering, the 25-year net savings usually lands between $25,000 and $40,000. That's an ROI of 140–225% over 25 years, or 4–7% annualized — comparable to a conservative stock portfolio but with lower volatility.
Two inputs make or break the calculation. First, the net metering rate: if your utility pays full retail for exports, savings are dramatic; if they pay avoided-cost (~$0.04/kWh), savings drop 30–50%. Second, inflation: at 3.5% annual rate increases, year-25 savings are 2.5× year-1 savings. At 0% inflation, savings stay flat in nominal dollars.
For sizing the system itself, use the Solar Panel Sizing Calculator first. For battery economics, use the Battery ROI Calculator. This tool answers the financial question: is solar worth it over the long haul?
- System size (kW) — total DC rating. Use the Solar Panel Sizing Calculator to find this.
- Peak sun hours — annual average for your location (NREL maps for US, SAREC for South Africa).
- System losses — leave at 14% unless you have specific data.
- Electricity rate — your current $/kWh. Find on your bill.
- Net metering rate — what you get for exports. Same as retail = 1:1 NM. Check your utility policy.
- Annual usage — 12-month total from your bills.
- Installed cost — turnkey quote from installer, BEFORE incentives.
- Incentives — The federal ITC expired Dec 31, 2025. Enter any state/local rebates. South African: section 12B.
- Inflation — leave at 3.5% (US 10-year average).
- Degradation — 0.5% for Tier-1 panels. 0.4% for premium (SunPower, LG).
The 25-year ROI is the most useful headline. Anything above 100% (i.e. doubling your money) is a strong investment. Below 50% suggests poor economics — usually because of low net metering rates or cheap local electricity.
When to Use This Calculator
Why 25 years?
25 years is the standard panel warranty period. Tier-1 panels are warrantied to produce 80–85% of nameplate output at year 25. The 0.5% annual degradation rate compounds: at year 25 a panel produces (1 - 0.005)^25 = 88.2% of original output. This degradation is built into the calculator. Most panels outlast their warranty — 30+ year lifespans are common — but projecting beyond 25 years gets speculative.
The three forces driving lifetime savings
First, panel degradation reduces production ~0.5% annually, so year-25 production is ~12% lower than year-1. Second, electricity inflation increases the value of each kWh saved — at 3.5% annual increases, year-25 kWh are worth 2.36× year-1 kWh. Third, net metering policy determines how much each exported kWh is worth, which can change over time. The first two are predictable; the third is political risk.
The net metering cliff
Net metering reform is the single biggest risk to solar economics. California's NEM 3.0 (April 2023) cut export rates from ~$0.30/kWh to ~$0.08/kWh, extending payback periods from 6 years to 10–12 years overnight. Other states are considering similar reforms. The calculator lets you model this: set the NM rate to your current policy, but be aware that policy can change. If your NM rate is currently 1:1 retail, consider sizing for self-consumption rather than export.
Inflation: the silent multiplier
US residential electricity prices rose 3.5% annually from 2013–2023. Some states (MA, CA, HI) saw 5%+. Over 25 years at 3.5% inflation, year-25 savings are 2.36× year-1 savings. Cumulative 25-year savings are 1.6× what they'd be at 0% inflation. This is why solar looks better the longer you hold it — the asset produces the same kWh but each one is worth more.
What the calculator doesn't model
Three things: inverter replacement (string inverters last 10–15 years, $1,500–$3,000 replacement around year 12), roof replacement (if your roof needs replacing, panels must be removed and reinstalled — $2,000–$5,000), and property tax effects (most US states exempt solar from property tax assessment, but verify). For a conservative estimate, subtract $5,000 from the 25-year savings to cover these contingencies.
Frequently Asked Questions
For most US homeowners with south-facing unshaded roofs and net metering, yes — 25-year net savings typically exceed $20,000 on a $17,500 system. The exception is shaded roofs, low-electricity-rate regions (Pacific Northwest), or no net metering. Run the calculator with your actual numbers.
Typically 6–10 years in the US with full net metering, 10–14 years with reduced net metering (NEM 3.0), and 8–12 years in South Africa. The calculator returns the exact payback year for your inputs.
Net metering is the policy where your utility credits you for solar electricity you export to the grid. 1:1 net metering (full retail credit) makes solar very profitable. Reduced net metering (wholesale rates) cuts savings 30–50%. Check your utility's current policy.
Yes, multiple studies (Lawrence Berkeley National Lab, Zillow) show solar increases US home values by ~4% on average. This isn't included in the calculator — add it to your mental ROI.
Panels continue producing at ~80% of original output for many more years — 30+ year lifespans are common. Inverters typically need replacement around year 12–15 ($1,500–$3,000). After year 25 the system is essentially free energy, minus maintenance.
Further Reading
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Energy & Solar Glossary
Plain-English definitions of every term used in this calculator and across the site.