Battery economics · two products separated
Solar battery calculator — savings or resilience?
Most installer proposals bundle a battery's TOU-arbitrage savings and its outage-resilience value into a single payback number — double-counting the same battery capacity. This calculator separates them so you know which product you're buying.
Updated May 16, 2026.
Battery + utility + outage profile
Verdict — savings vs resilience
Savings-only payback of 8.8 years is inside the 10-year warranty horizon but past most homeowners' break-even comfort window. The case stands up only when resilience value is treated as a meaningful component. Combined payback (savings + resilience): 8.7 years.
Most installer proposals stack both numbers into a single payback. We surface them separately so you can decide which product you're actually buying.
What this audit can't see
Demand-charge offset (commercial / TOU-D-style residential rates often have separate demand components). Round-trip efficiency losses (~10% lost in charge/discharge cycle). Battery degradation curve beyond warranty floor. Whether your installer will void warranty on aggressive cycling. Critical-load panel installation cost if not bundled. The economics above are a first-pass; verify with your specific utility's rate sheet.
How the calculator thinks.
Savings product (TOU arbitrage): annual savings = daily cycles × usable kWh × (peak − off-peak rate spread) × 365. The battery charges during off-peak hours (cheap grid power or midday solar surplus) and discharges during peak hours. Spread × cycles is the daily profit; multiply by 365 for the year. Flat-rate utilities have zero spread, so savings collapse to zero.
Resilience product (backup value): annual value = outage hours × estimated load during outage × your stated dollar-value per kWh of kept-running power. We approximate the load at 2 kW continuous — a refrigerator, a few lights, modem and router, occasional small appliance. The value-per-kWh is yours to set; many homeowners value backup power well above retail rate because the alternative is spoiled groceries, lost work, or medical-device interruption.
Why separate? Berkeley Lab data1 documents that after California shifted to net-billing compensation in April 2023, battery attachment on new residential PV jumped from 14% to roughly 60% — homeowners installing batteries as a savings product to capture the midday production they could no longer profitably export. Meanwhile in regions with frequent outages but flat utility rates (hurricane states, fire-prone areas), batteries are bought as resilience productswith weak savings cases. Same equipment, different buying decisions. Conflating them produces payback math that doesn't survive scrutiny.
For context on why CA shifted: see the net metering vs net billing guide.
- 1. Berkeley Lab, "Tracking the Sun, 2024 Edition" (Executive Summary, August 2024; data through year-end 2023; sample ~3.7M U.S. distributed PV systems). State-level median residential installed price in 2023: $3.20–5.20/W. Loan dealer-fee gap, verbatim: "a large portion of residential systems are loan-financed, and installed prices reported for these systems likely include dealer fees, adding anywhere from 5-50% to the total up-front price paid by the customer." Verified 2026-05-16. emp.lbl.gov · Tracking the Sun 2024 (PDF) ↩
Next: Run the full quote audit on the whole proposal.
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Hi, I'm the TrueSolarCost assistant. I answer questions about how to read a residential solar proposal, what the calculators on this site compute, and what the public-data benchmarks (NREL PVWatts, EIA, IRS, LBNL, DOE, DSIRE) mean for the numbers in your quote. I'm not a tax professional, CPA, structural engineer, or licensed installer — for tax-position decisions talk to a CPA, for roof-condition or structural questions talk to a roofer or engineer, for utility-rate or interconnection specifics talk to your utility.