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.
Split the battery story
A battery is usually worth it for backup before it is worth it for pure bill savings. The exception is a utility tariff where exporting solar is paid poorly but evening power is expensive. If the proposal claims both stories at full value, split them apart.
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.
Two products, one box.
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. DOE's battery-storage guidance2and NREL's resilience guidance3 both frame storage as a backup / resilience tool, not just an energy-arbitrage product.
Why separate? Berkeley Lab data1documents California's 2023 battery attachment rate on new residential PV at about 14% — pre-NBT baseline, against a 12% national average. After the April 2023 shift to net-billing compensation, attachment has risen markedly: homeowners install batteries as a savings product to capture the midday production they can 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) ↩
- 2. U.S. Department of Energy, "Should I Get Battery Storage for My Solar Energy System?" DOE describes solar-plus-storage systems as providing power when solar production is unavailable or when backup from the grid is not available. Verified 2026-05-16. energy.gov · battery storage ↩
- 3. National Renewable Energy Laboratory, "Solar Resilience" text version. NREL notes that when solar-plus-storage is wired for backup power, a critical-load subpanel is commonly used to serve essential loads. Verified 2026-05-16. nrel.gov · Solar Resilience ↩
Next: Run the full quote audit on the whole proposal.