TOU Arbitrage: When Battery Storage Actually Makes Financial Sense
TOU Arbitrage: When Battery Storage Actually Makes Financial Sense
The solar industry has a battery problem. Installers push batteries on every quote because they increase deal size by $8,000-12,000. Marketing materials emphasize backup power and "energy independence." But the financial case for residential batteries is narrower than the pitch suggests — it depends almost entirely on one variable: your utility's time-of-use (TOU) rate spread.
The Core Economics
A home battery stores solar energy generated during the day and discharges it during evening peak hours. The financial value comes from the difference between what you'd pay for grid electricity during peak and what the battery "cost" to charge (effectively free if charged from solar).
The math is simple:
Daily Arbitrage = Usable Capacity × Peak/Off-Peak Spread × Capacity Factor
For a 13.5 kWh battery (Tesla Powerwall-class):
- Usable capacity after round-trip efficiency (90%): 12.15 kWh
- Capacity factor (accounting for days with insufficient solar, partial cycles): 75%
- Effective daily discharge: 9.11 kWh
The spread is everything. At $0.10/kWh spread (weak TOU), daily arbitrage is $0.91. At $0.33/kWh spread (SDG&E in California), it's $3.01.
Annualized:
- Weak TOU: $332/year
- Strong TOU: $1,099/year
Against a net battery cost of ~$8,400 (after 30% ITC), weak TOU gives you a 25+ year simple payback. Strong TOU gives you ~8 years. The 25-year NPV swings from deeply negative to solidly positive based on this single variable. For the full NPV methodology behind these calculations, see our solar ROI guide.
Battery Cost Structure
Current residential battery economics (2025-2026):
| Component | Cost |
|---|---|
| 13.5 kWh lithium-ion battery | $8,500-10,000 |
| Installation labor + electrical | $1,500-2,500 |
| Permitting + inspection | $500-800 |
| Typical gross cost | $10,500-13,000 |
| Federal ITC (30%) | -$3,150 to -$3,900 |
| Net cost | $7,350-9,100 |
We use $12,000 gross / $8,400 net as a representative mid-point for analysis.
Battery costs are declining at approximately 8-12% per year. By 2028, the same 13.5 kWh system should cost $7,500-9,000 gross — improving ROI significantly. But waiting also means missing years of arbitrage revenue, so the optimal timing depends on your TOU spread.
Battery Degradation
Lithium-ion batteries degrade with use. A well-managed system loses approximately 2-3% of capacity per year. At 2.5% annual degradation:
| Year | Remaining Capacity | Usable kWh |
|---|---|---|
| 1 | 100% | 12.15 |
| 5 | 88.1% | 10.71 |
| 10 | 77.6% | 9.43 |
| 15 | 68.4% | 8.31 |
| 20 | 60.3% | 7.32 |
| 25 | 53.1% | 6.45 |
By year 15, the battery holds about 68% of its original capacity. Most warranties guarantee 70% capacity at year 10. We model a battery replacement at year 12 at 50% of original cost ($6,000), which resets capacity and extends the system life.
This degradation significantly impacts long-term NPV. A battery that generates $1,099/year in year 1 generates only $583/year by year 25 (from degradation alone, before accounting for rate escalation which partially offsets it).
Where Batteries Work: TOU Utilities Ranked
We analyzed every major TOU utility in the US. The key metric is the peak/off-peak spread — the wider the spread, the better the battery economics.
Tier 1: Strong Battery Case (Spread > $0.20/kWh)
| Utility | State | Peak | Off-Peak | Spread | Annual Arbitrage | Battery NPV |
|---|---|---|---|---|---|---|
| SDG&E | CA | $0.580 | $0.140 | $0.440 | $1,463 | $13,800+ |
| PG&E | CA | $0.450 | $0.120 | $0.330 | $1,099 | $7,200+ |
| SCE | CA | $0.420 | $0.110 | $0.310 | $1,032 | $6,100+ |
| HECO | HI | $0.520 | $0.250 | $0.270 | $899 | $3,800+ |
| Con Edison | NY | $0.428 | $0.148 | $0.280 | $932 | $4,500+ |
California dominates Tier 1 because NEM 3.0 eliminated favorable export credits, making self-consumption via battery the primary way to capture solar value. SDG&E's extreme spread makes batteries almost mandatory for solar customers.
Tier 2: Moderate Battery Case (Spread $0.10-$0.20/kWh)
| Utility | State | Peak | Off-Peak | Spread | Annual Arbitrage | Battery NPV |
|---|---|---|---|---|---|---|
| Eversource CT | CT | $0.380 | $0.180 | $0.200 | $666 | $700+ |
| APS | AZ | $0.245 | $0.072 | $0.173 | $576 | -$400 |
| NV Energy | NV | $0.225 | $0.068 | $0.157 | $523 | -$1,400 |
| DTE | MI | $0.258 | $0.118 | $0.140 | $466 | -$2,600 |
| SRP | AZ | $0.218 | $0.068 | $0.150 | $499 | -$2,100 |
Tier 2 utilities have marginal battery economics. The TOU arbitrage alone doesn't justify the investment. However, if you value backup power at $500-1,500/year (reasonable in areas with frequent outages), these utilities can cross into positive territory.
Tier 3: Weak Battery Case (Spread < $0.10/kWh)
| Utility | State | Peak | Off-Peak | Spread | Annual Arbitrage | Battery NPV |
|---|---|---|---|---|---|---|
| Xcel CO | CO | $0.195 | $0.098 | $0.097 | $323 | -$4,200 |
| Georgia Power | GA | $0.195 | $0.078 | $0.117 | $390 | -$3,500 |
| Austin Energy | TX | $0.168 | $0.078 | $0.090 | $300 | -$4,700 |
| ComEd | IL | $0.218 | $0.098 | $0.120 | $399 | -$3,300 |
| BGE | MD | $0.215 | $0.108 | $0.107 | $356 | -$3,900 |
For Tier 3 utilities, batteries don't pencil on TOU arbitrage alone. These customers are better served by solar-only installations with 1:1 net metering, which effectively uses the grid as a free battery.
Flat-Rate and Tiered Utilities
You can run these numbers for your specific utility with Elovane's free calculator.
For utilities without TOU rates (flat or tiered structures), battery arbitrage value is near zero. There's no peak/off-peak spread to exploit. The only financial value comes from:
- Net metering optimization — if your utility credits exports below retail rate, storing solar for self-consumption captures the retail/export gap
- Demand charge reduction — some utilities charge based on peak demand (kW), not just consumption (kWh). Batteries can shave peaks.
- Grid services programs — some utilities pay for batteries enrolled in virtual power plant (VPP) or demand response programs. Payments range from $30-75/month.
Without these, flat-rate customers should skip the battery and invest the $8,400 in additional solar panels or other improvements. Check your state's net metering policy to understand whether the grid effectively serves as your free battery.
The NEM 3.0 Effect
California's transition from NEM 2.0 to NEM 3.0 in April 2023 transformed battery economics for the state's three major IOUs. Under NEM 2.0, solar exports were credited at the full retail rate — effectively making the grid a free battery. Under NEM 3.0, exports are credited at "avoided cost" rates of $0.04-0.08/kWh.
This means a PG&E customer exporting a kWh at midday receives $0.06 instead of $0.32. The lost $0.26/kWh creates a massive incentive to store solar in a battery and self-consume during peak hours.
For NEM 3.0 customers, the battery value calculation includes both TOU arbitrage AND avoided export losses:
Battery Value = TOU Arbitrage + (Retail Rate - Export Rate) × Self-Consumed kWh
This nearly doubles the effective battery value in California, pushing payback periods below 5 years for SDG&E customers and below 7 years for PG&E and SCE.
The Decision Framework
Based on our analysis across all major US utilities:
Buy a battery if:
- Your utility has a TOU spread > $0.20/kWh (California, Hawaii, Con Edison)
- You're on NEM 3.0 or a similar low-export-credit policy
- You value backup power highly (medical equipment, frequent outages, remote location)
- Your utility offers a VPP or demand response program paying $50+/month
Skip the battery if:
- Your utility has flat rates with 1:1 net metering
- TOU spread is < $0.10/kWh with no supplementary value programs
- You're in a state where battery costs haven't declined to current averages
- You have limited capital and solar-only NPV is stronger than solar+battery NPV
Wait and evaluate if:
- Your utility is transitioning from 1:1 NEM to reduced export credits (many states are moving this direction)
- Battery prices are declining 10%+ per year in your market
- Your utility is launching new TOU rates or VPP programs
Model Your Specific Case
If you're in a fire zone, pair your battery investment analysis with wildfire hardening ROI analysis to understand how outage risk and insurance costs affect the backup power value of storage.
The analysis above uses representative costs and capacity factors. Your actual battery ROI depends on your specific utility rate, consumption pattern, solar production profile, and local battery costs. Run a personalized analysis with Elovane to see your exact numbers, including Monte Carlo sensitivity on the rate escalation that drives long-term battery value.