Key Takeaways

  • Very high rooftop solar penetration lowers midday export prices and can produce negative price events; aim to avoid exporting low-value energy.
  • Time-of-use (TOU) signals shift value to evening peaks; typical spreads are $0.03–$0.05/kWh midday vs $0.25–$0.40/kWh evening under current market figures.
  • Practical battery sizing for most homes: 5–15 kWh usable capacity, sized to shift 2–6 hours of excess midday output into evening consumption.

What You Need to Know

When many homes have rooftop solar, midday grid supply can exceed local demand. That pushes wholesale and export prices down, and in some markets creates near-zero or negative export values during peak solar hours. Export credits (or buy-back rates) paid to homeowners often fall into the $0.02–$0.06/kWh range midday under current market figures, while evening import rates under TOU plans can be $0.25–$0.40/kWh. The result: the value of exported solar is much lower than self-consumed or shifted energy.

Two grid-side effects matter for homeowners:

  • Export limits: utilities or inverters may cap exports (0–5 kW typical) to manage local flow, so extra generation may be curtailed.
  • TOU signals: utilities increase off-peak/peak differentials to incentivize shifting loads; this expands arbitrage opportunities for batteries.

Battery basics that affect sizing and savings:

  • Round-trip efficiency: modern lithium-ion batteries are ~85–95% efficient.
  • Typical battery sizes: residential units commonly offer 5–15 kWh usable capacity (example: single-module ~13.5 kWh).
  • Lifespan and cycles: expect 3,000–5,000 full cycles or 10–15 years depending on use strategy and warranty.

How to Save Money

  1. Assess your export window: Track your system for 2–4 weeks to measure midday export (kWh) and evening import (kWh). Many homes export 2–6 kWh during peak sun; use that as a baseline for battery size.
  2. Calculate value of shifting: Use the difference between buy and sell prices. For example, shifting 1 kWh from $0.04 export to avoid a $0.30 import saves ~$0.26 before battery losses. With 90% efficiency the net save is ~ $0.23/kWh.
  3. Pick a sizing strategy: For most American households, consider these approaches: Minimize wasted exports — If average midday export is 4 kWh/day, a 5–7 kWh usable battery will capture most excess for evening use. Maximize evening coverage — To cover typical 3–5 hours of evening peak for a medium house, choose 10–15 kWh usable capacity. Balance cost vs. benefit — With battery installed costs around $400–$800/kWh under current market figures, a 10 kWh system could cost $4,000–$8,000 before incentives. At a net saving of $0.20–$0.25/kWh shifted, shifting 10 kWh/day saves ~$700–$900/year, so simple payback is typically 5–12 years depending on prices and incentives.
  4. Account for export limits and incentives: If your system has a 5 kW export cap or a low export tariff, prioritize storage sizing that captures generation above local load up to that cap. Also factor in federal or state incentives that reduce upfront cost.
  5. Operate smart: Program battery to avoid exporting during low-value midday hours and discharge into evening TOU peaks. Consider a hybrid strategy that reserves 10–20% capacity for backup if outage protection matters.

Bottom Line

As rooftop solar penetration rises, midday export prices fall and TOU peaks become more valuable. Homeowners should size batteries to capture typical midday exports and discharge into evening peaks: 5–7 kWh usable for modest export capture, 10–15 kWh for broader evening coverage. Use measured export/import data from your site, apply current price spreads (e.g., $0.03–$0.06 export vs $0.25–$0.40 import), and factor battery efficiency and installed cost ($400–$800/kWh under current market figures) to estimate payback and optimal capacity.