Why Our Estimates Are More Conservative

Most plug-in solar brands advertise 1–3 year payback periods. We consistently show 5–7 years for most setups. Here's why our numbers are different — and why we believe ours are more accurate.

The Inputs That Matter

1. Solar Panel Output

A 600W panel system does not generate 600W continuously. Real-world output depends on:

  • Peak sun hours: The number of hours per day when sunlight intensity is equivalent to 1000W/m². This ranges from 3.2 hours/day in Alaska to 6.5 hours/day in Arizona.
  • Panel efficiency: Most plug-in solar panels are 20–22% efficient. Dust, angle, and temperature reduce this further.
  • System losses: Inverter losses, wiring losses, and battery round-trip efficiency (typically 85–92% for LFP batteries) reduce output.

Our formula: Daily kWh = Panel Watts × Peak Sun Hours × 0.80 (system efficiency factor)

For a 600W system in a 4.5 peak sun hour location: 600W × 4.5h × 0.80 = 2.16 kWh/day

2. Electricity Rate

We use EIA March 2026 state-average residential rates. We do not use utility-specific TOU peak rates unless the user specifically selects TOU mode — because not all users have TOU plans.

3. Self-Consumption Rate

Not all generated electricity displaces grid consumption. If you're away from home during peak generation hours, the battery may fill up and excess energy is lost (plug-in systems don't export to the grid in most configurations). We assume an 85% self-consumption rate for battery systems.

The TOU Optimization Factor

Users with Time-of-Use electricity plans can significantly improve their economics. TOU plans typically have peak rates 2–3x higher than off-peak rates. A system that charges at $0.12/kWh (off-peak) and displaces $0.35/kWh (peak) effectively earns $0.35/kWh in savings, not $0.18/kWh.

Our calculator includes a TOU mode that uses the spread between peak and off-peak rates for your state. This can reduce payback periods by 30–40% in high-spread states like California and Connecticut.

What We Don't Include

  • Degradation: LFP batteries degrade ~2–3% per year. We don't model this, which means our long-term savings are slightly optimistic.
  • Maintenance costs: Plug-in solar systems have minimal maintenance, but we don't include any maintenance cost assumptions.
  • Electricity rate escalation: Rates have risen ~6% per year recently. We use current rates, which means our long-term savings are conservative.

The Bottom Line

Plug-in solar is not a get-rich-quick investment. In most states at average electricity rates, payback is 5–7 years. In high-rate states with TOU optimization, payback can be 3–5 years. Note that the Federal ITC (30%) expired December 31, 2025 and is no longer available for new purchases. State incentives may still reduce your effective cost.

We'd rather give you an honest estimate than an optimistic one. Use our Savings Calculator to run the numbers for your specific situation.