How this calculator works
What the model counts, what it deliberately leaves out, and a worked example showing why the same battery gives different answers in different states. Data as of 2026-10-18.
Counted in the result
- Self-consumption uplift — in solar mode, each stored kWh is worth peak rate minus the export credit you give up.
- TOU arbitrage — in battery-only mode, each stored kWh is worth peak rate minus the off-peak charging rate.
- VPP earnings — your annual input, counted in both modes (dispatch payments do not depend on owning solar).
- Annual extra cost — any recurring cost you enter, subtracted every year.
- Degradation, escalation, round-trip losses — applied year by year across the analysis horizon.
- Incentives you enter — rebates reduce net cost; they are never auto-applied.
Excluded from the result
- Backup power — real value, not a cash flow. Shown separately, never in the payback ratio.
- Program performance payments — e.g. Connecticut's ten-year ESS payments: shown as VPP input, not defaults.
- Tax effects — Section 25D ended 2025-12-31; Section 48E belongs to third-party owners.
- Panel upgrades — include them in installed cost if your quote has them.
- Tariff redesign — today's structure is assumed to hold; that is an assumption, not a promise.
- Load changes — an EV or home office changes your evening peak, and the cap on discharge.
The same battery, two states
A 13.5 kWh battery on a $200 monthly bill, $15,400 installed. In California (SCE TOU-D-PRIME), exports earn about $0.06 while peak costs $0.61 — each self-consumed kWh is worth about $0.55, and the model shows a payback near 14.9 years with a negative ten-year net. In Texas on a representative flat plan, buyback credits exports at or near retail, so storing solar does not reduce the bill at all: the honest answer is $0 in bill savings, and the case rests on backup and VPP instead.
That contrast is the point of this site. A single-state calculator cannot show it, and a calculator that quietly assumes below-retail exports everywhere overstates savings in half the country.
The formula set
| effective_rates = custom prices (peak / off-peak / export) or state-pack values |
| — empty or invalid custom entries fall back to the state value, never zero |
| annual_usage = monthly_bill × 12 ÷ avg_retail_rate |
| discharge_y1 = min(battery_kwh × cycles × 365, annual_usage × evening_peak_share) × dMul |
| — dMul = 1.0 default; the usage scenario band uses 0.7 / 1.3 |
| discharge[t] = discharge_y1 × (1 − degradation)^t |
| solar-existing: saving[t] = discharge[t] × eff × (peak − export_rate) + VPP − annual_extra_cost |
| battery-only: saving[t] = discharge[t] × eff × (peak − off_peak) + VPP − annual_extra_cost |
| net_cost = installed_cost − rebate |
| payback = net_cost ÷ saving[0] |
| ten_year_net = Σ(t=0..9) saving[t] × (1 + escalation)^t − net_cost |
| break_even_price = Σ(t=0..9) saving[t] × (1 + escalation)^t |
| npv = Σ saving[t] × (1 + escalation)^t ÷ (1 + required_return)^t − net_cost |
| max_upfront_cost = the installed price at which NPV = 0 |
| sustained_payback = first year cumulative savings turn positive and stay non-negative |
| PRD v1.0.2: VPP earnings apply in both modes |
Uncertainty, shown not hidden
The largest unknown in any payback estimate is how much of your usage can actually shift into the battery. Instead of hiding that inside one number, the results screen shows a usage scenario band — lower (0.7×), central and higher (1.3×) transferable energy — with payback and ten-year net for each. The band describes energy, not better financial outcomes: the central row is the model's best estimate and the outer rows bracket it.
If your utility is not the state default, turn on use my own electricity prices in the input panel: peak, off-peak and export rates are prefilled from the state pack, and empty or invalid entries fall back to the state values rather than zero.
This page is educational. It is not financial, tax, or installation advice. Data as of 2026-10-18.