Data & method

Every number on this site, with its source and verification date. Data as of 2026-10-18.

Assumptions

Values the calculator applies when you do not override them
Assumption	Value	What it means
Evening peak share of usage	30%	Share of annual usage assumed to fall in the evening peak window — caps how much discharge can be shifted. Derivation: the SCE peak window is 5 h of 24 (~21% of hours); California residential load in the 4–9 p.m. window runs ~1.4× the daily average, giving ~30%. A planning assumption, not a metered value for your home — adjust the cycles input if your load profile differs.
Default cycles per day	1	One full charge/discharge cycle per day is the planning default for peak shifting.
Default round-trip efficiency	90%	Energy out divided by energy in; the remainder is lost as heat.
Default annual degradation	2%	Capacity fade applied each year, compounding across the horizon.
Default rate escalation	5%	Assumed annual increase in the value of avoided electricity purchases.
Analysis horizon	10 years	Window over which discounted savings are accumulated.

The complete model — nothing else is applied to your inputs
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

Southern California Edison — TOU-D-PRIME rates

peak $0.61 / off-peak $0.26 / super off-peak $0.24 / avg retail $0.345 per kWh

as of 2026-10-18 · updated Quarterly · Source

Export opportunity cost (avoided cost average)

$0.06/kWh (range $0.03–$0.21)

as of 2026-10-18 · updated Quarterly · Source

Evening peak share of usage

30%

as of 2026-10-18 · updated Annual · Source

Installed cost reference — California

$700–$1,300 per usable kWh · main panel upgrade $3,500–$5,500

as of 2026-10-18 · updated Quarterly · Source

Tesla Powerwall 3 — usable capacity and cost

13.5 kWh · 11.5 kW · $12,500–$16,500

as of 2026-10-18 · updated Quarterly · Source

Enphase IQ Battery 5P — usable capacity and cost

5 kWh · 3.84 kW · $5,000–$7,000

as of 2026-10-18 · updated Quarterly · Source

FranklinWH aGate — usable capacity and cost

13.6 kWh · 10 kW · $9,500–$11,500

as of 2026-10-18 · updated Quarterly · Source

Residential Clean Energy Credit (25D)

terminated