WA Solar + Battery Analysis — Upload your Synergy usage

1. Input data

Your CSV is parsed into 30-minute (timestamp, kWh) readings, combining the unbilled and billed columns. Only the most recent 365-day window is used — every individual data point, no averaging or sampling.

2. Solar generation model

Daily generation uses Perth-specific monthly yields (kWh per kW per day):

daily_gen_kWh = PERTH_YIELD[month] × solar_kW

That daily total is spread across 48 half-hour slots using a cosine-squared shape centred on 12:30, zero outside 6am–7pm:

slot_gen = daily_gen_kWh × SHAPE[hour, minute]   (Σ SHAPE = 1)

3. Simulation per half-hour slot

Solar only:

self_consumed = min(gen, usage)
grid_import   = usage − self_consumed
exported      = gen   − self_consumed

Solar + battery (SOC = state of charge; battery starts each day empty):

surplus = gen − self_consumed
deficit = usage − self_consumed

if surplus > 0:
    charge     = min(surplus, battery_kWh − SOC)
    SOC       += charge × 0.92            # round-trip efficiency
    exported   = surplus − charge

if deficit > 0:
    discharge   = min(deficit, SOC)
    SOC        -= discharge
    grid_import = deficit − discharge

Exports are tagged peak (3pm–9pm) or off-peak per slot. DEBS rates apply the same across every Synergy plan.

4. Annual bill per plan

import_cost   = Σ (import_by_hour[h] × rate(h, plan))
supply_cost   = daily_supply_charge × n_days
export_credit = export_peak × 0.10 + export_offpeak × 0.02
total_bill    = import_cost + supply_cost − export_credit

Plan rates (1 July 2025 schedule):

A1: flat 32.37c/kWh, supply 116.05c/day
Midday Saver: 8.62c (9am–3pm) · 53.84c (3pm–9pm) · 23.69c (9pm–9am), supply 129.23c/day
EV Add On: Midday Saver rates plus 19.38c (11pm–6am), supply 129.23c/day

5. Rebates and net system cost

Federal rebate is tiered from 1 May 2026 under the Cheaper Home Batteries Program (STC factor 6.8). Working dollar figures based on current STC spot price:

fed_rebate = (first 14 kWh)  × $252/kWh    # 100% of factor
           + (14–28 kWh next) × $151/kWh    # 60% of factor
           + (28–50 kWh next) × $38/kWh     # 15% of factor
           + (above 50 kWh)   × 0           # no rebate

wa_rebate   = $130 × min(battery_kWh, 10)   # WA Residential Battery Scheme, Synergy
battery_net = max(0, battery_gross − fed_rebate − wa_rebate)
option_B_total = solar_cost + battery_net

6. Savings and payback

solar_savings     = no_solar_bill − solar_only_bill
batt_savings      = no_solar_bill − solar_battery_bill
solar_payback_yrs = solar_cost     / solar_savings
batt_payback_yrs  = option_B_total / batt_savings
incremental_yrs   = battery_net    / (batt_savings − solar_savings)

The winner per scenario (No Solar / Solar Only / Solar + Battery) is the plan with the lowest total annual bill.

Assumptions and caveats

Your 365-day history is assumed to repeat each year (no load growth, no tariff escalation).
No solar panel degradation and no battery capacity fade over time.
Battery SOC resets to 0 at midnight each day (conservative — a real system holds charge).
Round-trip efficiency applied on the charge side only (92%).
Perth-specific yields; not accurate for other cities.
Generation shape is a smooth cosine curve — real output varies with cloud, shading, panel orientation.

Western Australia Solar + Battery Payback Analysis

System Parameters