How the number is made.

1. 01

   Bills to hourly load

   Each bill gives a month of energy, often a recorded peak and sometimes energy by time slot. The shape of the day starts from the working hours of the site, is reshaped until every ToD slot carries the share its meter recorded, and is scaled so each month carries the energy billed. Without slot readings, load outside working hours is tuned until the modelled peak matches the recorded demand. Where solar is already installed, its simulated output is added back so the model sees the load, not just the grid import.

2. 02

   Solar, hour by hour

   A typical meteorological year from PVGIS gives hourly irradiance, temperature and wind for the exact site. Irradiance is transposed onto the array with the Hay and Davies model, corrected for angle of incidence, converted with the same module model PVGIS uses, then clipped at the inverter. Samples are interpolated onto the local clock, which keeps half hour time zones exact.

   PVGIS, EU Joint Research Centre
3. 03

   The tariff, line by line

   Every bill is rebuilt from the regulator schedule: slabs, time of day windows, fixed and demand charges with the billing demand floor and excess demand rate, kVAh billing and what solar does to power factor, adders, duty, and the export arrangement with its netting, carry forward, settlement and network charges. The modelled bill is checked against the uploaded bills and any steady difference is carried per unit.

4. 04

   Battery dispatch, optimised

   The battery is dispatched by dynamic programming over all 8,760 hours. It charges from surplus solar or the cheapest grid hours, discharges into the site at the most expensive ones, holds demand below the monthly level it can sustain, keeps a reserve, rides through outages against the diesel cost, and never exports. A small threshold stops it cycling for spreads that do not pay for the wear.

5. 05

   Ageing and money

   Solar output falls by its yearly rate. Battery capacity falls with calendar time and with every full cycle against the rated cycle life of the cell, and the battery is replaced when it reaches the set capacity. Each year of savings comes from re-running the year at that state of ageing. Cash flows carry tariff growth, O&M, insurance, replacements, GST, written down value depreciation, tax and an optional loan.

6. 06

   Finding the size

   Solar and battery sizes are searched on a grid, refined twice around the leader, then battery durations are compared. The recommendation is the design with the highest lifetime value that meets the return asked for, inside the budget, the roof and the size limit of the export scheme. When nothing meets the target, the closest design is shown and marked.