Where do
these numbers come from?

1. Hourly solar production and electricity use

Each day has 24 hourly intervals. Solar follows an illustrative 06:00–18:00 curve. Daytime demand is distributed evenly over 12 hours; nighttime demand is split between 18:00–22:00 and the remaining hours using your settings. Direct solar use each hour is the smaller of generation and demand.

The model repeats January–December in a standard 365-day year. Illustrative seasonal factors average 1 across the year; they are not actual weather or roof-survey data. Monthly sun hours replace average yield and seasonality, while shade and degradation remain. Optional inverter capacity caps hourly output before use or battery charging. Temperature and general losses are already reflected in net equivalent yield. Do not deduct shade or clipping again if included in the input. Equivalent sun hours mean net kWh/kWp/day, not daylight duration. Start with 4 hours/day and refine using installer data.

Optional hourly profiles describe representative weekdays and holidays using 24 kWh weights each. The scale factor is monthly energy ÷ days ÷ weighted average daily total, preventing double counting against the bill. Holidays can use more than weekdays. Solar, battery and exports are calculated for each day type before aggregation. Zero weights on active days fall back to the daytime/nighttime split. Demand changes compound by (1 + annual change) to the power of elapsed years, both before and after solar.

2. Batteries and surplus electricity

Surplus solar charges the battery for nighttime use, up to one cycle per day, with round-trip losses. Dispatch is limited by usable capacity and nighttime load, serving evening demand first. There is no grid charging. Optional AC charging/discharge limits and additional reserve apply; reserve is deducted from entered usable capacity. Hourly power limits are applied before monthly totals. Shared hybrid-inverter power and DC storage before clipping are not modeled. Capacity degrades annually and resets on replacement.

Surplus earns no revenue until export is enabled. Hourly exports are capped at the entered kW. Revenue starts and stops with the contract period and is before taxes or case-specific fees.

3. Bills before and after solar

Full bills are calculated from grid imports rather than valuing all generation at an average rate. Both sides use the same tariff category, with no automatic reclassification. Ft and VAT are editable. TOU uses an approximate off-peak-day share, not each date in the utility calendar. Phase count does not change the formula; energy is aggregated across phases.

MEA residential normal codes are 1.1 / 1.2 and low-voltage TOU is 1.3.2. PEA equivalents are 1.1.1 / 1.1.2 and 1.2.2. Follow your bill. Enabling TOU applies it before and after solar. Imports at 09:00–22:00 on the non-off-peak-day share are Peak; the rest are Off-peak. The default uses the same daily load profile. Solar before 09:00 or on off-peak days offsets the off-peak rate.

Tariff growth affects energy charges and Ft only; service charges remain fixed. Subsidies, free-electricity entitlements and personal tax benefits are excluded. The model supports low-voltage residential tariffs, not large-business demand charges.

4. Financing

At 0% interest, principal is divided by the payment count. After a promotional rate, reducing-balance payments are recalculated from remaining debt and term. Principal and interest are rounded to satang, with the final payment adjusted to clear the balance. Daily interest, penalties, overpayments and refinancing are excluded. Flat-rate loans do not use a separate promotional period.

5. Net cash flow, payback and NPV

Month 0 deducts the down payment, or full cash price, plus fees. Loan principal is not deducted twice because repayments include it. Maintenance is zero during the specified free years, then the entered annual cost in today’s money is inflated from installation and divided by 12. For two free years, payments start in month 25 at base maintenance × (1 + inflation) squared ÷ 12. Free cleaning does not waive equipment replacement. Full replacement costs occur at the start of the year after service life: 12 years means month 145. These costs also inflate.

Sustained payback is the first month after the last negative balance within the horizon, not a temporary crossing before replacement costs. Three cost-based cards use full installation price: installation only, plus maintenance, and plus maintenance and inverter replacement. They offset these costs with bill savings and exports, excluding interest, fees and battery replacement. Charts and net cash flow include all owner expenses. Cumulative savings start at zero and add bill savings and exports before costs; this is neither net profit nor a yearly reset. “After repayment” averages the 12 months after the last payment, or the first year for cash purchases, and is omitted if the horizon is too short. NPV discounts monthly net amounts using the compounded annual discount rate. Resale and salvage value are excluded.

6. References for optional settings

Inverter and battery power-limit principles reference NREL SAM: output and clipping andbattery charge/discharge powerThis website uses a simplified hourly model. It does not run SAM or forecast local weather.

7. Analysis and AI

Numeric observations follow calculator rules. Future scenarios vary yield and tariff growth to show sensitivity, not forecasts. Copy Prompt copies the latest assumptions and results for your chosen AI tool. Clicking it sends no data to an AI service.

Use these estimates to narrow options. Before investing, verify the roof survey, time-based loads, quotation, financing agreement and utility conditions.