A time-of-use tariff charges different electricity prices during defined periods. Solar savings depend on which imports are avoided, when surplus is exported, and whether loads or batteries can move energy between low- and high-price windows. Annual generation alone cannot calculate the result.
Use interval data and the complete tariff schedule.
Read every tariff component
Record import rates by time, export rates, standing or fixed charges, demand charges where applicable, taxes, seasons, weekends, holidays, caps, and contract term.
The highest peak rate is not applied to every avoided kWh. Fixed charges often remain after solar.
Download current terms and note their effective date.
Match solar production to windows
Solar typically rises in the morning, peaks around midday under favourable conditions, and falls toward evening. Tariff peaks may occur later, when household demand rises and solar fades.
Roof direction can affect timing. West-facing panels may produce more late-afternoon energy than an equivalent east-facing section, while total annual output can differ.
Model each roof face using interval production.
Avoided import value
Solar used onsite avoids the import price at that moment. One kWh used during an expensive period can be worth more than one used off-peak.
Household self-consumption should be matched to the tariff interval. Applying the peak rate to all self-consumed generation inflates savings.
Consumption monitoring provides stronger evidence than a generic percentage.
Export value
Surplus receives the applicable export treatment, which may be fixed, time-varying, wholesale-linked, netted against imports, or uncompensated.
Export and import prices can differ substantially. Do not assume an exported kWh offsets an imported kWh one-for-one.
Policy and retailer terms can change during the system life.
Load shifting
Flexible loads can move into solar or lower-rate periods. Examples include laundry, dishwashing, water heating, pool pumps, cooling, and EV charging.
Automation can help, but load safety, comfort, appliance limits, and actual availability matter. Running wasteful loads merely to use solar does not create value.
Prioritise loads that would occur anyway.
Batteries
A battery can store midday solar or low-price grid electricity and discharge during expensive periods. The economic spread must cover round-trip losses, standby consumption, degradation, and equipment cost.
Reserve held for outages reduces daily tariff capacity. Exporting from storage can be attractive under some tariffs but must be permitted.
Model several operating strategies rather than assuming one full profitable cycle every day.
EV charging
EVs are large flexible loads when parked long enough. Direct daytime solar charging can increase self-consumption, while overnight off-peak charging may be cheaper or more convenient.
Compare solar export value with the later import price. Smart chargers can follow surplus or tariff schedules.
Use actual driving and parking patterns.
Demand charges
Some tariffs charge for the highest power demand during a period. Solar can reduce demand when production aligns, but a later household peak may remain unchanged.
Batteries can shave peaks if power, capacity, forecasting, and controls are sufficient. One missed peak can determine much of the billing period's charge.
Energy savings and demand savings should be calculated separately.
Seasonal schedules
Tariff windows and rates can change by season while solar and household demand also change. Summer cooling may align with solar; winter heating may occur when production is low.
Use at least a representative year. A single summer bill can misrepresent annual value.
Check daylight-saving treatment and billing timezone.
Choosing system orientation and size
A tariff can influence the value of east, south or north-equivalent, and west production depending on hemisphere and peak periods. It does not override shade, roof area, climate, or total yield.
Compare annual bill outcomes for plausible layouts rather than selecting orientation from the tariff alone. Tariffs can change before the roof does.
Tariff switching risk
A premium export or battery tariff may require a specific retailer, meter, device, or control program. Import rates and fixed charges can offset the advertised benefit.
Run the whole bill before switching. Review contract length, exit, rate variation, data access, and what happens when an incentive ends.
Do not design a decades-long system around one introductory offer.
A calculation workflow
- Obtain household interval consumption.
- Model solar production at the same interval.
- Apply self-consumption before exports.
- Add battery charging, loss, reserve, and discharge if relevant.
- Apply each import, export, fixed, and demand charge.
- Repeat by season and tariff scenario.
- Compare the annual bill with and without the project.
- Test tariff changes and household growth.
Common mistakes
- Applying peak import price to all solar generation.
- Ignoring fixed and demand charges.
- Treating export as equal to avoided import.
- Assuming a battery has no losses or wear.
- Using one average day for every season.
- Forgetting EV and heating schedule.
- Treating today's tariff as permanent.
Time-of-use solar savings come from timing, not merely energy totals. A transparent interval model shows whether panels, load shifting, or storage creates the strongest value for the household.