Balance household load, solar surplus and battery limits

Capacity, usable energy and power answer different questions

Nominal capacity is the stored-energy rating in kilowatt-hours. The calculator reduces it by the entered depth of discharge, reserve and round-trip efficiency to estimate energy available to the household. It divides target delivered energy by the same factors to estimate required nominal capacity. Check product specifications carefully because manufacturers may state usable capacity separately.

Power is the rate at which the battery or inverter can supply electricity, measured in kilowatts. A battery can contain enough energy for an evening but still be unable to start or run several appliances together. The power check subtracts the entered simultaneous protected load from selected continuous output. Surge limits, phase configuration, power factor and grid interaction require a real system design.

Use interval data instead of one electricity bill average

Australian Government solar guidance recommends understanding not only how much electricity is used but when it is used. A smart meter may record 15- or 30-minute intervals. Obtain at least 12 months where possible and separate weekdays, weekends, seasons and unusual periods.

For each interval, identify household load, solar generation, self-consumption, export and import. The useful charging opportunity is excess generation after on-site loads, network export limits and system losses. A large annual export total does not prove a battery can fill on a winter day. Similarly, a high evening bill does not prove solar surplus exists to charge it.

The inputs on this page compress that time series into daily averages. Use them for scenario screening, then ask a retailer or independent adviser to model the full interval dataset. Review assumptions for degradation, tariff escalation, outages, load growth and control strategy.

A battery needs an energy source and a control strategy

Government guidance explains that a battery stores excess solar for later use and may reduce curtailment or grid purchases. If the solar system is too small or daytime consumption uses most generation, a large battery can remain partly empty. The calculator caps daily shifted energy by entered solar surplus, selected delivered capacity and target evening load.

AC-coupled and DC-coupled systems route energy differently and can have different equipment, conversion and retrofit implications. An installer should assess compatibility with the existing inverter, metering, phase arrangement, export controls and network connection. Do not assume a battery from one product family can attach to any solar system.

Battery management may prioritise bill savings, backup reserve, time-of-use arbitrage or a virtual power plant. These objectives can conflict. A high reserve improves outage readiness but reduces everyday capacity. Document the control mode used in financial modelling and who can change it.

Backup operation must be designed, not assumed

Australian Government battery guidance notes that a battery can support outages only when configured for backup, and not every system does so. Some arrangements provide selected circuits; others may disconnect solar during an outage. Request a single-line diagram and written explanation of the proposed behaviour.

Test backup at commissioning under controlled conditions. Verify high-load exclusions, changeover, monitoring, shutdown and restoration. Keep emergency instructions accessible even if internet service or a mobile app is unavailable.

Calculate bill value from the tariff difference

Each solar kilowatt-hour stored rather than exported loses the export value but can avoid a later import. The calculator multiplies annual shifted energy by the entered import tariff minus export value, then subtracts annual service or subscription cost. If export value equals or exceeds avoided import value, the model reports no positive simple payback.

Time-of-use plans require period-specific modelling. A battery may charge from low-price grid energy or discharge during a demand event, while a virtual power plant may pay incentives and take control under agreed conditions. Retail plans can change, so keep tariff dates and model alternative scenarios.

Simple payback divides net upfront cost by net annual bill value. It ignores finance, discount rate, degradation, replacement, residual value, tariff inflation, tax and opportunity cost. Use discounted cash flow for an investment decision and test whether warranty throughput or term covers the assumed operation.

Verify incentives against the current programme and quote

The Australian Government’s battery information describes the Cheaper Home Batteries Program and its connection to the Small-scale Renewable Energy Scheme. Programme settings, eligible capacity, product listing, installation date and retailer paperwork affect the value. The calculator deliberately provides a dollar input rather than estimating certificates or a national discount.

Check the current government programme page on the date of purchase and obtain the proposed incentive as a line item in writing. Confirm whether the quoted installed price is before or after that value and whether state, territory, network or virtual-power-plant offers interact with it. Do not subtract the same benefit twice.

An incentive can improve upfront economics but does not make an oversized, unsafe or incompatible system suitable. Capacity should still follow measured load, solar surplus, power needs and future plans.

Product, location and installer selection are safety decisions

Government guidance says to check approved products and use appropriately registered, licensed and accredited installers. Battery location has specific constraints, including surrounding materials and separation from openings or habitable spaces under applicable requirements. A preferred blank wall on a sketch may not be an acceptable installation location.

Discuss temperature, weather, flooding, impact, ventilation, access, fire response and noise. Protect the equipment from vehicles and storage. Do not enclose or modify it outside manufacturer instructions. Keep model and serial details, commissioning results, warranty, shutdown procedure and emergency contacts.

Monitor warnings and unusual heat, odour, swelling, sound or damage in accordance with manufacturer guidance. Do not open a battery enclosure. After flooding, fire exposure or impact, isolate the area and obtain specialist advice.

Compare battery proposals with the same operating case

Reject comparisons that use different loads, tariffs or incentive treatment without disclosure. Ask who owns monitoring data, what requires a subscription, how faults are supported in Australia and what happens if an inverter or battery component fails. A lower headline capacity can outperform a larger system when it better matches the property and control objective.