Solar Battery Sizing: Start with Measured Surplus and Backup Loads
In this guide
Size around chargeable surplus and the load you want to serve
Short answer: to store solar for evening use, start with the surplus that can actually charge the battery during the day and the load you want to shift into the evening. Do not size from panel kWp or total daily generation alone. For backup, start with essential loads and the desired backup duration, then check continuous/peak output for appliances running at the same time.
Ask the proposal to show an auditable chain: surplus kWh during the charging window → usable battery capacity after its stated loss assumptions → load kWh it can supply, alongside output kW and duration. If recurring surplus is too small to fill the offered battery, or loads can be shifted into solar hours, compare no battery or a smaller capacity before buying. If surplus is adequate but output cannot support simultaneous loads, kWh capacity alone does not solve the requirement.
Separate solar surplus from backup needs
For self-use, start with inverter and bill records of generation and exports over comparable periods. Identify when surplus occurs and whether it recurs. For backup, list essential appliances, their power and how long they need to run. These two purposes do not necessarily require the same battery size.
Do not convert panel capacity in kWp directly into battery size or savings. Generation varies by site and conditions; surplus after daytime loads varies by household. Ask the provider to state the data and assumptions used.
If the loads are a kettle and rice cooker, record each cooking cycle
Appliance names alone do not determine capacity. Record the input power on each nameplate, normal operating duration and whether the two appliances run at the same time. If you can measure energy for a complete cooking cycle, use that reading; nameplate power and operating time are only a first estimate, not measured kWh consumption.
Tell the provider whether cooking happens while the panels are generating, after sunset or during a power interruption. For each case, ask the written proposal to separate the energy that must be supplied from the power that must be available at the same time. Panel capacity in kWp is not inverter output in kWac, and a battery’s nominal capacity is not its usable capacity. Do not accept a size that omits the assumed loads and duration.
If you can measure one cooking cycle, a first daily-energy input is measured kWh per cycle × cycles per day. Record keep-warm use separately if relevant. This is only the load input; the provider still needs to show generation at the use time, system losses and equipment capacity in the design.
| Appliance | Model and nameplate input | Energy per cycle if measured | Use time/duration | Run together? | Must work during outage? |
|---|---|---|---|---|---|
| Electric kettle | ________ | ________ kWh / not measured | ________ | yes / no | yes / no |
| Rice cooker | ________ | ________ kWh / not measured | ________ | yes / no | yes / no |
When requesting a price, ask the provider to state panel kWp, inverter kWac, supported continuous and peak output, nominal and usable battery capacity, backed-up circuits and the operating conditions used in its design. If blackout backup is not a requirement, say so and compare no-battery or load-shifting options for the same purpose.
TNB’s published calculation example — not your battery-size recommendation
TNB’s domestic example uses a 5 kWp system and estimates 525 kWh monthly generation; it assumes 75% (394 kWh) is self-consumed and 131 kWh exported [source]. These are TNB’s illustrative inputs, not a prediction for your roof, a three-phase sizing recommendation or a battery capacity. The example shows why proposals should separate generation, self-use and export. A battery assessment still needs your own records and selected backup loads.
Require precise capacity and compatibility details
A written offer should state the battery model, nominal and usable capacity, charge/discharge power, assumed efficiency or losses, relevant operating limits and warranty. Ask for exact inverter and battery models, communications/firmware conditions, the manufacturer’s compatibility list, and the electrical/protection work included. A brand name alone is not proof of compatibility.
If backup during an outage matters, request the backup-circuit diagram or list and the output conditions allowed by the exact model. Do not assume the system can support the whole home or every appliance at once.
Compare options in writing
Ask the provider to compare: (1) no battery, (2) shifting loads into solar-generation hours, and (3) a battery based on actual exports and selected backup loads. For a battery, request total installed cost, included items, output limits, loss assumptions, warranty and the method used to estimate benefit. Mark missing information as unknown. A seller’s displayed price or a future-price guess is not a quotation for your system.
This is educational information and does not provide an individual calculation or equipment recommendation. Also read the home battery guide and solar with or without a battery.
Battery sizing inputs
| Purpose | Record | Ask the provider to state |
|---|---|---|
| Self-use | Inverter generation/export and bill over comparable periods | When surplus occurs and the assumptions used |
| Backup | Essential loads, power and desired support duration | Circuits and output supported by the exact model |
| Equipment | Nominal/usable capacity and charge/discharge power | Losses, compatibility, limits and written warranty |
| Cost | Full installed cost and included work | Compare no battery, load shifting and battery with the same assumptions |
Frequently asked questions
How do I know what battery capacity fits?
Start with inverter records and bills showing actual surplus exports over comparable periods, plus the evening loads you want to support. Ask the offer to distinguish nominal from usable capacity and state power and assumed losses.
Do I need a battery as large as all my evening usage?
Not necessarily. List essential loads and the backup duration you want. A battery also has output and usable-energy limits; ask the provider to show which loads the exact model can support.
How should I compare cost and benefit?
Request full installed cost and written assumptions for usage, exports, efficiency/losses, lifetime and warranty. Compare with no battery or shifting loads into generation hours. This guide does not calculate payback or promise savings.
Can I add a battery to an existing system?
Do not infer compatibility from a brand or a ‘battery-ready’ label. Check the inverter and battery models, communications, firmware, system limits and manufacturer documents; without model-specific evidence, treat compatibility as unconfirmed.
I want solar to boil water and cook rice. What panel and battery capacity do I need?
There is no fixed panel or battery size from appliance names alone. Record each model, nameplate input power, measured energy per cooking cycle if available, normal duration and timing, and whether both run at once. Ask the provider to separate daily energy from simultaneous inverter power and state the assumptions for panel kWp, inverter kWac and usable battery capacity in a written offer.