Battery Sizing Optimiser / Simulator – SolarPlus Help Centre

1. What the tool does

The optimiser sizes a grid-connected battery so that a chosen sizing metric (simple pay-back, first-year ROI or minimum annual cost) is maximised.

This sizing tool is located on the Map Design view accessed by clicking the 'Battery' icon. It uses the panel layout as currently set in the map view as the basis for the battery sizing analysis.
It compares:

Solar-only (no battery)
Optimal battery size
The next smaller and larger battery (to show sensitivity)

and returns clear economic and energy KPIs together with monthly and hourly profile charts.

2. Inputs to the battery analyser model

Category	Details	Typical source
Energy profiles	8 760 hourly values of site load (kWh) and PV production (kWh).	Loads from typical profile or imported usage data, solar production analysis is performed from the panel layout within the map view
Tariff	Hourly tables of import $/kWh, export $/kWh and (if applicable) demand $/kW.	Retail contract or tariff database.
Battery pricing	$ / kWh (cells & PCS) • Fixed installation cost • Rebates (fixed or per kWh).	Supplier quotation, incentive scheme.
Lifetime & C-rate	Warranty years for annualisation; maximum charge / discharge rate (C-rate).	Datasheets.
Solar cap-ex (optional)	Total PV system cost if you want to evaluate PV + battery together.	Project budget.
Optimisation mode	Pay-back, ROI or Cost.	Select in UI.

3. How the optimiser works (in plain language)

Dispatch simulation – Steps through the year hour-by-hour:
- PV first serves the load.
- Any surplus charges the battery (up to its C-rate & 10 % reserve).
- Remaining surplus is exported.
- If there is a deficit the battery discharges (always, or only in peak-tariff hours) before buying from the grid.
- Round-trip efficiency is fixed at 95 %.
Billing – Energy charges (import – export) are summed; if the tariff has a demand component, the monthly peak kW is multiplied by the demand rate and added.
Economics – For every candidate battery size the tool calculates:
- Annual bill with battery
- Annual savings versus the baseline (see “Baselines” below)
- Capital cost, annualised cost, ROI, pay-back, total annual cost
Search – Candidate sizes from 4 kWh upwards are tested; the size with the best selected metric wins. The tool also reports the closest smaller and larger sizes that differ by at least 4 kWh.
KPIs – Additional sizing indicators (battery utilisation, PV self-consumption, share of renewables, etc.) are derived.

4. Baselines

Solar-only baseline (default): evaluates the battery as an add-on to an existing PV system.
Grid-only baseline: tick Include solar cap-ex to view the combined PV + battery business case; in this mode PV cost is added to the investment and grid-only is used for the “do-nothing” bill.

5. Results returned

Group	Indicator	What it tells you
Economic	Simple pay-back (y), ROI (%), Annual savings ($), Annualised cap-ex ($ / y), Total annual cost ($ / y).	Financial attractiveness.
Billing	Annual bill with export ($).	Residual cost after PV and battery.
Investment	Up-front investment ($).	Cheque you have to write.
Sizing	Battery utilisation (%), Battery-to-load ratio, Solar-to-battery ratio, PV self-consumed (%), Powered-by-renewables (%).	How well the battery matches site PV and load.
Flags	“OK” / “Undersized solar”.	Quick check that PV can fill the battery most days.

A separate section supplies:

Average daily solar by month – 12-point column chart.
Average hourly profile (load & PV) – 24-point multi-line chart.

6. Interpreting the table

Solar-only column shows the business case for installing PV alone.
Optimal is the mathematically “best” battery under the chosen metric.
Undersize / Oversize help you see how sensitive economics are to sizing – if the numbers barely change you have a flat optimum.
Check battery utilisation: values well below 60 % usually mean the battery is larger than the PV can charge or the load can use.
If Solar-to-battery ratio > 1 the flag warns that PV is likely to leave the battery under-charged in winter.

7. Practical tips

Use at least one full year of load data; if unavailable, create a representative 8 760 using normalised profiles.
Split export tariff periods if they differ seasonally – it affects the value of exporting surplus PV.
When tariffs include demand charges, set the resolution to match the way peaks are billed (hourly, 30 min, etc.).
If a site is likely to export very little (high self-consumption), set the export tariff to zero to avoid over-valuing exports.
Remember that the tool does not model battery degradation or round-trip efficiency deterioration; adjust lifetime or costs if these are important.

8. Key assumptions

One-hour time-step (sub-hourly peaks are not captured).
Fixed 10 % state-of-charge reserve.
Single round-trip efficiency of 95 %.
No battery degradation, O&M or finance costs.
Demand charge is based on the monthly highest net-to-grid kW (coincident with load, not 15-minute interval).
Single existing tariff scenario (does include TOU, Demand, Seasonal tariffs)
No smart energy factors considered

Review these against project requirements before final sign-off.

9. Deliverables

Summary table with Solar-only, Undersize, Optimal, Oversize.
Two charts:
- Average daily solar (by month) – shows seasonality.
- Average hourly load & PV – shows typical daily shape.
All KPIs listed in section 5 for each scenario.

What the “Minimum Bill” or 'Least cost overlifetime” optimisation looks for

(sometimes nick-named “minimum-bill” or “levelised-cost” sizing)

Step	What is measured	How it is used in the search

Step	What is measured	How it is used in the search
1	Annual energy bill with the battery import charges export credits (if switched on) demand charges	This captures the running cost of the candidate battery size/strategy for one full typical year.
2	Annualised battery investment Up-front battery CAPEX ÷ design life (years)	The purchase price is spread evenly over the expected lifetime so it can be compared on a per-year basis with the energy bill. No discounting or finance charges are applied – just straight-line amortisation.
3	Total annual cost (step 1 + step 2)	This is the single figure the optimiser cares about when optimisation mode = “cost”.
4	Pick the winner	Among all candidate battery sizes (and both “always” / “peak-only” dispatch strategies) the algorithm simply chooses the configuration with the lowest total annual cost.

Why use the “Cost” mode?

It answers the question: “Which battery leaves me with the smallest yearly out-of-pocket expense once I’ve spread the purchase price over its life?”
It naturally penalises oversizing (higher CAPEX) and undersizing (higher grid bill).

What it does not do

It doesn’t look at payback speed or percentage return – those are the “Payback” and “ROI” modes.
It doesn’t time-value money (no NPV/DCF) – everything is treated in today’s dollars.
It doesn’t add solar CAPEX unless Include Solar Capex is active; that option puts PV and battery on the same footing.

In short, the “cost” optimiser is a straightforward bill-plus-amortisation comparison: whichever option gives you the cheapest total cost per year wins.