Solar + BESS ROI: How to Calculate Payback
Solar + BESS ROI measures whether the savings and benefits justify the investment, usually expressed as a payback period — and it depends on load profile, tariff, solar generation, demand charges, battery size and operating strategy.
Main sources of savings
Solar + BESS creates savings from several areas:
- Solar energy savings (reduced grid purchase)
- Maximum demand reduction (lower demand charges)
- Higher solar self-consumption (storing excess solar)
- Power expansion value (delaying grid upgrades)
- Reliability and resilience value (reduced operational risk)
The payback formula
A simple calculation is: Payback Period = Total Project Cost ÷ Annual Net Savings. The more accurate version is: Annual Net Savings = Energy Savings + Demand Savings + Other Benefits − O&M Cost.
How the example works
A worked example combines the project cost (RM) with the annual solar savings (RM), the annual maximum-demand savings (RM) and the annual O&M cost (RM). Annual net savings = solar savings + demand savings − O&M cost, and simple payback = total project cost ÷ annual net savings.
The actual numbers depend entirely on your site, so Solunar models the RM figures per project rather than quoting a fixed price. Share your electricity bill and load profile and we will prepare a project-specific payback estimate.
Data needed to calculate ROI
To calculate ROI properly, the following is needed:
- 12 months of electricity bills and demand charges
- Half-hourly load profile
- Existing solar generation data and tariff category
- Operating hours and load pattern
- Battery size and discharge duration
- Project cost, O&M cost and financing cost
- Battery degradation assumptions and expansion plans
Why bigger is not always better
A larger battery does not always create better ROI. An oversized battery increases project cost; if it is not fully used, payback weakens. A correctly sized BESS matches the site’s load pattern, solar profile and savings objective. The EMS also matters — poor control may discharge at the wrong time and reduce savings, so software and control strategy are as important as battery capacity.
Frequently Asked Questions
Payback is calculated by dividing total project cost by annual net savings.
Solar energy savings, maximum demand savings, self-consumption improvement and avoided operational costs may be included.
Electricity bills, load profile, maximum demand data, solar generation data and project cost are needed.
No. The BESS should be sized based on real site demand and savings potential.
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