Size a battery from your measured surplus and your tariff's actual price spreads. If the bill does not show the spread or the surplus, the battery has nothing to earn.
By the numbers
A 90% round-trip efficiency battery needs about 111 kWh of charge to deliver 100 kWh back.
Source: BillTrends default model
BillTrends sizes surplus-capture BESS as monthly surplus divided by 30 days and by 0.90 round-trip efficiency, rounded up.
Source: BillTrends default model
The BESS tool's load-shift alternative spreads working-day surplus over 8 hours and 26 working days.
Source: BillTrends default model
BillTrends' current BESS capex placeholder is Rs 28,000 per kWh before GST and 18% GST, or Rs 33,040 per kWh with GST.
Source: BillTrends default model
BillTrends' current default rooftop-solar yield table totals 1,485 kWh per kWp per year before site-specific edits.
Source: BillTrends default model
The three honest earning modes
First, time-of-day arbitrage: charge in cheap windows, discharge in costly ones. A 90% round-trip efficiency battery needs about 111 kWh of charge to deliver 100 kWh back, so the earning exists only if your tariff spread pays for that loss.
Second, solar self-consumption: if you export daytime solar at a poor export rate and import evening power at a high rate, a battery moves those units from the bad price to the good one.
Third, peak management: discharging through short demand peaks can protect a lower contract demand. This depends entirely on how brief and predictable your peaks are.
- Arbitrage needs a real ToD spread after round-trip efficiency.
- Self-consumption needs measured export surplus, not assumed surplus.
- Peak shaving needs short peaks; long peaks need a bigger, costlier battery.
Sizing from the bill, not the brochure
For solar consumers, the right basis is the actual per-slot daytime export surplus across many months, taken from the bills, using a typical month rather than the best one. BillTrends sizes surplus-capture BESS as monthly surplus divided by 30 days and by 0.90 round-trip efficiency, rounded up.
For arbitrage, the usable spread is what remains after charging losses, discharge losses and battery degradation. A spread that looks attractive gross can be thin net.
- Use the median month's surplus, not the peak month's.
- Count round-trip efficiency and degradation in the earning, not beside it.
- Model the battery against the tariff in force; BillTrends' current evening ToD premium for screening is Rs 2.00/kWh, not a tariff quote.
When the answer is no
A flat tariff with no ToD spread, no solar surplus, and long broad demand peaks leaves a battery with nothing to do. In those cases the recurring bill leaks (power factor, demand rightsizing, ToD placement of load itself) are cheaper wins and should come first.
BillTrends sizes BESS from your measured export surplus and tariff, shows NPV, IRR and payback with the assumptions disclosed, and is equally willing to show a case that does not pay.
Related BillTrends pages
FAQ
Is BESS worth it if I already have rooftop solar?
It depends on what your exports earn. If your bills show daytime export surplus compensated at a poor rate while evening imports cost much more, a battery sized from that measured surplus can pay. If exports are well compensated, the case weakens.
How big a battery do I need?
Size it from your bills: the per-slot export surplus or the peak you want to shave, using a typical month. BillTrends derives this from your actual billed slots rather than a generic profile.
Does a battery remove demand charges?
It can reduce billed demand if your peaks are short enough for the battery to carry. Long or unpredictable peaks need disproportionately large batteries, and the economics usually break.