The payback model holds the battery at 238.74 kWh for ten straight years. The same product page publishes two throughput figures that disagree by a factor of 4.5, and neither of them is that.
Two published throughput figures on the same page
The published specification for the 125 kW / 265 kWh all-in-one cabinet, model BS07-265-ES-X, states a usable energy of 238.74 kWh at 90 percent depth of discharge, a cycle life of at least 8,000 cycles at 25 degrees C and 0.5C ending at 70 percent, and a cumulative discharge energy of 426.0 MWh.
Read the last two together. Eight thousand cycles at 238.74 kWh a cycle is 1,909.9 MWh of discharge. The published cumulative figure is 426.0 MWh, which is 1,784 cycles at the same usable energy. The gap is 4.5 times, and the cumulative figure is 22.3 percent of what the cycle rating implies.
The pattern repeats: the 225 kWh cabinet publishes 361.4 MWh, which is 1,784 cycles at its 202.56 kWh usable, and the 192 kWh unit publishes 309.8 MWh, which is 1,784 cycles at 173.62 kWh. All three equal about 1.61 times their rated energy, so the cumulative figure looks like a convention rather than a measurement. The published text does not state what it is cumulative over, and I will not guess.
That unstated basis is the largest number in a revenue model. Enter 8,000 cycles and the asset has 1,909.9 MWh to sell. Enter 426.0 MWh and it has 426.0.
Conditions: usable energy at 90 percent depth of discharge; cycle life as published. Source: published specification states, three cabinets in the current range, 2026.
What the fade curve does to usable energy
The end-of-life point is 70 percent, reached after at least 8,000 cycles, so the two published numbers pair into a fade rate: 30 percentage points over 8,000 cycles, or 0.00375 points a cycle.
At 330 cycles a year that is 1.24 points a year. Ten years takes the cabinet to 87.6 percent and 209.2 kWh a cycle instead of 238.74. At 600 cycles a year it is 2.25 points a year, 77.5 percent at year ten, and 185.0 kWh a cycle.
Set the cycle rating beside the warranty term. Eight thousand cycles at 330 a year is 24.2 years, and at 600 a year 13.3 years, against a published warranty of 5+5 years. On both duties the calendar term arrives first, by a factor of 2.4 at the lower duty and 1.3 at the higher. That is my reading of two published figures, not a claim that either is a guaranteed minimum.
Where a level-savings payback goes wrong
Worked example. Take a project whose level-savings model returns six years.
The level model sells 238.74 kWh a cycle, 330 cycles a year: 78.8 MWh a year and 787.8 MWh over ten years. With the fade above it delivers 739.1 MWh, so the level model claims 48.8 MWh more than the fade allows, or 6.2 percent, and the payback moves from 6.0 years to 6.2.
At 600 cycles a year the erosion scales with the duty: 1,432.4 MWh claimed against 1,271.3 MWh delivered, or 11.3 percent, and the payback moves from 6.0 years to 6.4.
If the model is fed the cycle rating while the site runs 330 cycles a year, the ten-year claim is overstated by 6.2 percent; if it is fed the cumulative discharge figure instead, that duty exhausts 426.0 MWh in 5.4 years and the payback argument is over before year six.
Capacity fade is a modest correction to the years and a larger correction to the energy a model may claim. The MWh error matters more, because throughput is what a warranty cap and an augmentation plan are written against. If the saving is set by a demand charge rather than by throughput, the correction does not apply.
Efficiency is published as a maximum
Max efficiency is published as 98.5 percent for this cabinet, as a maximum and as a beginning-of-life figure. No end-of-life efficiency figure appears on the page, so a ten-year model that holds 98.5 percent assumes the most favourable case rather than the stated one.
Two boundaries belong beside it. Round-trip efficiency runs from the battery terminals back to them, converter efficiency is measured at the converter, and system efficiency includes cooling and auxiliary load. Mixing the three overstates the saving.
Which revenue streams the fade reaches
Energy-limited revenue scales with usable kWh, so arbitrage and solar self-consumption fall directly with the numbers above.
Power-limited revenue is set by the kW the site holds below a target. A 20-minute peak often needs far less than a full cycle, so fade reaches it only when the peak's energy requirement approaches the energy still available.
Backup is a reserve rather than a throughput; fade shortens the ride-through duration. One fade rate, three revenue outcomes, and they belong on three lines of the model.
What to ask, and what is settled at signature
Ask what the cumulative discharge energy is cumulative over, and in what unit. A reply that repeats the figure without its basis leaves the largest number in the model undefined.
Ask which limit the warranty enforces first, cycles or years, and what the throughput cap is at that point. A reply of whichever comes first, with no throughput figure, is not an answer.
Ask for the retained-capacity test conditions (temperature, C-rate and depth of discharge) beside the 70 percent. Without them the figure cannot be compared with another supplier's.
Four things stop being negotiable at signature. The retention definition is fixed once the warranty is signed, so a 70 percent point stated at 25 degrees C and 0.5C cannot be reopened if the site later runs hotter or harder. The throughput cap is fixed once the commercial terms are signed. What the BMS counts as a cycle is fixed at commissioning. And the measurement boundary of the acceptance test is fixed once that test is signed off.
Ruibit distributes this equipment, so every figure above is the manufacturer's published number rather than cycle-test data held by the party selling it.
The assumption worth dropping is that degradation drives the answer. It is a modest correction to the years and a large one to the energy a model may claim; the published throughput basis decides which.
FAQs
1. Does degradation lengthen the payback period?
Yes, but by less than most models assume. Where savings are proportional to throughput, a fade of about 1.24 percentage points a year moves a six-year payback to 6.2 years. The larger error is the energy claimed: the same project overstates ten-year throughput by 6.2 percent, or 48.8 MWh.
2. How do cycle life and cumulative discharge energy differ?
Two throughput figures on the same page, disagreeing by a factor of 4.5. A cycle life of at least 8,000 cycles at 238.74 kWh usable implies 1,909.9 MWh; the cumulative discharge energy published beside it is 426.0 MWh, which is 1,784 cycles. The page does not state what that figure is cumulative over, so ask in writing.
3. Does capacity fade reduce demand-charge savings?
Barely, unless the peak is energy-limited. A demand charge is set by the highest kW imported, so the value depends on the power held below the target, not on the kWh available. A short 20-minute peak usually needs far less than a full cycle. Energy-limited revenue, such as arbitrage and solar self-consumption, falls directly with usable kWh instead.
4. Is efficiency also affected by degradation?
Not in the published data. Max efficiency appears as 98.5 percent, a beginning-of-life maximum; no end-of-life figure is published for the same unit. A ten-year model holding 98.5 percent for the whole horizon assumes the best case rather than the stated one.
