Battery Arbitrage Economic Case (Sezibwa)

Headline

The battery NFE already owns can earn money with no solar at all, simply by charging during the cheap off-peak window and discharging through the expensive evening peak. On the real site load this saves about 42,43,000 to 77,79,000 UGX per month,month which is roughly(about 11 to 20 percent of the energy bill,bill) depending on how much reserve is held back for backup. It is free money on owned hardware, but it is modest. Solar is the real story (see Solar Plus Battery Economic Case).

This case has no capex (the battery is already bought) and no solar, so weather does not affect it.

Method

This is a transparentTransparent scoping model using our own battery and tariff maths, run on the actual sitereal load, not OpenEMS. Load source is the site aggregate meter (meter_100), overabout 14421 days.kWh/day, Batteryvalidated isinternally against the SRNE10 unitcustomer atsubmeters (independent UEDCL bill still to confirm). Battery 8.9 kWh usable, 90 percent round round-trip, 5 kW charge and discharge cap. Tariff is UEDCL Code 10.2: off-peak 00:00 to 06:00 at 343 UGX per kWh,343, shoulder 06:00 to 18:00 at 577.11, peak 18:00753 toUGX/kWh. 24:00 at 753. Control policy is greedy with no forecast: charge from grid duringCharge off-peak toward full, discharge to cover loadthe through theevening peak down to a reserve floor, idle in shoulder.floor.

Why the site is a good arbitrage candidate

The load trough is overnight,overnight exactly(cheapest when power is cheapest,power) and the load peak is the evening (about 1.95 kW at 22:00), sitting inside the price peak. The site naturally consumes most when power is both most expensive and highest, so shifting that consumption to the off-peak window is worth real money.

The line shows grid power drawn across a day. Arbitrage adds a charge block in the cheap off-peak window (grid draw rises) and removes draw during the expensive evening peak (the battery covers it instead).

Results

Savings depend on how much of the battery is reserved for backup rather than used for arbitrage:

Reserve held for backup	Saving per month	Percent of energy bill	Peak kWh shifted per day	Cycles per day
0 percent (max saving)	77,400about 79,000	20.1about 20 percent	7.0	0.84
30 percent	58,400about 59,500	15.2about 15 percent	5.3	0.63
50 percent (safe backup)	42,500about 43,100	11.0about 11 percent	3.8	0.46

What it means

1. Arbitrage alone saves about 42,000 to 77,000 UGX per month (roughly 11 to 20 percent of the energy bill) on hardware NFE already owns, with zero solar. The percentage, not the shilling figure, is what tells you whether it is worth scaling. Modest but real.
2. The reliability dial is explicit and cheap on wear. Holding 50 percent for backup roughly halves the saving versus 0 percent, but at under one cycle per day on a 6,000 cycle battery that is 15 to 20 years or more of life. Arbitrage does not age the battery.
3. The logic that earns this (charge off-peak, discharge peak, hold a floor) is exactly what the inverter's own settings already do, so the native settings capture most of this today with no extra code. OpenEMS controllers only out-earn native settings once there is forecasting or solar to be smart about.
4. This confirms the strategic read: arbitrage is a nice-to-have, not the money story. Solar is (see Solar Plus Battery Economic Case, where the same battery plus a 4 kWp array offsets about 62 percent of the bill and pays back in under two years).

Investment metrics (15 percent cost of capital)

Metric	Value
Upfront cost	0 (battery already owned)
Payback	immediate (no new spend)
ROI / IRR	not applicable (no capital invested)
20-year NPV	about 3.9M UGX (about 2.9M at 22 percent)

Year-1There savingis aboutno 59,000return UGXratio perbecause month;no 20-yearmoney totalis aboutinvested; 10.8M UGX. This caseit is pure upside on owned hardware, so there is no return ratio; it simply adds free cashhardware that declines as the battery ages.

Metrics

What it means

1. Arbitrage alone saves about 43,000 to 79,000 UGX per month (11 to 20 percent of the bill), on hardware NFE already owns, with zero solar. The percentage, not the shilling figure, is what tells you whether it is worth scaling. Modest but real.
2. The reliability dial is cheap on wear: holding 50 percent for backup roughly halves the saving versus 0 percent, but at under one cycle per day on a 6,000-cycle battery that is 15 percentyears costor more of capitallife.
3. The logic that earns this (thecharge assumedoff-peak, financingdischarge rate).peak, Paybackhold a floor) is the time to recoverwhat the upfrontinverter's cost;own ROIsettings already do, so native settings capture most of it today with no extra code.
4. Strategic read: arbitrage is a nice-to-have, not the totalmoney lifetimestory. return on that cost; IRRSolar is (see the project'sSolar effectivePlus annualBattery return,Economic Case, where the same battery plus 4 kWp offsets about 56 percent of the bill and apays lender funds a project when its IRR comfortably exceeds the loan rate; NPV is the wealth created expressedback in today'sabout money.1.9 Solaryears).
stays strongly positive across any realistic rate (10 to 24 percent) because the payback is so short; the discount rate mainly affects the battery-expansion case.

Battery aging over 20 years

Because this case is entirely battery-dependent, it is the most exposed to aging. Taking the conservative assumption thatWith usable capacity fadesfading to 50 percent by year 20, the arbitrage30-percent-reserve saving falls from about 59,000500 UGX per UGX/month today (at 30 percent reserve) to about 30,000 by year 20. It roughly halves.

Over the full 20 years the undiscounted total is about 10.8M UGX; in today's money at a 15 percent discount rate it is worth about 3.9M UGX.in Sincetoday's themoney. battery is already owned (no capex), this is pure declining upside. The lesson carries into the solar cases: pairingPairing the battery with solar makes the income far less sensitive to aging, because daytime solar self-consumption does not depend on the battery.

For the joint solar-and-battery optimization that usesusing this same aging model, see Optimal Solar and Battery Allocation; for how cheap battery must get before expanding it pays, see Battery Price Break-Even.

Caveats

TheGreedy policy is greedy (charge full off-peak, discharge to floor, idleshoulder in shoulder)idle); a forecast-driven controller should land near this or slightly above. The model ignoresIgnores the fixed monthly service charge,charge which is (not offsettable,offsettable) and treats the battery as arbitrage-dedicated, whereas its real primary role is backup, which is why the reserve tradeoff matters.

Note: this battery-only arbitrage is the value captured BEFORE solar. It is not additive with the solar case. Once a solar array is installed it claims the single daily battery cycle (charging for free), so the solar case supersedes grid arbitrage rather than stacking on top of it. See the solar page for the test.