Battery Arbitrage Economic Case (Sezibwa)

Headline

The battery NFE already owns can earn money with no solar at all, by charging during the cheap off-peak window and discharging through the expensive evening peak. On the real site load this saves about 43,000 to 79,000 UGX per month (about 11 to 20 percent of the 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

Transparent scoping model on the real load, not OpenEMS. Load source is the aggregate meter (meter_100), about 21 kWh/day, validated internally against the 10 customer submeters (independent UEDCL bill still to confirm). Battery 8.9 kWh usable, 90 percent round-trip, 5 kW cap. Tariff UEDCL Code 10.2: off-peak 343, shoulder 577.11, peak 753 UGX/kWh. Charge off-peak toward full, discharge to cover the evening peak down to a reserve floor.

Why the site is a good arbitrage candidate

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

Results

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

Investment metrics (15 percent cost of capital)

There is no return ratio because no money is invested; it is pure upside on owned hardware that declines as the battery ages.

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 years or more of life.
3. The logic that earns this (charge off-peak, discharge peak, hold a floor) is what the inverter's own settings 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 money story. Solar is (see the Solar Plus Battery Economic Case, where the same battery plus 4 kWp offsets about 56 percent of the bill and pays back in about 1.9 years).

Battery aging over 20 years

Because this case is entirely battery-dependent, it is the most exposed to aging. With usable capacity fading to 50 percent by year 20, the 30-percent-reserve saving falls from about 59,500 UGX/month today to about 30,000 by year 20. It roughly halves. Over 20 years the undiscounted total is about 10.8M UGX; at a 15 percent discount it is worth about 3.9M in today's money. Pairing 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 optimization using this aging model, see Optimal Solar and Battery Allocation; for how cheap battery must get before expanding it pays, see Battery Price Break-Even.

Caveats

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