Optimal Solar and Battery Allocation (Sezibwa)

[!info] Conservative, bank-facing basisbasis. Tariff: UEDCL Code 10.2 Q3 2026 (revised666.5 after/ independent562.1 review)/ 429.7, configurable input). Turnkey installed capex andcapex, real day-to-dayweather. weatherReproducible (seededfrom variable-irradiance year)nfe_solar_model.py. These are the numbers a lender should see.

Headline

Sweeping both dimensions (solar size and battery capacity) to find the allocation with the best 20-year return gives a clear, slightly counterintuitive answer:

The most business-optimal allocation is solar-heavy and battery-light. Put the money into panels on the battery already owned. Adding battery capacity lowers the return.return at today's battery price.

• No feed-in (today): about 5 kWp on the existing 8.9 kWh battery. Turnkey capex about 6.3M UGX, IRR about 42 percent, simple payback about 2.23 years, IRRdiscounted payback about 453.1 percentyears, 20-year NPV about 10.9.8M UGXat 15 percent.
• With feed-ina (ifusable netexport metering arrives):arrangement: about 8 kWp on the same battery. Capex about 10.1M, payback about 2.1 years, IRR about 4846 percent, NPV about 19.6M18.5M.
• In both, buying a second or third battery unit lowers NPV.

Method

A 2-D sweep over solar (0 to 8 kWp) and battery (existing one unit, or plus one or two SR-SE10B units). For each combination the model runs the real load through 20 years and computes discounted savings minus turnkey capex.

Assumption	Value
Tariff	UEDCL Code 10.2 Q3 2026 (configurable)
Horizon	20 years
Battery aging	usable capacity fades linearly to 50 percent by year 20
Panel aging	0.5 percent per year
Weather	seeded variable-irradiance year (real day-to-day spread)
Discount rate	15 percent (also shown at 2210 percent)/ 15 / 20 / 25 percent
Solar capex	about 1.26M UGX per kWp turnkey
Extra battery capex	5.568M UGX per SR-SE10B (8.9 kWh usable)
Existing inverter + 1 battery	sunk (9.4M already spent)
Feed-in	none (base), or surplus sold at 343 UGX/kWh

Result

The greenest cells sit on the bottom row, the battery NFE already owns. Moving up (more battery) turns the map redder at every solar size. Moving right (more solar) raises NPV until spill, or keeps rising with feed-in.

Investment metrics and sensitivity

NPV shown across discount rates so you can see which allocation survives expensive capital (15metrics percentranked costNPV, ofIRR, capital)discounted payback, simple payback, then cumulative ROI):

22%

Allocation	Upfront	Payback	ROI (20-yr cumulative)Capex	IRR	NPVSimple at/ 15%disc. payback	NPV at@10%	@15%	@20%	@25%
5 kWp (no feed-in,export, today)	6.3M	42%	2.23 / 3.1 yr	723 percent15.4M	45 percent	10.9.8M	6.0M4M	4.1M
8 kWp (ifwith netusable metering arrives)export)	10.1M	46%	2.1 / 2.8 yr	811 percent28.5M	48 percent18.5M	19.6M12.3M	11.3M8.2M

Both survive a 25 percent cost of capital comfortably. (The undiscounted 20-year cumulative ROI is large, ~680 to 780 percent, but it is not a decision metric; NPV and IRR carry the case.)

Findings

1. Do not buy more battery for return. At every solar size, adding a battery unit lowers 20-year NPV; two extra units go negative. The battery capex is not recovered within the horizon.
horizon
2. Why:at batterytoday's is expensive per kWh (5.57M for 8.9 kWh is about ~626,000 per kWh) versus solar; the conservative aging assumption erodes its contribution; and the discount rate weights the faded later years lightly.UGX/kWh.
3. Solar income is resilient to battery aging.aging; Battery-battery-only savings halve over 20 years;years, the solar cases decline far less,less.
because
4. Value, daytimenot penetration. Beyond ~4 to 5 kWp, extra panels spill and returns diminish unless storage gets cheaper or export becomes usable.

Battery replacement strategy

The model conservatively assumes the battery simply fades to 50 percent over 20 years. A real long-horizon plan should choose one of three strategies, and the economics differ:

• Operate the original battery for all 20 years (what the model assumes — the pessimistic bound; usable capacity, and so the evening-shift slice, keeps shrinking).
• Replace once when usable capacity hits a chosen threshold (say ~70 percent around year 10 to 12): restores capacity at the cost of a fresh battery, worth it only if battery prices have fallen (see the break-even case).
• Augment gradually by adding cells as load grows rather than replacing.

Importantly, the recommended allocation (solar-heavy, existing battery) is the least exposed to this: solar self-consumption does not depend on the battery.battery,

so battery replacement is a Feed-inreliability/backup changesdecision, not a driver of the size, not the shape. If net metering arrives, the optimum shifts to more solar (about 8 kWp) because surplus is sold rather than spilled, but the answer is still to spend on panels, not battery.

return.

The one important caveat for the bank case

This prices energy savings only. It does not price the battery's real job, backup and reliability during outages. So for a lender: solar (and the existing battery)Solar is the return story, financed on the payback and NPV above;story; additional battery, if wanted,battery is a resilience decision justified separately, not folded into the ROI case.separately.

Recommendation

Fund about 5 kWp of solar on the existing battery now (about 6.3M turnkey, payback about 2.2 years, IRR about 4542 percent). If neta meteringusable arrives,export arrangement materialises, scaling to about 8 kWp becomes thebetter. better allocation. Treat any extra battery as reliability capex with its own justification.

For how cheap battery must get before expanding it pays, see Battery Price Break-Even. Companion cases: Battery Arbitrage (alone) and, Solar Plus Battery.