Solar Sizing Re-run, Sept 2026 (600 Wp panels)
This page re-runs the solar sizing from Solar Plus Battery Economic Case (Sezibwa) on three months of live OpenEMS load. It answers one question: is 7 to 8 panels (600 Wp each) still the best PV size for Sezibwa, and does it cut the energy bill in half?
Answer
Yes. With 600 Wp panels:
- 6 panels (3.6 kWp) cut the energy bill by half.
- 7 panels (4.2 kWp) cut it by 55%.
- 8 panels (4.8 kWp) cut it by 58% and give the best 20 year return (highest NPV).
Past about 5 kWp each extra panel adds little. The battery is already full by midday, and without grid export the surplus is wasted.
What changed since August
| Input | August model | This re-run |
|---|---|---|
| Load data | Modbus logger archive, Mar to Aug 2026 | OpenEMS aggregate meter DTSU666 (serial 221123297561), 1 Jul to 28 Sep 2026, 76 days of data |
| Average load | 21.3 kWh/day | 25.8 kWh/day |
| Baseline energy bill | about 374,000 UGX/month | about 454,000 UGX/month |
| Panel size | kWp only | 600 Wp panels |
The model code and economics are otherwise unchanged. One correction was made: the daily average now divides by the hours actually covered by data, not the calendar span, so data gaps (for example the site internet outage of 25 to 26 Sep) no longer understate the load.
Cost of an 8 panel install (at cost)
Priced from the Rincol catalogue (August 2026 buy prices), with the railing at 421,000 UGX per 4 panel kit:
| Item | Qty | Cost (UGX) |
|---|---|---|
| 600 Wp mono panel (GS600N-144M) at 320,250 | 8 | 2,562,000 |
| Railing kit (4 panels per kit) at 421,000 | 2 | 842,000 |
| 6 mm2 UV solar cable at 12,000/m | 40 m | 480,000 |
| PV combiner box (MC4) | 1 | 80,000 |
| DC circuit breaker 20 A | 1 | 25,000 |
| Surge protector (a DC rated one is still to be sourced) | 1 | 35,000 |
| Earthing (rod, clamps, conductor) | 1 | 80,000 |
| MC4 connectors (estimate) | 6 pairs | 48,000 |
| Labour and transport | 1 | 250,000 |
| Total | 4,402,000 |
Panels and railing are 3.4M, and the rest of the balance of system and labour adds about 1.0M. The inverter (3.84M) and battery (5.57M) are already paid for, so these are the only new costs.
Results (600 Wp panels, no grid export, at cost)
| Panels | kWp | Saving per month (UGX) | Bill cut | Solar wasted | Cost (UGX) | Simple payback | IRR | NPV at 15% (UGX) |
|---|---|---|---|---|---|---|---|---|
| 5 | 3.0 | 199,406 | 44% | 4% | 3.44M | 1.4 yr | 69% | 10.9M |
| 6 | 3.6 | 225,904 | 50% | 10% | 3.76M | 1.4 yr | 71% | 12.4M |
| 7 | 4.2 | 247,923 | 55% | 16% | 4.08M | 1.4 yr | 72% | 13.6M |
| 8 | 4.8 | 263,462 | 58% | 22% | 4.40M | 1.4 yr | 71% | 14.3M |
| 9 | 5.4 | 273,637 | 60% | 28% | 5.14M | 1.6 yr | 63% | 14.3M |
| 10 | 6.0 | 281,205 | 62% | 33% | 5.46M | 1.6 yr | 61% | 14.5M |
| 11 | 6.6 | 287,350 | 63% | 38% | 5.78M | 1.7 yr | 59% | 14.6M |
| 12 | 7.2 | 292,746 | 64% | 42% | 6.10M | 1.7 yr | 56% | 14.6M |
Cost is built per panel count: panels, one railing kit per 4 panels, and a fixed 40 m of cable, combiner, breaker, surge protector, earthing, connectors and labour. 4 panels is left out because a 4 panel string (about 175 V) is below the inverter's 200 V MPPT minimum. The SRNE ASP48120SH3 has 2 MPPT inputs of up to 9 kW each at 200 to 650 V, so 5 to 14 panels fit on one string.
For a turnkey quote at Rincol selling prices, add roughly 20 to 25% (the August 7 panel quote was 5.02M at sell prices against 4.16M at cost).
Battery only (the existing SRNE, no panels) saves about 34,000 UGX/month (8%) on the model's arbitrage policy.
How to read it
Recommendation
Install 8 panels of 600 Wp (4.8 kWp) on the existing SRNE inverter and battery, for about 4.4M UGX at cost. That cuts the energy bill by 58% (about 263,000 UGX/month) and pays back in about 1.4 years. 7 panels (4.1M, 55%) is the cheaper option. Going above 8 only makes sense with grid export (net metering) or more battery storage.
Assumptions and caveats
- Weather is modelled, not measured: 5.2 peak sun hours, 0.78 performance ratio, and a seeded day to day variation. Measured site irradiance would firm this up.
- Tariff is UEDCL Code 10.2 Q3 2026 (off peak 429.7, shoulder 562.1, peak 666.5 UGX/kWh). The Q4 schedule takes effect on 1 Oct and should be checked.
- Capex uses August 2026 Rincol catalogue buy prices and the 421,000 railing kit. The catalogue has no DC rated PV surge protector yet (the 35,000 line is the AC one), so budget more for that. Confirm current prices before buying.
- Load covers July to September only, so there is no seasonal variation.
- Battery use: with solar, the battery is charged from solar and discharged through the shoulder and peak periods. It replaces the current grid arbitrage (off peak charge, 21:30 to 23:30 discharge). A 30% reserve is held for backup.
- Per phase limit: the inverter delivers at most 4 kW per phase on battery (see the 27 Sep outage report). Solar does not change that limit.
Reproduce
The model is versioned in the nfe-modbus-energy-logger repository under scripts/solar_sim/ (issue #89, PR #90, in review). From the repo root:
python3 scripts/solar_sim/sweep_bom.py scripts/solar_sim/m100_openems.csv
python3 scripts/solar_sim/nfe_solar_model.py scripts/solar_sim/m100_openems.csv
m100_openems.csv holds the site aggregate energy in 15 minute steps (site totals only, no per customer data).