Solar Build #2: Planning a SANS-Friendly Hybrid Setup (MUST PV18-5048 + Meritsun LFP)
Part 1: Design and planning
Back in June 2015 I documented my first solar install here on the blog (Part 1: The brief overview – Going Solar). I've now started a second build, and this time I wanted to get the design right on paper before touching a single cable. I started with a diagram from ChatGPT. It was a reasonable starting point, but when I checked it against SANS 10142-1 (the South African wiring code) it had some serious gaps. This post covers what I changed and why.
Disclaimer: This is my personal project log, not professional advice. In South Africa a registered electrician has to verify the installation and issue a Certificate of Compliance (CoC). Your insurer will almost certainly ask for it too.
The kit
| Component | Model | Key specs |
|---|---|---|
| Inverter | MUST PV18-5048 VHM (off-grid hybrid) | 5 kW, 48 V, 118 A DC in, 80 A MPPT charger, PV 64–130 V MPPT / 145 V max |
| Batteries | 2 × Meritsun LFP100-48 | 48 V, 100 Ah (about 5 kWh each, about 10 kWh total), 100 A working current, 15S |
| Battery breaker | VVOKA WJM1-125/2300Z DC-MCCB | 125 A, 2-pole, 550 V DC, 20 kA breaking capacity |
| PV | About 2.5 kW of panels | About 37 V Voc, about 9 A Isc each |
| AC surge protection | ACDC KM20-40 (already in my DB) | 275 V, 40 kA |
What the AI diagram got wrong
The ChatGPT diagram had a sensible AC layout (a grid bypass and an interlocked changeover switch). Here's what was missing or wrong:
- No PV side at all. It showed no panels, no DC isolator and no DC surge protection.
- No earth leakage protection anywhere.
- No neutral-earth bond on battery. This is the big one, and I explain it below.
- Battery wiring: unequal cable lengths, a confusing extra blue DC cable, and no clear fusing per battery.
- Labelling: a "Manual Bypass ATS" (ATS means automatic transfer switch, so the label contradicts itself), and no breaker ratings or cable sizes.
- Several separate earth spikes instead of one main earth terminal.
Lesson: AI tools are handy for a first sketch, but check everything against the standard.
What's changed since my 2015 build
| 2015 build | 2026 build | |
|---|---|---|
| Inverter | 5 kVA Chinese off-grid MPPT hybrid, 48 V | MUST PV18-5048 VHM, 5 kW, 48 V (still off-grid, no export) |
| Batteries | 4 × 12 V 100 Ah lead-acid deep cycle in series (48 V, ~4.8 kWh, only ~50% usable) | 2 × Meritsun 48 V 100 Ah LiFePO4 in parallel (~10 kWh, most of it usable) |
| PV | 4 × 250 W "12 V" panels in series, ~1 kW | ~2.5 kW, 3 × 37 V Voc panels per string, several strings in parallel |
| Design focus | Sizing, cost and payback | Protection and SANS 10142-1 compliance |
Some things haven't changed. Run on solar by day, use the grid at night and fall back to batteries during load shedding. Size the system for what you actually use, and put only the essential loads on the inverter. Both still work.
What has changed: - Lithium instead of lead-acid. I no longer need the 50% depth-of-discharge rule I wrote about in 2015. Lithium cycles deeper and lasts longer, but it needs proper DC protection (that 20 kA breaker) and the correct charge settings for 15S. - The MPPT window instead of "12 V panels". In 2015 I thought of panels as 12 V or 24 V blocks. With MPPT, what matters is the string's open-circuit voltage against the inverter's limit (145 V here), including on a cold morning. - Protection and earthing are designed in from the start. These are the earth leakage unit, the neutral-earth bond on battery, separate neutrals, DC isolators and surge arresters. My 2015 cost table left out the DB and electrical work. This time it's the core of the design. - A correction to my 2015 note on kVA: multiplying kVA by 0.8 is a rough rule. Modern inverters such as the MUST state their rating in watts, so use that figure.
The design
1. PV strings: watch the 145 V limit
The MUST's MPPT range is 64–130 V, but the number that can damage the inverter is the 145 V maximum PV input. Panel open-circuit voltage rises in the cold, so plan for a winter morning:
- 3 panels × 37 V = 111 V at 25 °C, about 122 V at −5 °C. ✅
- 4 panels would be about 163 V on a cold morning. ❌ That's above the maximum.
So I'm using 3 panels per string and several strings in parallel. Every string has the same panel model, orientation and tilt.
The combiner box: - One 16 A, 2-pole, DC-rated breaker per string. With 3 or more strings in parallel, a faulty string can be back-fed by all the others. - +/− busbars, then a Type 2 DC surge arrester (a low-voltage PV type), then a 40 A DC isolator, then the inverter. - 4 mm² solar cable per string, and 6 mm² to the inverter.
2. Batteries: busbars, equal cables and a proper DC breaker
- Both batteries go to +/− busbars with equal-length, equal-size cables so they share current evenly.
- From the busbars to the 125 A 2-pole DC-MCCB, then into the inverter, using 35–50 mm² cable.
- The 20 kA breaking capacity matters. Two LiFePO4 batteries can push thousands of amps into a short circuit, and a cheap DIN-rail breaker might not be able to break it.
- DC breakers are often polarised: the LINE side goes to the battery and the LOAD side to the inverter.
- If a battery has no breaker on its front panel, fuse its positive lead close to the battery.
The 15S gotcha: My Meritsun serial numbers start with MSP15S, which means 15 cells in series. The pack is fully charged at about 54 V, while many inverter defaults assume 16S. I'm setting the battery type to USER. The values below are to be confirmed with Meritsun: - Bulk about 52.5–53 V, float about 51.5–52 V, cut-off about 45–46 V - Total charge current ≤ 100 A. The MUST can charge at up to 140 A (solar plus AC), which is more than the 125 A breaker can carry.
3. AC side: a separate solar DB and a changeover switch
My existing DB has a 63 A main switch, lights, a 30 mA earth leakage unit and the stove and plug circuits. The solar DB sits about 1 m away.
- House DB: a 40 A breaker feeds the inverter's AC input (10 mm² cable).
- Solar DB: inverter output, then a 25 A breaker, then a 30 mA earth leakage unit.
- Changeover switch: 2-pole (L + N), GRID – OFF – SOLAR, break-before-make.
- Essential circuits only on solar: lights, the fridge, TV, Wi-Fi and computers. The inverter can supply 22 A (5 kW), so the stove and geyser stay on grid only.
4. Separate neutrals
The grid neutral and the solar neutral stay completely separate. They only meet inside the changeover switch, and every essential circuit has both its L and its N switched over. If you mix the neutrals, earth leakage units trip for no obvious reason and the bonding scheme below stops working.
5. The neutral-earth bond: the part most diagrams leave out
When the grid fails and the inverter runs on battery, its output has no neutral-earth reference. Without that bond, the solar DB's earth leakage unit can't trip on an earth fault. The protection looks fitted but doesn't actually work.
My plan is an N-E bonding relay in the solar DB: - A normally-closed contact, with its coil powered from the grid - Grid live: the coil is energised and the bond is open (the grid supplies the N-E reference) - Grid down: the coil drops out and the bond closes on the inverter side
Check your inverter manual first. If the inverter bonds N-E internally on battery, a relay would give you a double bond.
6. Earthing
There is one main earth terminal in the house DB. The panel frames and rails, battery rack, inverter and solar DB all bond back to it. There's no separate earth spike for the solar side.
Diagrams
[Insert the single-line diagram here. – upload image in Blogger]
[Insert the multi-line wiring diagram here: L, N, PE and DC +/− shown separately. – upload image in Blogger]
Commissioning checklist
- ☐ Polarity checked on all DC breakers (LINE side to the source)
- ☐ String voltages measured before connecting the inverter (each one below 145 V)
- ☐ Inverter battery type set to USER, with the 15S voltages and a charge current of 100 A or less
- ☐ Earth leakage test on SOLAR with the grid off: it must trip
- ☐ Bonding relay confirmed open when on GRID
- ☐ "Dual supply / embedded generation" labels at the meter, both DBs, the changeover switch and the inverter
- ☐ AC and DC cables in separate trunking
- ☐ CoC issued by a registered electrician
- ☐ Checked with the municipality whether the system has to be registered (it's off-grid with no export)
Next up
Part 2 will cover the physical install: mounting the panels, building the combiner box, the battery bay and the first power-up. I'll post photos and the real measured string voltages.
Have you built something similar or spotted something I've missed? Let me know in the comments.
