The question every business owner asks, in some form: how long before this pays itself back? Most online "solar ROI calculators" simplify the answer to one number, and that number is usually wrong. Real solar ROI depends on six variables, and your specific situation moves each one. Here's how to actually think about it.
The simple ROI formula
Payback (in years) = Effective cost ÷ (Annual savings × Conservative factor)
Where:
- Effective cost = System price − Section 12B tax saving
- Annual savings = Monthly bill × Solar offset % × 12
- Conservative factor ≈ 0.85 (accounts for cloudy days, demand charges, soiling)
Worked example: R50 000/month bill, mid-size commercial
A small factory in Gauteng pays R50 000/month for electricity on a commercial tariff blending to R4.10/kWh.
| Calculation | Value |
|---|---|
| Monthly consumption | R50 000 ÷ R4.10 = 12 195 kWh |
| Target solar offset | 70% (the sweet spot) |
| Required system size | ~59 kWp |
| Install cost (incl. setup) | R815 000 |
| Monthly saving | R35 000 |
| Annual saving | R420 000 |
| Section 12B saving (27%) | ~R220 000 |
| Effective cost | R595 000 |
| Payback (with conservative factor) | ~1.7 years |
After year two, the system runs free for the next 23 years. That's R420 000 a year in saved electricity — over R10 million in lifetime savings on this one install.
The six variables that move ROI
1. Your blended tariff
The bigger your effective R/kWh, the faster solar pays back. Demand charges and Time-of-Use peaks push your blended rate well above the "domestic R3.20/kWh" people assume. Most commercial customers are paying R4.00 – R5.50/kWh effective. Check your last bill. Divide total (ex-VAT) by kWh used. That's your real number.
2. Your daytime load profile
If 60–80% of your demand falls between 7am and 5pm, you're a perfect solar candidate. If most of your load runs overnight (24/7 cold store, night-shift bakery), you need more battery, which pushes payback out.
3. System size
Bigger commercial systems get cheaper per kWp. A 50kWp install costs around R13 500/kWp installed. A 250kWp install can drop to R12 000/kWp. Larger systems also have proportionally smaller fixed costs (SSEG paperwork, project management) so they're more efficient on a per-kWp basis.
4. Your tax position
Section 12B gives you 27% back if you're a normal company. If you're a partnership, sole prop, or trust, your effective rate may differ. Speak to your accountant before assuming the 27%.
5. Diesel offset
If you currently run a diesel generator during load shedding, solar replaces that fuel cost as well. Diesel at R23/L plus generator wear and maintenance adds R8 000 – R20 000+ per month to your real electricity cost on top of Eskom. That additional saving compresses payback dramatically — often shaving 6–12 months off the headline figure.
6. Roof orientation and shading
A flat warehouse roof in Joburg gets 5+ peak sun hours/day average. A shaded urban office roof might get 3.5. The difference is 40% on your annual production. Roof matters.
The "less obvious" ROI levers
Beyond payback, solar buys you three things people don't put in spreadsheets:
- Predictable energy costs for 15+ years. Eskom tariffs go up; your solar bill doesn't.
- Operational resilience. A hybrid system with battery means load shedding stops eating your day. That's harder to put a rand value on, but it's real.
- A capital asset on your balance sheet that increases the value of your property.
When solar doesn't pay back well
Three scenarios where solar isn't the right move:
- Heavy night-time load with no daytime use. Solar can't help much. Battery is the only answer and it's expensive on its own.
- You're moving in 18 months. Payback at 1.5–3 years is excellent, but you need to be in the building long enough to harvest it.
- Your tariff is genuinely under R2/kWh. Rare in SA but possible on subsidised industrial rates. Worth checking before assuming solar helps.
Get your actual ROI
Send us your last three utility bills. We'll model your exact tariff, your exact load profile, and your exact payback period — not a generic calculator estimate. Inside 24 hours.