Commercial solar system sizing
A business using 60 000 kWh a month, 60% of it in daytime, needs about 258 kWp of solar to cover its daytime use — far less than the 968 kWp its roof could hold. Add a battery for 4 hours of critical-load backup and the recommendation grows to 294 kWp with 175 kWh of installed storage. These are the figures My Solar Plan's own sizing engine produces.
Hypothetical business, real engine. The consumption and backup targets are illustrative inputs. The roof is the 5 000 m² factory roof from the factory roof example, in Johannesburg, and the rate is City Power's 2026/27 large-power-user low-voltage energy rate.
Step 1: size solar to daytime use
60 000 kWh a month with 60% used in daytime is about 1 180 kWh of daytime use on an average day. Dividing by what one kWp produces per day at this site gives 258.1 kWp. The roof could fit 967.5 kWp, so the roof is not the limit here — the load is. Covering the whole roof would mostly produce electricity the business cannot use when it is generated, which My Solar Plan does not count as a saving.
Step 2: size a battery for backup
Backup is sized from what has to keep running. Here the normal site load is 150 kW and the business chooses the Critical operating level, which My Solar Plan treats as 25% of normal load (37.5 kW). The other levels are Reduced (50%), Most (75%) and Full (100%); a known backup load in kW overrides the percentage.
- Usable storage = 37.5 kW × 4 hours = 150 kWh.
- Installed storage = usable ÷ (90% depth of discharge × 95% discharge efficiency) = 175.4 kWh.
- Recharging it from solar each day needs 166.2 kWh (usable ÷ (95% charge × 95% discharge efficiency), the full round trip), so the solar recommendation rises to 294.4 kWp.
The two options side by side
| Solar only | Solar + backup battery | |
|---|---|---|
| Recommended solar | 258.1 kWp | 294.4 kWp |
| Battery (usable / installed) | — | 150 / 175.4 kWh |
| Solar production per year | 432 000 kWh | 493 000 kWh |
| Grid consumption replaced | 60% | 68% |
| Indicative saving on per-kWh charges per year (excl. VAT) | R1 310 000 | R1 476 000 |
The battery raises the share of consumption covered because it carries surplus solar into the evening. Its main purpose here is resilience: the business case for the battery itself, including its cost, is reviewed with Red Energy rather than shown as a payback. Both savings figures use the same average-day model and exclusions as the warehouse example: only per-kWh charges are valued, and days the site is closed are not deducted, so a business that shuts at weekends would save less than shown.
Step 3: from a recommendation to a design
My Solar Plan's recommendation is a starting point. When a plan continues with Red Energy:
- The site inspection confirms the electrical supply (single- or three-phase), roof condition, shading, cable routes and where the inverter and distribution board sit.
- The design is matched to an approved Red Energy equipment configuration. Panel kits come in fixed sizes, so a design between sizes is rounded down to the largest approved configuration that fits.
- The inverter is chosen from the supply phase, system size and whether a battery is needed.
- Equipment is priced, and only then is a payback period calculated.
Inputs that change the answer most
- Daytime share of use. Shift-based manufacturing that runs through the day suits solar better than a site that uses most of its electricity at night.
- Measured consumption. A monthly kWh figure from the electricity account is more reliable than an estimate from the rand amount. Red Energy can also use interval data from a load logger.
- Expected growth. A planned expansion can be entered as load growth so the system is sized for it.
- Backup scope. Backing up everything for many hours needs far more storage than keeping critical loads running through a typical outage.
For the money side of the same calculation, see the warehouse savings example.
Frequently asked questions
What size solar system does a business need?
Size solar to the electricity the business uses while the sun is up, not to the size of the roof. My Solar Plan divides average daytime consumption by how much one kilowatt-peak produces on an average day at the site, adds whatever is needed to recharge a battery, and caps the result at what physically fits.
How big a battery does a business need for load shedding?
Decide which loads must keep running and for how long. My Solar Plan multiplies the backup power by the hours of autonomy wanted, then allows for the battery's usable depth of discharge and discharge efficiency to get the installed capacity. A known backup load in kW takes priority over a percentage of the normal site load.
Does a backup battery also save money?
On a flat tariff My Solar Plan counts a battery as saving money only when it stores surplus solar for evening use. On a time-of-use tariff it models the battery serving Peak periods. Battery cost and the full battery business case are reviewed with Red Energy.
Build an indicative plan for your own site
My Solar Plan runs the same engine on your property: start with the address, confirm the roof, describe how the business uses electricity, then save the plan privately and refine it with Red Energy.
Start with my propertyLast reviewed 2026-10-13.