Grid-tied, hybrid or off-grid solar: choosing the right system for a plant

Plantmargin Engineering · 28 September 2026 · 2 min read

Every solar proposal starts with one choice that decides almost everything else: is the system grid-tied, hybrid or standalone? The same roof can carry any of the three, but they behave differently, cost differently and are sized by different rules.

Grid-tied: the lowest cost per kWh

A grid-tied system feeds the plant alongside the utility. The plant uses solar first and buys the rest from the grid. There is no battery, so the investment is lowest and the payback quickest.

What owners often miss: a grid-tied system switches off when the grid fails. Inverters must disconnect within about two seconds of losing the grid (anti-islanding, tested to IEC 62116) so that line workers are not put at risk. If outages are frequent, the solar energy lost during them should be counted in the proposal.

Two options make a grid-tied system work harder:

• Zero export or net metering. Without approval to export, a zero-export controller limits the inverters so nothing flows back. Where net metering is approved, surplus is credited, in Tanzania under the EWURA net-metering rules and small power producer framework.
• A PV-generator controller. On a site with a generator, this keeps solar running during outages while holding the generator above its minimum load.

Hybrid: solar, a battery and the grid

A hybrid system adds a battery and a hybrid inverter. Surplus solar is stored for the evening, the battery can trim the evening demand peak, and the critical loads keep running in an outage from a separate backup board.

Size the battery from the backup need first, not from the solar surplus:

battery kWh ≥ critical load kW × hours of backup ÷ (usable depth of discharge × inverter efficiency)

Example: 60 kW of critical load for 4 hours, with 90% usable and 95% efficiency, needs at least 60 × 4 ÷ 0.855 = 281 kWh. Check the power too: the battery and inverter must deliver the critical load plus a margin for motor starts.

Standalone (off-grid): the site's own micro-grid

With no utility at all, solar, a battery and a grid-forming inverter set the site's voltage and frequency. A generator usually backs it up for long cloudy spells. Three sizing rules apply that do not exist on the grid:

1. The worst month, not the average. The array is checked against the month with the least sun.
2. Days of autonomy. How long the battery alone must carry the load. One day is common with a generator; two or more without one.
3. Motor starting. A motor started direct-on-line draws roughly three times its rating. The inverter's surge rating must cover the largest motor, or that motor needs a soft starter or a drive.

The savings of an off-grid system are measured against the alternative, usually running on diesel alone.

Which one fits

Plantmargin engineers design all three from the plant's own metered load, with a proposal that lists the checks behind each choice.