Grid import, export and backup
The key idea
A grid connection has both a capacity and an availability. A large import limit supplies nothing during an outage, while an export limit can leave surplus generation unused.
Four different inputs
Import capacity limits power purchased at one time. Export capacity limits power sent back. Availability states whether exchange is possible at that time. Tariffs price the permitted energy or other billing quantities.
None of these inputs determines the others. A purchase rate is not a reliability measure. A 30 kW connection is not 30 kWh of energy, and it is not a guarantee of 30 kW in every hour.
The common mistake is to use a large grid rating as a substitute for backup during modeled outages.
Inspect an hour, then the day
This example has solar and load but no battery or generator. It makes any grid-dependent shortfall visible. The outage begins at 16:00.
At 18:00 the solar supply is zero and demand is 24 kW. Increase the import ceiling while the outage remains active. Nothing changes for that hour. Remove the outage and the ceiling can then determine how much load is supplied.
At midday, lower the export ceiling. The excess solar that cannot serve load or leave the site is curtailed.
- Purchased energy
- 140.7 kWh
- Exported energy
- 76.0 kWh
- Unmet energy
- 80.7 kWh
Daily purchase charge $35; export credit $6. These energy-only amounts do not include all bill items.
Read the values and assumptions
| Hour | Grid | Load | PV | Import | Export | Unmet | Curtail |
|---|---|---|---|---|---|---|---|
| 0 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 1 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 2 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 3 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 4 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 5 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 6 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 7 | On | 12 | 9.32 | 2.68 | 0.00 | 0.00 | 0.00 |
| 8 | On | 12 | 18.00 | 0.00 | 6.00 | 0.00 | 0.00 |
| 9 | On | 12 | 25.46 | 0.00 | 10.00 | 0.00 | 3.46 |
| 10 | On | 12 | 31.18 | 0.00 | 10.00 | 0.00 | 9.18 |
| 11 | On | 12 | 34.77 | 0.00 | 10.00 | 0.00 | 12.77 |
| 12 | On | 12 | 36.00 | 0.00 | 10.00 | 0.00 | 14.00 |
| 13 | On | 12 | 34.77 | 0.00 | 10.00 | 0.00 | 12.77 |
| 14 | On | 12 | 31.18 | 0.00 | 10.00 | 0.00 | 9.18 |
| 15 | On | 12 | 25.46 | 0.00 | 10.00 | 0.00 | 3.46 |
| 16 | Off | 12 | 18.00 | 0.00 | 0.00 | 0.00 | 6.00 |
| 17 | Off | 24 | 9.32 | 0.00 | 0.00 | 14.68 | 0.00 |
| 18 | Off | 24 | 0.00 | 0.00 | 0.00 | 24.00 | 0.00 |
| 19 | Off | 24 | 0.00 | 0.00 | 0.00 | 24.00 | 0.00 |
| 20 | On | 24 | 0.00 | 15.00 | 0.00 | 9.00 | 0.00 |
| 21 | On | 24 | 0.00 | 15.00 | 0.00 | 9.00 | 0.00 |
| 22 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
| 23 | On | 12 | 0.00 | 12.00 | 0.00 | 0.00 | 0.00 |
Model note · Constructed 24-hour teaching day with one-hour means, 12 kW base demand and 24 kW from 17:00 to 21:59. Analytic solar shape, no battery or generator. Outage starts at 16:00; unavailable grid imports and exports are zero. No simultaneous import/export. Illustrative rates: import $0.25/kWh, export $0.08/kWh; no fees, tax or demand charge. This is not a tariff quotation or a grid-interconnection study.
The arrows describe the selected hour. The totals cover all 24 one-hour samples. Changing the inspection hour moves the readout without changing the daily totals.
Why the totals differ
Purchased energy is the sum of permitted imports. Exported energy is the sum of permitted sales. Unmet energy is demand left after direct solar and imports. Curtailed energy is solar left after direct use and exports.
A battery could absorb some surplus and later reduce imports or shortfall, but its power, stored energy and dispatch would then matter. A generator could help only if its capacity, fuel, controls and connection permit it. Those components are deliberately absent from this explorer.
The math, if you want itOptional — the page reads completely without it
Solar serves the load first, up to whichever is smaller:
direct solar supply
D = min( Pload , Psolar )
When the grid is available, the exchange is capped by the ceilings:
import, when the grid is available
import = min( limitimport , Pload − D )
export
export = min( limitexport , Psolar − D )
During the outage both are zero. Whatever remains is unmet on the load side and curtailed on the solar side:
the load side
Pload = D + import + unmet
the solar side
Psolar = D + export + curtailed
With this rule, import and export cannot both be positive in one sample. Multiplying kW by the one-hour interval gives kWh. At 18:00 with no outage and a 15 kW ceiling, 24 − 15 = 9 kWh is unmet in that hour.
Price only the flows you model
The teaching bill multiplies imported kWh by 0.25 USD/kWh and exported kWh by 0.08 USD/kWh. It omits demand charges, fixed charges, tax, tiers and settlement rules.
Net imported energy is not automatically the billed quantity. Different import and export rates can make two cases with the same net energy have different charges. Use the actual tariff structure when building a financial scenario.
Keep interconnection checks separate
An export ceiling entered in a model is not permission to export. Actual grid connection can require utility agreements, protection, voltage/frequency controls, fault-duty assessment and other project-specific checks.
Likewise, this energy balance assumes that local solar can serve the site during the selected outage. Real grid-following equipment may disconnect. Check the actual islanding capability and control design before treating modeled local supply as outage backup.
See it in Phasor
Review the grid purchase/sell limits, tariff and availability assumptions. Run the plant and inspect its hourly grid exchanges and unmet demand. Follow with the required network and interconnection checks; the annual energy result does not set protection.