From midday solar to evening light
A fictional building uses some midday solar immediately and stores some for later. Follow the boundaries that determine what is available after sunset.
Begin with available sunlight
Imagine a fictional building with rooftop solar, suitable battery equipment, and a working grid connection. In daylight, the photovoltaic array converts light into DC electricity. Output depends on sunlight and equipment conditions. A rated capacity is not a promise of constant output, and the example does not prescribe a particular installation or predict its generation.
Panel output changes with available light.
The available primary energy sets the starting resource.
Read the complete journey
- 1
Begin with available sunlight
Imagine a fictional building with rooftop solar, suitable battery equipment, and a working grid connection. In daylight, the photovoltaic array converts light into DC electricity. Output depends on sunlight and equipment conditions. A rated capacity is not a promise of constant output, and the example does not prescribe a particular installation or predict its generation.
Notice: Panel output changes with available light.
Explore Solar panels & inverters
Why: The available primary energy sets the starting resource. - 2
Connect compatible electrical forms
The installation uses power electronics to connect its solar source, storage, and AC loads. Some designs couple a battery on a DC side; others use an AC connection with further conversion. These different arrangements affect the conversion path. Each must respect equipment limits, and no converter multiplies the energy available to the building.
Notice: The electrical form can change at a converter.
Explore Alternating & direct current
Why: Compatible voltage and current conditions let different equipment exchange power. - 3
Use some power immediately
Appliances in the building use power while the solar array produces it. When local production supplies some of that demand, measured grid imports can fall even if appliance consumption stays unchanged. The meter shows a boundary flow rather than every internal flow. Permission to export any surplus and applicable metering arrangements depend on local rules.
Notice: Lower imports can coexist with unchanged appliance use.
Explore Meters, power & energy
Why: Separating internal use from boundary flow prevents a misleading energy comparison. - 4
Keep part of the energy for later
In this illustration, suitable controls direct some available energy into storage. Charging is a load, and the amount accepted depends on charge state, power rating, and operating limits. Some energy is lost in the process. The presence of a battery does not guarantee it was fully charged before the evening demand begins.
Notice: Stored energy rises only when net charging occurs.
Explore Energy storage
Why: The evening resource depends on earlier charging and losses. - 5
Supply a load after sunset
When sunlight is unavailable, the store can discharge within its power limit and remaining usable energy. A modest load may be sustained longer than a large one. This journey supplies no runtime figure because capacity, charge state, efficiency, and load have not been specified. Other building demand may still require power from the wider grid.
Notice: Discharge reduces the remaining usable energy.
Explore Energy storage
Why: Duration depends on an energy balance as well as a power limit. - 6
Separate everyday storage from backup
Now consider a different condition: the wider grid becomes unavailable. The earlier solar-and-storage description alone does not establish backup capability. Independent operation requires a designed boundary, suitable controls, and equipment able to maintain a local supply. Selected loads and resource limits matter, so a system that shifts energy through time may have a different resilience role.
Notice: An island-capable boundary is an additional designed function.
Explore Microgrids & island operation
Why: Energy storage and safe independent operation solve different problems.
Why can lower daytime grid imports fail to predict how long a building can operate during an outage?
Reveal the explanation
Imports show a boundary flow, not stored usable energy or island capability. Outage operation also depends on charge state, load, equipment limits, and a design capable of independent supply.
Sources & further reading
Original explanations informed by these sources. Diagrams are conceptual; local systems differ.
- Photovoltaics and electricity U.S. Energy Information Administration · Not stated · Source & rights record
- Solar Integration: Inverters and Grid Services Basics U.S. Department of Energy · Not stated · Source & rights record
- Solar Integration: Distributed Energy Resources and Microgrids Basics U.S. Department of Energy · Not stated · Source & rights record
- Energy storage for electricity generation U.S. Energy Information Administration · 2023-08-28 · Source & rights record
- Measuring electricity U.S. Energy Information Administration · Not stated · Source & rights record
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