INTERACTIVE EXPLAINER
Energy Bank
Change power, time, demand and storage. Can the energy last 24 hours?
Balance one day
Starts empty. Output is steady, then off; demand is steady all day. Storage efficiency is assumed 90% each way.
Some device demand is unmet.Demand: 1,440 Wh over 24 hours.
1,040.4 WhSupplied to device
399.6 WhUnmet demand
Energy inside the store
Hours from the start. Dashed line: output stops at hour 6. The line follows the constant-rate assumptions, not a forecast.
Where the source energy goes
- Generated
- 1,200 Wh
- Supplied to device
- 1,040.4 Wh
- Still stored at hour 24
- 0 Wh
- Storage-path losses
- 159.6 Wh
- Unused source energy
- 0 Wh
Generated = supplied + still stored + losses + unused. Unmet demand is an additional need, not generated energy.
What this model assumes
Available power is flat for the first selected hours and zero afterward. This is usable power at the model’s shared energy boundary, not a panel’s nameplate rating or a real solar-day profile. The device demand is constant. There is no grid or second source, and the store is empty at the start.
Energy goes directly to the device first. Surplus can charge storage up to its energy capacity. Charging is 90% efficient and discharge is 90% efficient: the combined storage path returns 81% of energy sent through it. These fixed efficiencies are illustrative. Unused surplus is curtailed; it is not counted as storage loss.
Battery charging/output power limits, upstream solar conversion, weather, temperature, standby draw, ageing, start-up surges and changing loads are omitted. An adequate energy total does not prove a real system can deliver the required power. This is an energy-accounting lesson, not an equipment-sizing or backup-reliability recommendation.
Read the hourly energy table
Energy values cover each one-hour interval. Stored energy is the amount remaining at the end of that interval.
| Hour | Generated | Supplied | Unmet | Stored at end |
|---|---|---|---|---|
| 0–1 | 200 | 60 | 0 | 126 |
| 1–2 | 200 | 60 | 0 | 252 |
| 2–3 | 200 | 60 | 0 | 378 |
| 3–4 | 200 | 60 | 0 | 504 |
| 4–5 | 200 | 60 | 0 | 630 |
| 5–6 | 200 | 60 | 0 | 756 |
| 6–7 | 0 | 60 | 0 | 689.3 |
| 7–8 | 0 | 60 | 0 | 622.7 |
| 8–9 | 0 | 60 | 0 | 556 |
| 9–10 | 0 | 60 | 0 | 489.3 |
| 10–11 | 0 | 60 | 0 | 422.7 |
| 11–12 | 0 | 60 | 0 | 356 |
| 12–13 | 0 | 60 | 0 | 289.3 |
| 13–14 | 0 | 60 | 0 | 222.7 |
| 14–15 | 0 | 60 | 0 | 156 |
| 15–16 | 0 | 60 | 0 | 89.3 |
| 16–17 | 0 | 60 | 0 | 22.7 |
| 17–18 | 0 | 20.4 | 39.6 | 0 |
| 18–19 | 0 | 0 | 60 | 0 |
| 19–20 | 0 | 0 | 60 | 0 |
| 20–21 | 0 | 0 | 60 | 0 |
| 21–22 | 0 | 0 | 60 | 0 |
| 22–23 | 0 | 0 | 60 | 0 |
| 23–24 | 0 | 0 | 60 | 0 |
A rate and an amount answer different questions.
Watts (W) describe power: the rate of energy transfer. Watt-hours (Wh) describe energy. A steady 60 W for 6 hours needs 360 Wh. A storage capacity in Wh does not, by itself, tell you its maximum output power.
Try changing one thing
- Reset. Raise available power from 200 W to 400 W, keeping the 800 Wh store. There is still unmet demand after the source stops.
- Now raise storage to 2,000 Wh. Under these exact assumptions, the device receives its full 24-hour demand. Some energy remains stored at the end.
- Set storage to zero. Extra energy made earlier cannot help later in this example.
For a related heat-and-power balance, read how refrigerators move heat. Its steady average-load example is also a teaching model, not an appliance rating.
Sources and scope
The U.S. Energy Information Administration explains W and Wh. The U.S. Department of Energy explains solar storage, conversion losses and the difference between storage power and energy capacity. The flat output profile and 90% efficiencies here are chosen teaching assumptions, not measurements from those sources.