1. The battery
Ah
×
battery
It's on the label: “100 Ah”, “12 V”. Two golf-cart batteries are 2 × 6 V. The label only shows “reserve” minutes? Convert it to Ah.
50%
I don't know how full it is
For a 12 V lead-acid or AGM battery: disconnect everything, wait a few hours, then read the voltage with a multimeter.
| Resting voltage | Charge |
|---|---|
| 12.7 V | 100% |
| 12.5 V | 80% |
| 12.2 V | 60% |
| 12.1 V | 50% |
| 11.8 V | 30% |
| 11.5 V | 10% |
For lithium, the voltage barely moves: trust the battery's monitor or app.
2. What do you charge it with?
We calculate for a panel in the best conditions: well aimed, no shade, a sunny summer day, with an MPPT controller.
$ / litre
3. The answer
—
What if you had…
How we calculate
- The missing energy: number × voltage × Ah × the empty part. A 12 V 100 Ah battery at half is 600 Wh.
- Losses: a battery doesn't give back everything you put in. We count 20% losses for lead-acid, 12% for AGM, 5% for lithium.
- The panel: even at its best, it gives about 80% of its watts (heat, wire, controller). A sunny summer day is about 5 h of full sun.
- The charger: its amps × the charging voltage (about 14.4 V for a 12 V battery).
- The end of the charge: a lead-acid or AGM battery accepts less and less current as it fills. The last 20% takes a few hours, no matter how big the charger. Lithium takes nearly everything until the end.
- Gas: a small quiet generator burns about 0.3 L/h with a charger; a contractor generator runs at fixed speed and burns about 1 L/h, even almost unloaded.
These are estimates for a battery in good shape. An old battery fills faster… because it holds less.
Want to know how many panels and batteries your project needs? Try the full calculator.