Sizing a small solar battery charger
Small solar is not whole-house solar. It is the generator that will not start, the gate that will not open, the logger in a field, the trailer nobody checks. For those, a 10–40 watt panel and a controller will outperform a much larger system you did not need.
Start from daily amp-hours, not panel watts
Convert everything to amp-hours per day at system voltage. Milliamps × hours ÷ 1000 = amp-hours.
0.250 A × 24 h = 6 Ah/day.
Battery: at least 2× daily load → 12 Ah minimum; 30–35 Ah preferred for cloudy spells.
Panel: 20–40 W depending on latitude. Controller: yes.
Standby: (55/60) × 24 × 0.150 = 3.3 Ah/day
Transmit: (5/60) × 24 × 2 = 4.0 Ah/day
Total: 7.3 Ah/day → 18 Ah minimum battery, 40–55 Ah preferred; 30–50 W panel.
Panel output is not nameplate output
Panels are rated at peak sun with near-perpendicular incidence. Budget 4–6 productive hours per day, not 24. Anything before or after that window produces reduced power.
| 12 V panel | Approx. amps |
|---|---|
| 2 W | 0.25 A |
| 5 W | 0.33 A |
| 10 W | 0.60 A |
| 20 W | 1.20 A |
| 30 W | 1.67 A |
| 40 W | 2.40 A |
| 50 W | 3.30 A |
| 80 W | 4.40 A |
| 110 W | 6.30 A |
Controllers: when you need one
5 watts and up: use a controller. It regulates charging so the battery is not overcharged, and it blocks reverse current at night. Panels under that size usually have a blocking diode built in and are effectively self-regulating as maintainers.
One gotcha: many controllers will not pass voltage until they see a viable battery. Testing a controller on the bench with no battery attached looks like a dead controller when it is fine.
Orientation and tilt
Face the panel true south (not magnetic south) and tilt it to roughly your latitude — 25° to 46° in the continental US. Latitude plus 15° biases for winter, minus 15° for summer. Trailers and other mobile applications get parked in any direction, so a flat mount is the practical choice even though it is not optimal.
What solar is not good at
Trolling motors and golf carts come up constantly, and they are usually poor fits: the amps and voltages demand a physically large, expensive array. Solar maintenance on a stored battery — yes. Bulk charging a cart from empty every weekend — not realistically.
Run DC devices directly where you can. Every inverter adds 10–20% loss, so a 12 V fan beats a 120 V fan through an inverter every time.
Parts mentioned in this guide
Common questions
Can I skip the controller on a 5 W panel?
Often yes, if the panel has a blocking diode and the battery is large relative to the panel. Above 5 W, use a controller — the risk of overcharging a small battery is real.
My panel reads 20 V open-circuit on a 12 V system. Is it broken?
No. Open-circuit voltage of 16–22 V is normal; it collapses to battery voltage under load. That is why you cannot judge a panel by its unloaded voltage.
How long should the run from panel to controller be?
As short as practical, and size the wire for the current. Longer runs lose more; going up in system voltage (24 or 48 V) reduces loss more effectively than thicker wire alone.