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Three energizer sizes, calculated
Running 24 hours a day. The battery is lead-acid at 50% usable for 3 days without sun, and the panels are sized for 4 and for 2.5 peak sun hours.
| Energizer | Power draw | Energy per day | Battery for 3 days | Panel at 4 h of sun | Panel at 2.5 h |
|---|---|---|---|---|---|
| Small energizer, 0.1 A | 1.2 W | 29 Wh | 14 Ah | 10 W | 15 W |
| Medium energizer, 0.25 A | 3 W | 72 Wh | 36 Ah | 24 W | 38 W |
| Large energizer, 0.5 A | 6 W | 144 Wh | 72 Ah | 48 W | 77 W |
An electric fence is a small load, so the panel is small too. What matters most is the battery and the panel staying healthy through weeks of poor weather, which is why the winter column is the one to plan around.
How it is calculated
- Energy per day (Wh) = power draw in watts × hours per day. Watts = amps × 12 V.
- Battery (Wh) = energy per day × days without sun ÷ usable depth of discharge. Ah = Wh ÷ 12 V.
- Panel (W) = energy per day ÷ (peak sun hours × 0.75). The 0.75 covers heat, dirt, wiring and charge controller losses.
- Runtime on your battery = battery Ah × 12 V × usable depth ÷ energy per day.
Round the panel up to the next size you can buy. It does not model the extra draw when a fence is heavily loaded, or battery ageing.
Using it anywhere
- Watts, amps and amp-hours work worldwide, and the sun levels apply to any country.
- Choose the sun level for the shortest, cloudiest month when the fence matters most.
- Most solar energizers use a 12 V battery. If yours uses 6 V or another voltage, enter the draw in watts.
Why solar fences fail
- Sizing for summer. Days get short and cloudy just when animals are most likely to test a fence.
- Shade. A tree or tall grass over the panel can cut charging a lot.
- A worn battery. A tired battery holds far less than its rating.
- Vegetation on the wire. Weeds touching the line drain the energizer and can raise its draw.
- Poor grounding. A weak earth makes the fence ineffective whatever the battery.
Common questions
What size solar panel do I need for an electric fence?
For a medium energizer drawing about 3 W around the clock, the fence uses about 72 Wh a day. With 4 peak sun hours you need about 24 W of panel, and with 2.5 sun hours about 38 W. Round up to the next panel size, and size for your cloudiest season.
How big a battery does a solar electric fence need?
For the same 3 W energizer to run 3 days without any sun on a lead-acid battery at 50% usable, you need about 36 Ah at 12 V. A lithium battery at 90% usable needs about 20 Ah.
How long will a 12 V battery run an electric fence?
A 55 Ah lead-acid battery at 50% usable holds about 330 Wh, so it runs a 3 W energizer for roughly 4.6 days without charging. Enter your battery in the calculator to see its runtime for your energizer.
Does a longer fence use more power?
The energizer’s own draw is set mostly by its design. A longer or weedier fence can make it work harder in some models, so check the spec sheet, and leave margin on both the battery and the panel.
Do I need a charge controller?
Yes for most panels above a few watts. A controller protects the battery from overcharging. Many small solar fence kits include one in the panel or energizer, so check what yours has.
Why does my solar fence stop working in winter?
Short, cloudy days give the panel little energy, and cold weather reduces the battery’s capacity. If the battery is too small or the panel is shaded, it runs down. Size the panel for your winter sun and keep the panel clear of snow and shade.
Related calculators
For a larger solar system, use the solar and battery calculator. To pump water on a farm, try the solar water pump calculator, and to check how long solar takes to charge a battery, the solar charge time calculator.