We’ve got a little cabin we use on weekends, no grid power. Just want lights, phone charging, and maybe a small fridge. How do I figure out what size battery bank makes sense without way overbuilding?
Great question, and a pretty common starting point for folks heading down the off-grid path. The good news is you can size this pretty reasonably without a lot of guesswork if you work through it step by step.
Start with your load list
Write down every device you want to run and roughly how many hours a day you’ll use it. Something like:
- 4 LED lights × 10 watts × 5 hours = 200 watt-hours
- Phone charging × 2 = ~20 watt-hours
- Small 12V or efficient mini fridge = maybe 30–50 watt-hours per hour, but it cycles, so maybe 300–400 watt-hours over 24 hours
Add those up and you’ve got your daily watt-hour number.
Then do the battery math
A general rule of thumb: size your bank so you’re only drawing down to 50% capacity (that protects lead-acid batteries and keeps lithium healthy too). So if you need 700 watt-hours a day, you want at least 1,400 watt-hours of usable capacity — which gets labeled as a 1.4 kWh bank.
For a weekend cabin with modest loads like yours, a lot of folks land somewhere in the 1–2 kWh range and do just fine.
Don’t forget your charging source
Battery size and solar panel size go hand in hand. If you’ve got two cloudy days in a row, a bigger bank buys you time. A couple of 100–200 watt panels and a decent MPPT charge controller can top off a modest bank pretty reliably.
One honest suggestion
For the actual wiring, panel sizing, and charge controller setup — it’s worth a conversation with someone who does off-grid solar installs. Not because it’s rocket science, but because getting the charge controller sized right and the wiring done safely makes a real difference in how long your gear lasts.
What kind of battery chemistry are you leaning toward — lead-acid, AGM, or lithium? That’ll shape the sizing conversation a bit too.