# What size portable battery do I need?

I need to power a 10 W device to operate 16 hours a day on a 50% use capacity. So 10 W device running 8 hours a day I guess. I need to run this device from a battery pack as no AC is available. How many days can I run on what size of battery? Ideal is 14 days before recharge, how big a battery will I need?

• Do you have any access to sunlight, so solar can help top-up the battery across the daytime ? Dec 8, 2023 at 2:23

10 W * 8 hours/day * 14 days = 1120 Wh battery.

Do yourself a favor and get a larger one than you need. Especially lead-acid suffers from deep discharge so a two times overrating would be advisable. Lithium-ion is less sensitive, but ideally you would dimension to operate between 20-80 % SoC, implying a 1.4 overrating factor.

• Definitely oversize your battery. Especially if it has to survive under less-than-ideal temperature conditions as well. In winter, it's not uncommon to see commercial-grade Lithium-ion solutions hold 30% less charge, which means you need to over-engineer your battery to hold at least 143% of what you think it should under ideal conditions.
– Mast
Dec 7, 2023 at 20:45
• Then there's durability of a battery. How many months/years should it survive? Depending on the battery you get, expect 5-15% degradation each year.
– Mast
Dec 7, 2023 at 20:46

Simply multiply the numbers together.

10 W * 8 h * 14 = 1120 Wh.

• And realize that a lead-acid or gel battery can deliver perhaps 50% of the nameplate rating. Lithium is better under ideal conditions, but suffers much more from cold (unless it has built-in heating, which also impact actual Ah delivered). Dec 7, 2023 at 22:34