EGbatt Guide · Sizing

Undersized banks are the most common, most expensive mistake in storage design. This guide takes you from a plain list of loads to a bank size you can actually buy — with a worked example you can copy.

Updated 2026 · 6 min read · Written by EGbatt application engineers

Step 1 — List your loads in Wh per day

For every device write watts × hours/day. A fridge (150 W × 6 h = 900 Wh), lights (40 W × 5 h = 200 Wh), laptop (60 W × 4 h = 240 Wh). Sum them. Do not guess — measure or read the label.

Step 2 — Choose autonomy days

Autonomy is how many sunless/windless days you must ride through. Off-grid homes often use 2–3 days; grid-backup may use 1. More autonomy = bigger, costlier bank.

Step 3 — Apply depth of discharge

You only use part of the bank. At 80% DoD, divide daily Wh by 0.8. This is where the DoD/life trade-off lives — deeper daily DoD means a smaller but shorter-lived bank.

Step 4 — Worked example

InputValueMath
Daily load1,340 Whmeasured above
Autonomy2 days× 2 = 2,680 Wh
Usable at 80% DoD÷ 0.8= 3,350 Wh needed
Choose—round to a 3.5–4 kWh 48V bank

Step 5 — Size inverter and charger

Inverter continuous rating must exceed your largest simultaneous load; surge rating must cover motors. Charger/PV must refill the used Ah within a day of sun. Pick the voltage from the inverter size, then confirm cable.

Common mistakes

  • Undersizing the inverter surge for fridges/pumps.
  • Ignoring temperature (cold cuts usable capacity).
  • No margin for future loads — size 20% headroom.
  • Forgetting that the BMS sets the real limits.

Key takeaways

Measure loads

Watts × hours/day, summed — no guessing.

Pick autonomy

2–3 days off-grid is typical.

Divide by DoD

At 80% DoD use ÷ 0.8.

Add headroom

20% margin for future loads.

Talk to an EGbatt engineer

Tell us your load, voltage and autonomy. We will size the pack, quote it, and flag the traps before you buy.

Error: Contact form not found.

Or open the full contact page

Frequently asked questions

Size for typical daily load plus autonomy; add 20% headroom rather than designing around rare peaks.

Ah = Wh ÷ system voltage. A 3,350 Wh bank at 48V is about 70 Ah usable.

Choose a modular bank (stackable modules) so you can add capacity without redesigning.