Sizing an off-grid battery is the single step most first-timers get wrong — and the one that decides whether your lights stay on through a cloudy week. The good news: it comes down to three numbers and one formula. Here is the whole method, a worked example, and a free calculator that turns the answer into a real product.
Forget brand names and "100Ah" labels for a moment. Every off-grid sizing starts with three values:
Miss any one of these and you either overspend or, worse, sit in the dark. Let's take them in order.
The most reliable source is your electricity bill. Take your monthly kWh and divide by 30:
| Monthly use | Daily average | Notes |
|---|---|---|
| 300 kWh | 10 kWh/day | Efficient, smaller home |
| 600 kWh | 20 kWh/day | Typical family home |
| 900 kWh | 30 kWh/day | Large / all-electric |
If you are building new and have no bill, estimate from appliances: a fridge (~1.5 kWh/day), lights and electronics (~3-5), water pump or well (~2-4), and heating/cooking if electric (high — consider gas or propane to keep the battery small).
"Autonomy" is how many days the battery runs with no solar input. It is your insurance against weather:
More autonomy means a bigger, costlier bank — but it is cheaper than a generator you fire up every week.
A battery's nameplate capacity is not all usable. Discharge too deep and you kill cycle life. This is where chemistry matters:
| Chemistry | Recommended max DoD | Usable fraction |
|---|---|---|
| Lead-acid (flooded) | 50% | 0.50 |
| LiFePO4 (lithium) | 80-90% | 0.80-0.90 |
Because LiFePO4 safely uses 80%+ of its nameplate, a 10 kWh LiFePO4 pack gives you ~8 usable kWh, while a 10 kWh lead-acid bank gives only ~5. That difference is why the formula divides by usable DoD, not nameplate.
Let's size for an efficient home: 10 kWh/day, 2 days autonomy, 80% usable LiFePO4.
10 kWh/day × 2 days ÷ 0.80 = 25 kWh of battery capacity.
That 25 kWh should be the usable target. Since LiFePO4 runs to 80% DoD comfortably, you'd spec a ~25 kWh LiFePO4 bank (or a bit more for winter margin). Compare that to lead-acid: 10 × 2 ÷ 0.50 = 40 kWh nameplate — a much heavier, shorter-lived bank for the same real cover.
Use the formula above — capacity = daily kWh × autonomy days ÷ 0.80 — and read your starting point straight from the table. All figures assume an 80% usable LiFePO4 bank at 2 days of autonomy.
| Your daily use | Autonomy | Recommended LiFePO4 capacity | Fits NovaBESS |
|---|---|---|---|
| 8 kWh/day (efficient home / cabin) | 2 days | ~20 kWh | 2 × HomeStack 10 kWh |
| 15 kWh/day (typical family home) | 2 days | ~38 kWh | 4 × HomeStack 10 kWh (or 2×15 + 1×10) |
| 25 kWh/day (large / all-electric) | 2 days | ~63 kWh | HomeStack stacks to 60 kWh + winter margin |
| 30 kWh/day (high-load villa) | 2 days | ~75 kWh | Multiple HomeStack banks (parallel 60 kWh+, add a 2nd) |
Want 3 days of autonomy or live in a cold, low-sun climate? Add about 25% to the capacity column. The point is the same as before: build to the number, then pick the modules that meet it.
Once you have a target kWh, match it to a modular LiFePO4 line rather than a fixed box. NovaBESS published specs (July 2026):
| Product | Capacity | Key specs | Fits a target of |
|---|---|---|---|
| HomeWall (wall-mounted) | 2.5 / 5.1 / 10.2 kWh | 5,000 cycles @80% DoD, 15-yr design life, CE/UN38.3/UL/IEC, LCD | 2.5-10 kWh |
| HomeStack (rack/stackable) | 5 / 10 / 15 kWh per stack, parallel to 60 kWh | CAN/RS485, IP54, -25°C to +50°C, UN38.3 | 5-60 kWh |
| PowerBox (portable) | 4 / 8 / 16 kWh | CE/UN38.3/UL/IEC, movable | 4-16 kWh mobile/cabin |
Back to the 25 kWh example: two HomeStack 15 kWh stacks parallel to 30 kWh (leaving headroom for winter and future load growth), or one HomeStack 15 kWh plus a HomeWall 10.2 kWh. The point is you build to the number, then pick the modules that meet it — not the other way around.
If you are sizing a grid-tied backup battery, the math looks similar but the targets are very different. Off-grid has no grid to bail it out, so it must carry every sunless day on its own.
| Factor | Off-grid | Grid-tied (backup) |
|---|---|---|
| Charging source | Solar, wind or generator only | Grid + solar |
| Autonomy needed | 2–3 days (survive sunless spells) | 1 day (essentials only) |
| Undersize risk | Blackout — no grid to fill the gap | Minor — grid covers the rest |
| Typical size | 20–60+ kWh | 5–15 kWh |
| Expandability | Critical — start modular, grow later | Helpful, not urgent |
That is why an off-grid bank is usually 3–5× larger than a same-home backup bank. Size for the worst week of weather, not the average day.
If you would rather not hand-crank the formula, the NovaBESS battery calculator does it in 30 seconds. Switch it to Off-grid mode, enter your monthly use and any solar generation, and it returns a recommended capacity mapped to a concrete model — then offers a tailored quote with distributor pricing.
The calculator's off-grid logic multiplies your net daily use (consumption minus solar) by 1.5, which is equivalent to roughly 1.5 days of autonomy at 100% usable — a sensible starting point you can then pad for climate. It is the same formula above, pre-solved.
Open the free off-grid calculator →
Battery capacity (kWh) = daily kWh × days of autonomy ÷ usable DoD. A typical off-grid home using 10 kWh/day with 2 days of backup and an 80% usable LiFePO4 bank needs about 25 kWh. A free calculator maps the result to a concrete model.
Most efficient off-grid homes use 8-15 kWh/day, so a 15-30 kWh battery covers 1-2 days. Large all-electric homes can use 30+ kWh/day and need 40-60 kWh or more. Start from your actual utility bill, not a guess.
Yes. Modular LiFePO4 systems parallel to grow. NovaBESS HomeStack stacks 5 kWh modules and parallels stacks to 60 kWh, so you can start with one stack and add later without replacing the first one.
An off-grid battery only stores energy, it does not make it. You need a charging source: solar panels, wind, a generator, or grid-tie (which is no longer "off-grid"). Size the array to refill the battery between usage days, especially in winter.
Use the same formula — battery capacity (kWh) = daily kWh × days of autonomy ÷ usable DoD. For an 80% usable LiFePO4 bank, that is roughly daily use × autonomy days × 1.25. An efficient 10 kWh/day home with 2 days of backup needs about 25 kWh; a 15 kWh/day home needs about 38 kWh. Start from your real bill, not a rule of thumb.
Most off-grid cabins use 5–15 kWh/day. At 2 days of autonomy on an 80% LiFePO4 bank that means about 12–38 kWh. A small 5 kWh/day cabin runs on roughly 12–15 kWh (e.g. one HomeStack 10–15 kWh stack); a 15 kWh/day cabin needs about 38 kWh (four HomeStack 10 kWh modules). Add ~25% for cold climates or 3 days of backup.
Specifications reflect NovaBESS published product data as of July 2026 (HomeWall: 5,000 cycles @80% DoD, 15-year design life, CE/UN38.3/UL/IEC; HomeStack: 5/10/15 kWh modules to 60 kWh, CAN/RS485, IP54, -25°C to +50°C, UN38.3; PowerBox: 4/8/16 kWh, CE/UN38.3/UL/IEC). Sizing examples are illustrative; actual requirements depend on your load profile, climate and charging source. Confirm exact configuration with the distributor in your market before purchase.
Go deeper on sizing, cost and what a battery can actually run:
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