家庭用バックアップ蓄電計算機

停電に備えて必要な蓄電池の容量は？

重要負荷（冷蔵庫・ポンプ・インターネット・照明）を合計し、時間/日と目標日数を設定して、蓄電量と1日あたりの太陽光充電量を算出します。

Home backup calculator

Outage coverage planner

Critical load

Load hours per day

Target backup

days

Installed solar

Peak sun hours

h/day

Battery capacity

kWh

Battery DoD

Inverter efficiency

Estimated coverageSolar positive

Daily load16.2 kWh

Daily solar20.7 kWh

Usable battery10.9 kWh

Target coverage161%

Battery available

Battery needed

Solar installed

How the math works

Daily critical kWh = Critical watts × Hours per day ÷ 1000.
Required battery kWh = Daily kWh × Backup days ÷ (Battery DoD × Inverter efficiency).
Solar recharge per day = Solar kW × Peak sun hours × Performance factor (default 0.82, matching 18% system loss).
Net daily draw = Daily kWh − Solar recharge. If negative the system is solar-positive and only needs enough storage for the worst-case overnight.

Worked examples

Suburban home — 900 W critical for 18 h × 2 days, no solar

900 × 18 = 16.2 kWh/day × 2 ÷ (0.90 × 0.90) ≈ 20 kWh battery → roughly 1.5× Tesla Powerwall 3 (13.5 kWh each).

Same loads, 5.6 kW solar at 4.5 PSH

Solar recharge ≈ 5.6 × 4.5 × 0.82 = 20.7 kWh/day. Net daily draw is negative; one Powerwall (13.5 kWh usable) covers overnight comfortably.

Off-grid cabin essentials — 450 W for 12 h × 3 days

450 × 12 = 5.4 kWh/day × 3 ÷ (0.9 × 0.9) ≈ 20 kWh LiFePO4 stack, ideally paired with 3 kW PV.

Home battery backup FAQ

How do I figure out my critical loads?

Walk the breaker panel and identify must-have circuits: refrigerator (150 W avg), well pump (1000–2000 W intermittent), furnace blower (500 W), internet (40 W), a few LED lights (50 W). Sum the steady-state watts.

Do I need a transfer switch?

Yes for whole-home backup. Tesla Powerwall, Enphase IQ, and Franklin aPower ship with smart load centers. DIY stacks need a manual or automatic transfer switch installed by a licensed electrician.

Can solar alone keep me powered?

Only with batteries. Grid-tied solar without batteries shuts down during outages to protect line workers. AC-coupled batteries or hybrid inverters are required for daytime backup.

How many Powerwalls do I need?

For partial backup (essentials + fridge) most homes use 1 Powerwall (13.5 kWh). Whole-home with AC and EV charging usually needs 2–3 stacked. Use the calculator to confirm.

What's a realistic discharge depth?

LiFePO4 systems (Powerwall 3, Enphase IQ, Franklin) cycle to 100% safely. The 90% default in the calculator reserves headroom for cold-weather de-rating and end-of-life capacity loss.

計算機ディレクトリ