Solar Battery Sizing Calculator

What Your Results Mean

Start with your daily energy use in kilowatt-hours — your electricity bills state it, or the Solar Panel Sizing Calculator helps you break it down by appliance. Be honest about what the battery actually has to carry: in a typical solar setup, daytime loads run straight off the panels, and only the evening and overnight consumption touches the battery. Sizing the bank on the whole day's total when half of it never reaches the batteries just buys capacity you'll never cycle.

The calculation then chains two corrections onto that number. First, the inverter loses some energy converting the bank's DC into household AC — 90% is a typical planning figure, so delivering 10 kWh at the sockets takes about 11.1 kWh out of the bank. Second, batteries are not drained flat: depth of discharge (DoD) is the share you actually use, and staying above the rest protects cycle life. 80% is typical for lithium batteries, 50% for lead-acid — lead-acid ages quickly when cycled deeper. Both figures vary by product, so treat them as planning defaults and check the manufacturer's data.

The voltage choice is about current, not energy. Amp-hours are the bank's kWh converted at the system voltage: Ah = kWh × 1,000 ÷ V. The same 13.9 kWh bank is about 1,157 Ah at 12 V, 579 Ah at 24 V, or 289 Ah at 48 V. Currents scale the same way — delivering 3 kW at 12 V means 250 A flowing through the cables, which demands copper most sheds can't accommodate — so higher-voltage banks are the norm above a few kWh. Small sheds and campervans commonly run 12 V; whole-house banks are typically 48 V.

Backup days — autonomy — set how long the bank carries you without sun. One day is typical where the array recharges daily and the grid (or a generator) can ride out failures. Off-grid sites, cloudy winter climates and loads that must never stop push toward two or three days. Note the trade: more autonomy means a bigger bank that cycles shallower on ordinary days, which is gentler on the batteries but means more capacity sitting idle most of the year.

The daily throughput row is the figure that drives battery life. Batteries are rated for a number of cycles, and how hard you cycle them each day sets how many years those cycles last — a bank worked flat-out daily ages much faster than one loafing through shallow cycles. Lead-acid banks also lose usable capacity in cold weather, which varies by chemistry and climate; a winter margin is wise where freezing temperatures are normal.

What the Results Mean

• Usable Energy Needed: daily use × backup days, corrected for inverter losses — what the bank must actually deliver.
• Battery Bank Capacity (kWh): the nominal bank size once depth of discharge is factored in — the headline figure for comparing products.
• Bank Size (Ah): the same capacity expressed at your chosen voltage, for comparing battery units rated in amp-hours.
• Daily Throughput: the energy cycled through the bank each day — the number that drives cycle life.
• Total: the bank capacity in kWh — the one-line answer to "what size bank?".

The Math Behind the Calculator

Each step of the chain divides by an efficiency or a usable fraction, which is why the bank grows as it absorbs each loss.

The inverter efficiency covers DC-to-AC conversion; 90% is a typical planning default for a modern inverter at mixed loads. The depth of discharge reserves the untouched share of the bank — 20% on lithium at the 80% default, half the bank on lead-acid at 50%. Amp-hour conversion is the plain energy-to-charge relation at the system voltage. What the chain deliberately does not include: charge-controller losses, round-trip losses inside the battery itself, temperature derating and wiring drops — real system designs account for all of them.

Worked Example: 10 kWh/day, One Day of Backup

A household uses 10 kWh per day and wants one day of backup, with a 90%-efficient inverter, an 80% depth-of-discharge lithium bank, and a 48 V system.

1. Usable energy: 10 × 1 ÷ 0.90 = 11.1 kWh that must come out of the bank.
2. Bank capacity: 11.1 ÷ 0.80 = 13.9 kWh nominal (rounded from 13.89).
3. Amp-hours at 48 V: 13.9 × 1000 ÷ 48 ≈ 289 Ah — call it 290 Ah.
4. Lead-acid comparison: at 50% DoD the same load needs 11.1 ÷ 0.50 = 22.2 kWh — 60% more nominal capacity for the same job.
5. Voltage comparison: that 13.9 kWh bank is about 1,157 Ah at 12 V or 579 Ah at 24 V — which is why 48 V is the practical choice at this size.

Notice how the corrections stack: 10 kWh of delivered energy becomes a 13.9 kWh bank — roughly 40% more than the headline daily use — before any margin for cold weather or battery ageing. Anyone comparing a "10 kWh battery" against a "10 kWh daily load" is comparing two different things.

Safety: This Is Electrical Work

Assumptions & Limitations

Every number above rests on these constants. If your system differs, change the matching input — or read the linked guides for the full reasoning.

• 90% inverter efficiency and 80% / 50% depth-of-discharge figures are typical planning defaults — actual values vary by product
• No charge-controller losses, round-trip battery losses, temperature derating or wiring voltage drop included
• Amp-hours are nominal at the selected voltage; real banks are built from series-parallel strings that need balancing
• Daily use should reflect what the battery actually carries — evening and overnight loads in a solar setup
• Quantities only — this site deliberately does not price batteries, inverters or installation

Practical Next Steps

• Read: How Many Solar Panels Do I Need? Pair the bank with an array that can recharge it.
• Read: Electrical Service Load Guide Work out the loads behind your daily kWh figure.
• Plan the whole solar job Panels, battery and roof work sequenced together.