Solar system calculator
Size the inverter and battery bank for backup power — enter your backup load and outage hours, get kW, kWh storage and module count.
Quick answer: A 1,500 W backup load for 6 hours a day needs about a 1.9 kW inverter (25% headroom for surges) and 10.5 kWh of lithium storage — that's 3 × 5.12 kWh modules. The same backup in lead-acid needs ~18.9 kWh of bank.
Backup needs
Ballpark, not gospel. Real loads vary — measure your critical appliances with a kill-a-watt meter before buying, and have an installer confirm wire gauges and breaker sizing.
Battery types at a glance
| Battery type | Usable depth | Typical cycles | Best for |
|---|---|---|---|
| Lithium (LiFePO₄) | 90% | 3,000–6,000 | Daily cycling, backup, off-grid |
| Lead-acid / AGM | 50% | 500–1,200 | Occasional outages, tight budgets |
| Tubular lead-acid | 50% | 1,000–1,500 | Common in South Asia for inverter backup |
Usable depth is the fraction of nameplate capacity you can actually draw. The calculator already applies it, so its kWh output is the bank size to buy, not the energy you use.
Grid-tie vs hybrid vs off-grid
| Setup | Batteries | Works in outage | Best for |
|---|---|---|---|
| Grid-tie | None | No | Cutting the bill where the grid is reliable |
| Hybrid | Yes | Yes | Savings plus load-shedding / outage backup |
| Off-grid | Yes, 2–3 days | Yes | No grid access at all |
Solar battery size calculator
The tool above doubles as a solar battery size calculator: enter your backup load and outage hours and it returns the battery bank in kWh, already adjusted for usable depth. Sizing from daily consumption instead? The dedicated solar battery size calculator works from kWh per day and days of autonomy, and converts to amp-hours at 12, 24 or 48 volts.
Solar inverter size calculator
It is also a solar inverter size calculator: backup watts × 1.25 headroom for motor start-up surges. A 1,500 W load needs a ~1.9 kW inverter — round up to the nearest standard size (2 kW, 3 kW, 5 kW). Undersize it and the first air-conditioner start-up will trip the system.
Questions people actually ask
Straight answers, no fluff. Then run your own numbers above.
How do I size an inverter for my home?
Add up the watts of everything you want backed up, then add 25% headroom for motor start-up surges. A 1,500 W backup load needs about a 1.9 kW inverter — round up to the nearest standard size, typically 2 kW or 3 kW.
How many kWh of battery do I need?
Multiply backup watts by outage hours, then divide by usable depth and inverter efficiency. A 1,500 W load for 6 hours needs about 10.5 kWh of lithium storage (90% usable) or 18.9 kWh of lead-acid (50% usable).
Lithium or lead-acid batteries for solar backup?
Lithium (LiFePO₄) costs more upfront but gives 90% usable depth and thousands of cycles. Lead-acid is cheap but only 50% usable — discharging below half destroys its life — so you need nearly double the bank for the same backup.
What is the difference between grid-tie, hybrid and off-grid systems?
Grid-tie has panels plus a grid inverter and no batteries — it shuts down in outages. Hybrid adds batteries and keeps working. Off-grid has no utility connection at all and needs days of battery autonomy plus a backup generator for cloudy stretches.
How many 5.12 kWh battery modules do I need?
Divide your target storage by 5.12 and round up. The default 1,500 W / 6 hour example needs ~10.5 kWh of lithium storage, which is 3 × 5.12 kWh modules.
What are days of autonomy?
How many cloudy days your battery bank covers with no solar input. One day suits grid-tied backup; true off-grid setups use 2–3 days plus a generator.
Can I run an air conditioner on a solar backup system?
Yes, but it dominates sizing. A 1.5-ton AC draws ~1,500–1,800 W running with a large start-up surge — budget its watts honestly in the load input or it will trip the inverter.
How many solar panels recharge this battery bank?
As a rule of thumb, size the array at 2–5 kW per 10 kWh of battery so the bank refills on a normal solar day. The country calculators size panels from your electricity bill.
What does a solar battery size calculator tell me?
The bank size to buy in kWh — usable depth already applied — plus amp-hours at your system voltage and a battery count. This page computes it from backup load and outage hours; the dedicated battery calculator works from daily kWh and days of autonomy.
How does a solar inverter size calculator work?
It totals the watts you want backed up and adds ~25% headroom for start-up surges, then rounds up to a standard inverter size. The "Inverter size" row above is exactly that calculation, live.
Sizing the panels too? The country solar calculators size the array from your bill. Then the solar cost calculator prices the whole job, and the solar ROI calculator checks whether it pays back.