How do I size a solar battery backup?

Start from the backup question: what must keep running, and for how long? List your appliances and give each one the number of outage hours it must survive (fridge 24 h, TV 3 h) - the required energy is the sum of each appliance's kW x hours. Then divide by inverter efficiency and the battery's usable depth of discharge (DoD). For example 2.0 kW for 6 hours = 12 kWh AC, divided by 94% inverter efficiency = 12.8 kWh DC usable, divided by 90% DoD = about 15.6 kWh nominal LiFePO4 capacity (the calculator adds a ~10% reserve margin on top of the DoD-limited capacity). The calculator does all of this live.

I already have batteries - how long will they run my loads?

Use the 'I already have batteries' option in the Configure Your Battery Bank section. Enter how many batteries you have, the voltage of each and the amp-hour rating, plus the chemistry. The tool converts that into a series/parallel layout against your inverter's battery voltage, then reports the nominal kWh, the usable AC energy after depth of discharge and inverter efficiency, the runtime in hours at the load you entered, and exactly how many additional batteries are needed to cover the full requirement. Spare batteries that cannot fill a complete series string are flagged.

What is the difference between listing appliances and entering my load directly?

They are two separate methods and are calculated independently. 'I know the load directly' takes one kW figure and one outage duration, and sizes from load x hours - use it when you have an installer quote or a meter reading. 'I know my appliances' takes a list where each appliance carries its own outage hours, so a fridge that must run 24 h and a TV that runs 3 h produce a smaller, more accurate energy requirement than applying one duration to everything. The per-appliance hours are added together: required energy = sum of (kW x hours).

What does Simple mode leave out?

Simple mode sizes Full-load backup: every appliance you list is included, and it removes the three ranking controls (Priority, Backup status, Day/Night) because with every load in scope they cannot change the result. Engineering mode restores the objective choice - Emergency (only always-on Essential loads), Night-time (Essential + Important + night-period loads) or Full-load - and opens the Assumptions Engine, the chemistry comparison, the engineering analysis and the methodology.

Which battery should I choose - LiFePO4 or lead-acid?

For most backup installations LiFePO4 is recommended because it cycles 6,000+ times, allows ~90% depth of discharge and runs at ~95% round-trip efficiency - so a smaller physical battery delivers the same usable energy for much longer. Lead-acid (tubular) has a lower first price but roughly 50% usable capacity, 80% efficiency and 1,200 cycles. The calculator lets you adjust both chemistries' assumptions and compare the two side by side.

What voltage battery system do I need?

Residential LiFePO4 systems are normally 51.2 V nominal (16 × 3.2 V cells - commonly sold as a '48 V' battery). Lead-acid banks are usually 12 V for small systems, 24 V for medium and 48 V for larger ones. The calculator auto-selects the voltage from your capacity and lets engineers override it in Advanced settings.

How much solar do I need to recharge a backup battery?

The recharge need is the energy the battery used, restored through the full charging path - 95% round-trip and 85% charging efficiency. For a 2 kW load over a 6 h outage that is about 15.8 kWh/day at the panels. The calculator reports that kWh/day figure, which is location-independent. It deliberately does not state a kWp panel count, because peak sun hours differ by country, season, tilt and shading - use the Solar System Size Calculator with your location to get the exact array size, then enter your existing system size to check whether it can keep the battery charged.

What size inverter do I need for battery backup?

The inverter must cover your continuous load plus the peak simultaneous load. For a 2.0 kW typical load with a 4.1 kW peak, a 5 kW inverter is the smallest sensible choice. Motor loads (refrigerator, pump, AC, washing machine) can draw 2× nameplate at startup, so check the inverter's surge rating, not just continuous kW.

Battery Backup Calculator | Size a Battery & Test the One You Own

Solar Battery Backup Calculator

How much battery do you need - and what can the one you already own run?

This tool answers two questions. First, how much battery capacity your load needs - list your appliances and give each one the number of outage hours it must survive, or type your load straight in as kW plus hours. Second, what the batteries you already have can actually run - enter their specs and get the runtime, the series/parallel layout and how many more batteries close the gap. You get battery kWh and Ah, voltage, modules, currents, inverter size and the recharge energy per day, with every number labeled with its source. All results update live as you type.

Calculator mode

Simple: full-load backup, no ranking to understand. Engineering: choose the objective (emergency, night or full) and open every assumption, the chemistry comparison and the full analysis.

1 Objective 2 Loads 3 Solar 4 Tech

Step 2 - What loads will run on battery, and for how long?

Pick the method that matches what you know. Listing appliances: every appliance gets its own Hours in outage value - the hours it has to survive when the grid fails, so a fridge can be 24 h while a TV is 3 h. Direct load: type one kW figure and the hours it must run. In Engineering mode you also rank each appliance by Priority (🔴 Essential, 🟡 Important, ⚪ Optional), Backup status (Always = runs the whole outage; During backup = switched on by choice) and When (Day/Night) - those three decide which loads the chosen objective sizes. Tick Motor for fridges, pumps, ACs and washers - they need startup surge.

Hours are preset per appliance type - change any of them to match how long it must actually run in an outage.

Step 3 - Do you already have solar?

If yes, we check whether your existing array can recharge this battery - a headline answer, not just a panel count.

Existing system size (kWp)

Not sure how big your system is, or how much it will actually produce? Get the exact array size for your specific location from the Solar System Size Calculator → Peak sun hours change with country, season, tilt and shading, which is why this page reports the recharge energy in kWh/day instead of guessing a panel count. To verify the hourly and monthly output of a given array size, use Energy Yield Estimation →.

Step 4 - Battery technology

Auto recommends LiFePO4 for most backup systems; lead-acid remains an option where first price matters. The recommendation is not based on an arbitrary kWh threshold - it separates calculation from advice.

Auto (recommended)

Sizes with LiFePO4 and shows the lead-acid comparison underneath.

Lithium LiFePO4

Deep DoD, high efficiency, 6,000+ cycles - sized as a modular 51.2 V stack.

Tubular lead-acid

Lower first price, ~50% usable capacity, 1,200 cycles - sized as 12 V×200 Ah units.

Advanced Settings (engineers - replace typical assumptions with your datasheet values)

Battery system voltage

Auto: 51.2 V for LiFePO4; 12/24/48 V for lead-acid by bank size.

Battery chemistry - default planning assumptions (not product specs - replace with your datasheet)

LiFePO4 DoD %

LiFePO4 RTE %

Lead-acid DoD %

Lead-acid RTE %

Inverter eff %

Reserve %

Motor starting multiplier

Startup surge applied to motors (fridge, pump, AC, washer) on top of the running load. Datasheet-based installers can enter a precise value.

Inverter surge capability

How much above nameplate the inverter carries for a few seconds while motors start. The inverter is sized to satisfy continuous AND surge loads - replace with the datasheet value.

Apply temperature derating

Temperature factor (90% = derate to 0.9) for cold/hot sites.

Solar recharge assumptions

Peak sun hours

Charge eff %

Panel wattage W

Recharge in days

Peak sun hours refine the recharge size - Solar Metrix yield tools can give you location-accurate values.

Battery module & BMS - datasheet overrides

BMS rating A

Module voltage V

Module capacity Ah

Empty = defaults (LiFePO4: 51.2 V × 100 Ah; lead-acid: 12 V × 200 Ah). A BMS rating lets the tool flag whether your BMS can handle the peak discharge current.

Your Backup Plan

The homeowner answer first - details and engineering below.

🔋 Battery Size

⏱ Backup Time

⚡ Inverter

☀️ Recharge Need

Enter your loads above to see your backup plan.

Configure Your Battery Bank

Two questions, one section. Size the bank for me converts the sizing result above into a real series/parallel layout. I already have batteries works the other way: enter the batteries you own and we tell you what they can run and how many more you need. Both are separate calculation layers - neither changes the required bank or the module count above.

Required bank (from sizing) -- / --

Inverter battery voltage

Rules: series adds voltage only (12 V × 4 = 48 V - each battery carries the FULL string Ah, never split: 4 × 50 Ah in series is still 50 Ah, not 200) · parallel adds Ah only · every string uses identical batteries - never mix different Ah sizes in one string (a string is limited by its smallest battery).

Battery voltage

Battery capacity

Compare battery options

Enter the battery capacities you are considering (Ah, comma-separated). Each is evaluated independently on the same sizing result - click a row to apply that configuration.

What This Means - At a Glance

How Long Will It Last? Runtime, Energy Flow & Discharge

Estimated battery discharge assuming a constant load - not a real battery model. Actual SOC may differ because battery voltage, temperature, discharge rate, BMS limits and battery aging affect available capacity.

Autonomy Scenarios - How Long Will the Battery Last?

Same battery, different loads: what happens if you switch the AC on, or run only essentials.

What if - fixed loads

Chemistry Comparison - Same Duty, Both Options

What the sizing looks like with each chemistry, everything else equal. The recommended option is highlighted.

Advanced Engineering Analysis (For Engineers & Installers)

Load Analysis

Continuous load

Essential load

Peak load

Starting surge

Recommended inverter: --

Battery Analysis

Nominal system voltage	--
Nominal capacity required	--
Configured capacity (modules)	--
Usable energy	--
Depth of discharge	--
Round-trip efficiency	--
Nominal capacity	--
Bank configuration	--
Continuous battery current	--
Peak battery current	--
Required BMS discharge current	--
Peak discharge C-rate	--
Design utilization	--
Battery cable	--
Cycle life / calendar life	--
Battery energy losses (annual)	--

Solar Recharge Analysis

Estimated daily recharge

Estimated solar (kWp)

Panels

Assumptions Engine - Every Number, Labeled

Each result carries its source - User input Calculated Assumption - so you always know which numbers are yours, which are derived, and which are typical values you should replace with datasheets. Nothing here is treated as a universal constant.

How the Math Works

1. Backup requirement (the core question). Battery capacity is sized from what must run, and for how many hours - not from your total day+night energy. Day energy matters for solar recharge context, not for storage sizing.

2. Per-appliance outage hours. In the appliance method every load carries its own Hours in outage value, and the required energy is the sum of each load's kW × its own hours. A 0.15 kW fridge at 24 h contributes 3.6 kWh; a 1.5 kW air conditioner at 2 h contributes 3.0 kWh. The two input methods are calculated independently and never mixed: the direct kW method takes one load and one duration instead.

3. The energy chain (no double counting): E_ac = Σ (load kW × its own hours) → E_dc = E_ac ÷ inverter efficiency → E_charged = E_dc ÷ battery RTE → solar input = E_charged ÷ charging efficiency. Each loss appears exactly once: charger losses, battery round-trip losses, inverter losses, useful AC.

4. Nominal capacity: nominal = E_dc ÷ (DoD × temperature factor) × reserve margin. The battery is bought at its nominal rating; the usable portion is nominal × DoD × temperature.

5. Loads and inverter: the sizing load follows your objective: Emergency = always-on Essential appliances (Backup status: Always); Night = Essential + Important plus night-period loads; Full = every appliance you entered - which is what Simple mode always uses. Peak load = the in-scope running load plus the startup surge increments of the two largest motors (motors are auto-flagged for fridges, pumps, ACs and washers) - conservative for simultaneous starts. The recommended inverter is the smallest standard size that satisfies both conditions: at least 1.1 x the continuous load, and at least the peak load within its surge capability (assumption: 1.5x nameplate for a few seconds, configurable in Advanced).

6. Solar recharge: daily recharge = E_charged ÷ charging efficiency (restored in your chosen number of days). That kWh/day figure is location-independent, and the tool deliberately does not convert it into a kWp panel count - peak sun hours change with country, season, tilt and shading, so a panel count from a default setting would be a guess. If you entered existing solar, we compare its estimated daily production (kWp × PSH) against the recharge need and report a 🟢/🟡/🔴 verdict; if you did not, use the Solar System Size Calculator with your location to get the exact array size.

7. Assumptions are yours to change. DoD, RTE, inverter efficiency, reserve, temperature derating, motor starting multiplier, inverter surge capability, peak sun hours, charging efficiency and panel wattage all live in Advanced Settings. The defaults are default planning assumptions, not product specifications - replace them with your battery's datasheet before final design.

All data stays in your browser. No information is sent to any server.

Quick Answers (No Jargon)

How big a battery do I need to buy?

How long can this battery run my home?

What size inverter do I need?

How much solar do I need to recharge it?

LiFePO4 or lead-acid - which should I choose?

What can my battery actually run?

Now: Will It Pay for Itself?

Your backup design carries straight into the Battery ROI calculator - battery size, chemistry, backup load and cycle assumptions are transferred automatically.

Check the money side of this battery

10-year cost comparison, payback, LCOS - with your battery size and loads already filled in.

Next: the Question Your Answer Raises

These three are picked from what you have entered so far - not a fixed list.

Everything in one page

Open the Complete Setup Calculator

Analyze every aspect of your solar project, sizing, roof space, battery, yield, string design and ROI, in one place, and generate a downloadable report of your results.

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