It works backwards from the outage, not forwards from your bill. You say what must run and for how many hours; the tool sums the energy, converts it to a battery bank you can actually buy, sizes the inverter against the worst simultaneous startup, and tells you how much solar energy per day it takes to refill the bank.
Two independent input methods. List appliances and give each one its own outage hours, or type a direct kW load plus a single duration. They are calculated separately and never blended.
Every number is labelled as calculated, assumption, or your own input, so you always know which figures to replace with your battery's datasheet before you buy.
The two questions this tool answers
- How much battery capacity do I need? Sizing load × outage hours → AC kWh → divided by inverter efficiency → DC kWh → divided by usable depth of discharge and temperature → multiplied by a reserve margin → nominal kWh → rounded up to whole modules.
- What can the batteries I already own actually run? Their series/parallel layout, their usable AC energy, the runtime at your load, and the number of additional batteries needed to close the gap.
Method 1: list your appliances, each with its own hours
Choose “I know my appliances” and add a row per appliance: quantity, power in kW (or amps, which are converted at your system voltage), whether it is a motor, and — the field that matters most — how many hours of the outage it has to survive.
That last field is the reason this method is more accurate than a single duration. A fridge has to run the whole way through, a television for a few hours, a water pump for as long as the water lasts.
fans 0.30 kW × 8 h = 2.40 kWh
TV 0.10 kW × 3 h = 0.30 kWh
required AC energy = 6.30 kWh
The tool then divides that by the sizing load to give a weighted-average autonomy for the headline figures, but the requirement itself is the honest per-appliance sum. When one load dominates — the 24-hour fridge — the bank is genuinely sized for 24 hours on that load.
Method 2: type your load directly
Choose “I know the load directly (kW)” when you have a meter reading, an installer quote, or a single figure you trust. You enter the load in kW, optionally a peak, and the hours it must run.
Peak load is optional here. If you leave it blank the tool uses 1.25× your load, which is a reasonable allowance when you do not know your motor loads individually.
Simple mode and Engineering mode
That single sentence is the honest description of the two modes. In Simple mode the objective is fixed to Full-load backup: every appliance you list is sized. Because the ranking axes cannot change that outcome, they are hidden rather than shown as decoration. Switch to Engineering and the objective chooser, the ranking controls, the assumptions engine, the chemistry comparison and the full methodology all open up.
The three ranking axes
These matter only in Engineering mode, and only when the objective is not Full-load.
| Axis | Values | What it controls |
|---|---|---|
| Priority | 🔴 Essential · 🟡 Important · ⚪ Optional | How critical the appliance is. Drives Emergency sizing when the status is Always; Important loads join in for Night-time and Full-load; Optional loads only count in Full-load. |
| Backup status | Always · During backup | Whether the appliance runs the entire outage (fridge, router, security) or is switched on by choice (TV, air conditioner, washer). |
| Period | Day · Night | When the appliance is used. Night-period loads are pulled into Night-time sizing even if marked Optional. |
How the objective picks its scope
| Objective | Which loads size the system |
|---|---|
| Emergency backup | Only appliances that are Essential and Always. This is the strictest scope — it is why the tool can tell you “no loads in scope” if nothing is marked that way. |
| Night-time backup | Essential + Important, plus any load marked as a night-period appliance. |
| Full-load backup | Every appliance you entered, from Essential down to Optional. This is the Simple-mode default. |
Running load, peak load and motor surge
Three different numbers come out of your load list, and they are used for different jobs.
| Number | How it is built | What it sizes |
|---|---|---|
| Sizing load | The in-scope appliances' running power, added together. | The battery. Energy, capacity and runtime all follow from this. |
| Peak load | The running load plus the startup increments of the two largest motors. A motor's increment is its startup draw minus its running draw. | The inverter, because that is the worst case while motors start. |
| Continuous requirement | Sizing load × 1.1. | The inverter's steady-state rating, with running headroom. |
Taking the two largest motor increments together is deliberately conservative: it models a fridge and a water pump starting at the same moment while everything else in the house is already running. Motors are auto-flagged for fridges, pumps, air conditioners and washing machines, and the starting multiplier (2× by default) is editable.
The energy chain, once and only once
E_dc = E_ac ÷ inverter efficiency
E_ch = E_dc ÷ battery round-trip efficiency
solar = E_ch ÷ charging efficiency
nominal = E_dc ÷ (DoD × temperature factor) × reserve margin
Each loss appears exactly once. That is the whole point of the chain: charger losses, battery round-trip losses, inverter losses, then the useful energy that reaches your appliances. You are never charged twice for the same inefficiency.
The four verdicts
| Verdict | What it checks | When it warns |
|---|---|---|
| Chemistry | Both chemistries sized on the same duty at the same nominal kWh; the lower total cost of ownership wins (cycles × life). | It is a recommendation, not a constraint. LiFePO4 plans at 90% DoD / 6,000 cycles, lead-acid at 50% / 1,200. |
| Battery utilisation | Required energy against the usable energy of the bank you actually have. | Above ~85% the operating margin is tight; above 95% it is critical. |
| Solar recharge | Your existing array's estimated daily production against the daily recharge need. | 🟢 comfortably above · 🟡 roughly matches · 🔴 short of the need. |
| Inverter startup | Your peak demand against the inverter's surge capability. | When peak demand may exceed what the inverter carries for a few seconds. |
How much solar — and why no panel count
The tool reports the recharge requirement in kWh per day, which is a location-independent number: it depends on your loads and the charging losses, not on where you live.
It deliberately does not convert that into a kilowatt-peak figure. Peak sun hours change with country, season, tilt, shading and orientation, so a panel count derived from a default setting would be a guess dressed up as a result. Enter your existing system size in Step 3 and you will get a real green/amber/red verdict against it; if you do not know your system size, the tool points you to the Solar System Size Calculator for your location, and the Energy Yield Estimation tool to verify what a given array actually produces.
Configure Your Battery Bank: two questions
Once the sizing result exists, the same section answers a second question, in either direction.
- Size the bank for me — pick a battery voltage and amp-hour rating and the tool converts the required bank into a real series and parallel layout, with a wiring diagram, the resulting bank voltage, amp-hours and kilowatt-hours, and the capacity margin over the requirement. It also compares several amp-hour options side by side.
- I already have batteries — enter how many you own, their voltage, their amp-hours and the chemistry. The tool wires them against your inverter's battery voltage and reports the layout, the usable AC energy after depth of discharge and inverter efficiency, the runtime at the load you entered in Step 1, and exactly how many more batteries close the gap. Spare batteries that cannot complete a series string, and battery voltages that do not divide your inverter voltage, are both flagged.
usable AC kWh = nominal kWh × DoD × temperature factor × inverter efficiency
Which assumptions to replace with your datasheet
Anything labelled assumption is a typical planning value, not a product specification. In Advanced Settings you can replace all of them: depth of discharge, round-trip efficiency, inverter efficiency, reserve margin, temperature derating, motor starting multiplier, inverter surge capability, peak sun hours, charging efficiency, module voltage and module amp-hour rating.
The result is that a datasheet-driven design is one where a qualified engineer has replaced the yellow assumptions. The tool's job is to show you exactly which numbers are still guesses.
Frequently Asked Questions
Why does the calculator ask for hours on every appliance?
What is the difference between Simple and Engineering mode?
Why is a TV marked Essential still left out of the emergency sizing?
How is peak load worked out?
What does the recharge figure mean, and why is no panel count given?
What does the utilisation warning mean?
How do I know which assumptions to replace with my datasheet values?
The full calculator map
Each tool answers one question. This is the whole set, and what each one is for — so you can pick the next step instead of opening calculators at random.
Free, instant and interactive — enter your loads and outage hours, then check the results against the rules on this page.
- Free & instant
- No signup needed
- Every number labelled with its source
This guide explains the Solar Metrix Battery Backup Calculator. Numbers here are illustrations — your results depend on your actual inputs, location and datasheets. Always confirm final sizing with a certified installer.
Last updated: September 2026 | Browse all guides