Solar Battery Size Calculator
Find the home, solar or inverter battery you need to back up your appliances or your daily use for hours or days: its size in kWh and Ah, how many batteries, and in Advanced, the inverter and the solar panels to recharge it.
Presets use typical watts. The rating plate or a plug-in meter gives yours.
Depth of discharge is how much of the battery you let it use. Use your battery maker's figure if you have it.
Match it to your inverter. 12 V suits small backups; 24 V or 48 V suit bigger loads.
Autonomy adds whole days with no sun and no grid (for off-grid homes). Batteries give less in the cold: put the share your maker says is lost at your coldest temperature.
Leave the peak load and motor blank to use your appliance list.
Type the price from your quote or shop: total battery price ÷ its kWh. No price is assumed.
How your battery size is worked out
Battery = energy ÷ inverter efficiency ÷ depth of discharge.
- 1
Energy to back up
5 appliancesfor 8 h= 2,388 Wh
- 2
Allow for inverter loss
2,388 Wh ÷ 90%= 2,653 Wh used
- 3
Depth of discharge
2,653 ÷ 90% DoD= 2.95 kWh
- 4
Amp-hours and batteries
2,948 Wh ÷ 12 V= 245.7 Ah→ 3 × 12 V 100 Ah
Read more: the formula, a worked example, chemistry, wiring, inverter surge and solar recharge
A battery is sized from the energy it must give during the backup. Watts × hours gives that energy in watt-hours. The inverter loses some of it as heat, and you should not run the battery flat, so the battery has to be bigger than the energy you use. Dividing by the bank voltage turns the size into amp-hours, the figure most batteries are sold by.
The formula
energy (Wh) = each appliance's watts × how many × hours on, added up
or daily kWh × 1000 × share × backup hours ÷ 24
+ extra days of autonomy × energy a day
usable energy = energy ÷ inverter efficiency
battery size (Wh) = usable energy ÷ depth of discharge ÷ (1 − cold derate)
amp-hours = battery size (Wh) ÷ bank voltage
in series = bank voltage ÷ one battery's voltage
in parallel = amp-hours ÷ one battery's Ah, rounded up
inverter = peak load × (1 + headroom); VA = watts ÷ power factor
surge = peak load + (surge − 1) × biggest motor
solar kW = usable energy ÷ charging efficiency ÷ (peak sun hours × performance ratio)
Worked example
For 8 hours you want to run a fridge (150 W, its motor on about 35% of the time, so 2.8 h), two ceiling fans (75 W each, 8 h), four LED bulbs (9 W each, 8 h), a Wi-Fi router (10 W, 8 h) and a TV (100 W, 4 h). The energy is 150 × 2.8 + 150 × 8 + 36 × 8 + 10 × 8 + 100 × 4 = 420 + 1,200 + 288 + 80 + 400 = 2,388 Wh.
A 90% efficient inverter needs 2,388 ÷ 0.9 = 2,653.3 Wh from the battery. A LiFePO4 battery used to 90% must hold 2,653.3 ÷ 0.9 = 2,948.1 Wh (2.95 kWh). At 12 V that is 2,948.1 ÷ 12 = 245.7 Ah, so three 12 V 100 Ah batteries in parallel (300 Ah, 3.6 kWh). The same backup with lead-acid at 50% needs 2,653.3 ÷ 0.5 = 5,306.7 Wh, 442.2 Ah at 12 V: five 100 Ah batteries.
Everything on at once is 150 + 150 + 36 + 10 + 100 = 446 W. With 25% headroom the inverter should handle 446 × 1.25 = 557.5 W, which is 557.5 ÷ 0.8 = 697 VA. When the fridge motor starts it can pull about 3 × 150 W, so the inverter must take a surge of 446 + 2 × 150 = 746 W. The battery then gives up to 446 ÷ 0.9 ÷ 12 = 41.3 A.
To put the 2,653.3 Wh back in a day at 95% charging efficiency takes 2,653.3 ÷ 0.95 = 2,793 Wh. At 5.5 peak sun hours (an example only; use your own) and an 80% performance ratio, that needs 2.793 ÷ (5.5 × 0.8) = 0.635 kW, which is 2 panels of 400 W (0.8 kW).
Typical figures used (all editable)
| Battery type | Depth of discharge | Charging efficiency |
|---|---|---|
| Lithium iron phosphate (LiFePO4) | about 90% | about 95% |
| Lead-acid (flooded, tubular, AGM, gel) | about 50% | about 85% |
These are rules of thumb, not a maker's rating. Your battery's datasheet gives its own depth of discharge, cycle life at that depth and charging efficiency. A deeper discharge than the maker advises shortens a battery's life, especially lead-acid.
Hours on during the backup
With the backup in hours, each appliance counts the hours it is switched on during that time (never more than the backup). A fridge or freezer motor cycles on and off, so its preset counts about a third of the time. With the backup in days, each appliance counts its hours a day × the days. Extra days of autonomy add your energy a day for each one; with a backup in hours, a day is taken as the same use spread over 24 hours.
Batteries in series and parallel
Batteries in series add up their voltage; batteries in parallel add up their amp-hours. Four 12 V 100 Ah batteries in series make one 48 V 100 Ah string (4.8 kWh). Two such strings in parallel make 48 V 200 Ah (9.6 kWh). Use the same type, size and age of battery throughout a bank. Lithium batteries labelled 12 V are 12.8 V nominal; this page uses 12, 24 and 48 V, which gives a slightly larger, safer answer in amp-hours.
Inverter surge
Fridges, freezers, fans, pumps, air coolers, washing machines and ACs have motors or compressors. They can draw several times their running watts for a split second as they start. Only the biggest one is counted, since motors rarely start at exactly the same moment. Inverter ACs and fridges start softly and need less; old pumps and fixed-speed compressors can need more. Check the inverter's surge (peak) rating.
Tips
- Back up only what you need in a power cut: lights, fans, router, fridge. Leave heaters, kettles, irons and geysers off the backup.
- Higher bank voltages mean lower current, thinner cables and less loss. Match the inverter's battery voltage.
- Keep batteries cool and ventilated: heat shortens their life, and cold cuts their capacity.
- Solar panels also have to run your daytime loads, so a home that relies on solar needs more panels than the recharge alone. Our solar panel calculator sizes a full system.
This is an estimate for planning. Real backup time also depends on battery age, temperature, how loads change and the inverter's own idle draw. Have a qualified installer check your system.
Common questions
How big a battery do I need?
Add up the energy you want to run in watt-hours (watts × hours), divide by the inverter efficiency (about 90%) and then by the depth of discharge (about 90% for LiFePO4, 50% for lead-acid). A fridge, two fans, four LED bulbs, a router and a TV for 8 hours use 2,388 Wh, so you need 2,388 ÷ 0.9 ÷ 0.9 = 2,948 Wh, about a 3 kWh lithium battery.
How do I turn kWh into amp-hours (Ah)?
Divide the watt-hours by the battery bank voltage: Ah = Wh ÷ V. A 2,948 Wh battery is 2,948 ÷ 12 = 245.7 Ah at 12 V, 122.8 Ah at 24 V or 61.4 Ah at 48 V. The energy is the same; a higher voltage just needs fewer amp-hours and thinner cables.
Why does a lead-acid battery need to be twice as big?
Lead-acid batteries last much longer if you only use about half of their charge each time, so a 50% depth of discharge is typical. Lithium iron phosphate (LiFePO4) batteries are commonly used to about 90%. For the same backup, a lead-acid bank is about 1.8 times the size of a lithium one (90 ÷ 50). Use your battery maker's figure if it is different.
Should I use a 12 V, 24 V or 48 V battery bank?
Small backups such as lights, fans and a router often run on 12 V. As the load grows, the current at 12 V gets large (a 2,000 W load pulls about 185 A from a 12 V bank), so bigger systems use 24 V or 48 V. Match the bank voltage to your inverter.
What size inverter do I need?
Add the watts of everything that can be on at once, then allow some headroom (25% is a common rule of thumb). Motors in fridges, fans, pumps and ACs draw several times their running watts for a moment when they start, so the inverter's surge rating must cover the biggest motor starting. Inverters sold in VA need the watts ÷ power factor (about 0.8).
How many solar panels does it take to recharge the battery?
Divide the energy you took out of the battery, plus charging losses, by your peak sun hours and the performance ratio (about 80%). Putting back 2.79 kWh at 5.5 sun hours needs 2.79 ÷ (5.5 × 0.8) = 0.63 kW, which is 2 panels of 400 W.
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