In short
- A battery’s energy is Ah × volts: a 100 Ah battery at 12 V stores 1,200 Wh; a 100 Ah one at 48 V, 4,800 Wh.
- In series the volts add up and the Ah stay the same; in parallel the Ah add up and the volts stay the same. Always with identical batteries.
- The inverter is chosen by what you run at the same time: from a few hundred watts for lights and chargers to several thousand with air conditioning and a water pump.
- We give information so you know what to ask for. Installation is always for an electrician or installer.
How much energy each battery stores
Capacity is usually given in amp-hours (Ah). To get the energy it stores, multiply by its volts: Wh = Ah × V. Then subtract the part you shouldn’t use: for lithium we use 80%, as Australia’s national science agency CSIRO recommends so they last longer; for lead-acid, 50%, a cautious choice according to PVEducation. And if the appliance runs through an inverter, it uses some energy converting: our calculator assumes 85%.
| Battery | Energy (Ah × V) | Usable lithium (80%) | Hours with a fridge | Hours with a 100 W device |
|---|---|---|---|---|
| 50 Ah at 12 V | 600 Wh | 480 Wh | 4.5 | 4.1 |
| 100 Ah at 12 V | 1,200 Wh | 960 Wh | 9.1 | 8.2 |
| 200 Ah at 12 V | 2,400 Wh | 1,920 Wh | 18.1 | 16.3 |
| 300 Ah at 12 V | 3,600 Wh | 2,880 Wh | 27.2 | 24.5 |
| 100 Ah at 24 V | 2,400 Wh | 1,920 Wh | 18.1 | 16.3 |
| 100 Ah at 48 V | 4,800 Wh | 3,840 Wh | 36.3 | 32.6 |
| 200 Ah at 48 V | 9,600 Wh | 7,680 Wh | 72.5 | 65.3 |
How the hours are worked out: the fridge is a 200 W fridge-freezer which, according to the official table from Argentina’s energy regulator (ENRE), runs 45% of the time, so it uses 90 W on average, and through the inverter 90 ÷ 0.85 = 106 W comes out of the battery. The 100 W device draws 100 ÷ 0.85 = 118 W. For example, with 100 Ah at 12 V: 960 ÷ 106 = 9.1 hours. If your lithium battery says 12.8 V rather than 12, multiply by 12.8: a 100 Ah one is 1,280 Wh. With lead-acid, the usable energy is half the energy column (50%).
Note that 100 Ah isn’t always the same: 100 Ah at 48 V stores four times as much energy as 100 Ah at 12 V. To compare batteries, always compare Wh.
Which inverter you need for what you run
The inverter isn’t chosen by the battery, but by what you’ll have on at the same time (its continuous power) and by the start-up kick of the largest motor (its surge power). Here are four cases, with wattages from the ENRE table and motor start-up from the U.S. public utility EWEB:
| What you run at the same time | Continuous | Surge | Battery amps at 12 / 24 / 48 V |
|---|---|---|---|
| 4 LED bulbs, a fan, 2 chargers and the TV | 226 W | 226 W | 22 / 11 / 6 A |
| The above plus a fridge-freezer | 426 W | 1,026 W | 42 / 21 / 10 A |
| Fridge, 6 LEDs, 2 fans, 3 chargers, TV, microwave and washing machine | 1,839 W | 2,439 W | 180 / 90 / 45 A |
| The above with 8 LEDs and 4 chargers, plus a 2,200-frigorie (about 8,700 BTU) inverter air conditioner and a ½ HP water pump | 3,120 W | 4,173 W | 306 / 153 / 76 A |
The amps are the continuous watts ÷ 0.85 ÷ the battery voltage: for example, 1,839 ÷ 0.85 ÷ 24 = 90 A. That’s why the bigger the inverter, the higher the voltage should be: 306 amps at 12 V calls for very thick cables. To give you a sense of sizes, NREL’s guides mention small 100 to 200 W inverters for simple uses, and the home battery system the NLR laboratory uses as its reference is 5 kW. Look for one whose continuous and surge ratings on the spec sheet exceed yours with some headroom. We explain it in more detail in the inverter article.
What connecting batteries in series and in parallel means
When one battery isn’t enough, several are joined into what’s called a battery bank. Penn State University’s solar energy course explains it like this:
- In series: the negative terminal of one joins the positive of the next. The volts add up and the Ah stay the same. Two 12 V, 100 Ah batteries give 24 V and 100 Ah.
- In parallel: positive to positive and negative to negative. The Ah add up and the volts stay the same. The same two batteries give 12 V and 200 Ah.
- Both at once: four 12 V, 100 Ah batteries, in two series pairs joined in parallel, give 24 V and 200 Ah.
In the first two cases the energy is the same: 24 × 100 = 12 × 200 = 2,400 Wh. What changes is the voltage, and therefore the amps flowing through the cables and the inverter and controller you need, which must match the bank’s voltage.
What to respect when connecting batteries
- Identical batteries: Penn State always talks about identical batteries in a bank. Same type, same capacity and same voltage; the prudent thing is for them also to be the same brand, model and age. If in doubt, ask the manufacturer.
- Few parallel strings: the same course advises having as few parallel strings as possible, because voltage differences between them waste energy, and in large installations, connecting more batteries in series rather than adding parallel strings.
- Lithium batteries have their own protection system: the manufacturer’s manual says whether they can be connected in series or parallel, and how many.
- A wiring mistake can cause a short circuit with an enormous current. The bank’s connections, fuses and cables are work for a qualified electrician or installer.
How much battery you need is worked out by the power outage battery calculator or, if you have panels, the solar panel calculator. Which type to choose: home battery types.
Sources and references
- M. A. Chaaban, Penn State University, AE 868: Commercial Solar Electric Systems. Battery Systems and Design Considerations.
- CSIRO (Australian Government), 2021. Batteries for Homes.
- C. Honsberg and S. Bowden, PVEducation. Lead Acid Batteries.
- Ente Nacional Regulador del Gas y la Electricidad (Argentina). Consumo básico de electrodomésticos (in Spanish).
- EWEB (Eugene Water & Electric Board). Generator Sizing Considerations.
- National Renewable Energy Laboratory (NREL), 2000. Renewable Energy for Microenterprise (small inverters).
- National Laboratory of the Rockies (NLR, formerly NREL), Annual Technology Baseline 2024. Residential Battery Storage.
- National Renewable Energy Laboratory (NREL), November 2000. Renewable Energy for Rural Schools (an inverter is over 90% efficient near its maximum and less at low loads; the 85% is our own cautious figure).
Frequently asked questions
How many hours does a 200 Ah battery last?
It depends on the voltage and what you connect. A 200 Ah battery at 12 V stores 2,400 Wh; if it’s lithium and you use 80%, that leaves 1,920 Wh, which gives about 16 hours with a 100 W device through an inverter, and about 18 with a fridge-freezer.
Is it better to connect batteries in series or in parallel?
They store the same energy; what changes is the voltage. In series the voltage goes up and fewer amps flow through the cables, which is why it’s preferred in large installations; Penn State advises having few parallel strings. The bank’s voltage must match the inverter and controller.
Can I combine a new battery with an old one?
It’s not a good idea: a bank should use identical batteries, and the prudent thing is for them to be the same age too. If they’re lithium, the manufacturer’s manual says how they can be connected. Have an electrician check it.
