In short
- A 1,000 W inverter doesn’t need “a 1,000 battery”. Battery capacity depends on what you run and for how many hours.
- What the inverter does set is the current: at full power, a 1,000 W inverter draws about 98 amps from a 12 V battery, 49 from a 24 V one and 25 from a 48 V one.
- That’s why large inverters run at 24 or 48 V, and the inverter and the battery must be the same voltage.
- Capacity: Wh = watts you use × hours ÷ 0.85 ÷ the usable share of the battery (80% for lithium, 50% for lead-acid). Ah are those Wh ÷ the volts.
Words you’ll see
- Watts (W), volts (V), amps (A), Wh and Ah: we explain them slowly in watts, kWh, volts and amps. The rule used most here: watts = volts × amps.
- Inverter: turns the battery’s current into the current at your outlets. See which inverter to choose.
- Usable share (depth of discharge): how much of the battery you can use without damaging it: 80% for lithium and 50% for deep-cycle lead-acid.
The inverter’s watts don’t set the battery’s capacity
It’s the most common mix-up. The inverter tells you how much you can run at once; the battery, for how long. A 1,000 W inverter with a small battery won’t last long; the same inverter with a large one will last much longer. So the right question is: what will I have running, and for how many hours?
What the inverter does set: the current
An inverter draws from the battery as many amps as the watts it supplies, divided by the battery’s volts, plus a bit more for what it loses converting. According to the U.S. National Renewable Energy Laboratory (NREL), an inverter is over 90% efficient near its maximum and less efficient at low loads; here we use 85%, to be on the safe side. The maths:
Amps = watts ÷ 0.85 ÷ battery volts
| Inverter at full power | 12 V battery | 24 V | 48 V |
|---|---|---|---|
| 500 W | 49 A | 25 A | 12 A |
| 1,000 W | 98 A | 49 A | 25 A |
| 1,500 W | 147 A | 74 A | 37 A |
| 2,000 W | 196 A | 98 A | 49 A |
| 3,000 W | 294 A | 147 A | 74 A |
| 5,000 W | 490 A | 245 A | 123 A |
| 6,000 W | 588 A | 294 A | 147 A |
The more amps, the thicker the cables have to be and the more they heat up. In our calculators we use this rule: the lowest voltage that keeps the current at 100 A or less. That gives 12 V up to about 1,000 W, 24 V up to about 2,000 W and 48 V above that. It’s our own rule to keep the cables manageable, not a standard: your installer will adjust it to your case.
The battery has to be able to deliver that current
Having lots of Ah isn’t enough. According to Australia’s government science agency (CSIRO), the capacity a battery delivers changes with the current it’s discharged at. Each battery’s spec sheet gives its maximum continuous discharge current (in amps): it must be higher than the figure in the table for your inverter and voltage. If one battery isn’t enough, several are connected in parallel; we explain how in batteries in series and parallel.
How much capacity: working it out by hours
Battery Wh = average watts you use × hours ÷ 0.85 ÷ the usable share (0.8 for lithium, 0.5 for lead-acid). And Ah = Wh ÷ volts.
Careful with “the watts you use”: it isn’t the inverter’s rating, but what your appliances use on average. A fridge, for example, isn’t running all the time. The power outage battery calculator works it out appliance by appliance.
Worked example: an 8-hour power cut
You have a 1,000 W inverter and, during an 8-hour power cut, what you run uses 300 W on average:
- Energy: 300 W × 8 h = 2,400 Wh.
- With the inverter’s losses: 2,400 ÷ 0.85 = 2,824 Wh.
- Lithium battery (80% usable): 2,824 ÷ 0.8 = 3,529 Wh. At 12 V: 3,529 ÷ 12 = 294 Ah; at 24 V, 147 Ah.
- Lead-acid battery (50% usable): 2,824 ÷ 0.5 = 5,647 Wh. At 12 V, 471 Ah.
- Current: if at some point the inverter runs flat out (1,000 W), at 12 V the battery has to deliver 98 A.
The differences between lithium and lead-acid, and which one makes sense, are in battery types.
Common mistakes
- Choosing the battery by the inverter’s watts. Choose it by what you use and for how long.
- Using all of a lead-acid battery’s capacity. It gets damaged: count on only half.
- Mixing voltages. A 24 V inverter needs a 24 V battery (or bank).
- Forgetting the current. A battery that can’t deliver the amps the inverter draws will make it shut off, even when it still has charge.
- Using a car battery. The U.S. Department of Energy advises against it for these systems: they’re shallow-cycle and get damaged if discharged more than 20%.
When to call an installer
To set it up. Tens or hundreds of amps flow between the battery and the inverter: the cables, fuses and connections must be chosen and installed by an electrician or installer. With these numbers you’ll know what to ask for.
Sources and references
- National Renewable Energy Laboratory (NREL), November 2000. Renewable Energy for Rural Schools (inverter efficiency depending on the load).
- CSIRO (Australian government), 2021. Batteries for Homes (capacity changes with discharge current; depth of discharge).
- PVEducation. Lead Acid Batteries (the usable share of a lead-acid battery).
- U.S. Department of Energy, Energy Saver. Balance-of-System Equipment Required for Renewable Energy Systems (deep-cycle batteries; not car batteries).
Frequently asked questions
What battery do I need for a 1,000 W inverter?
It depends on what you run and for how many hours. For the inverter to deliver its 1,000 W, the battery has to supply about 98 A at 12 V (49 A at 24 V). For an average of 300 W over 8 hours you need about 3,529 Wh of lithium: 294 Ah at 12 V.
Can a 3,000 W inverter run on a 12 V battery?
It can, but at full power it would draw about 294 A, which calls for very thick cables and a battery able to deliver it. That’s why inverters that size usually run at 24 or 48 V: 147 A or 74 A.
How many Ah do I need?
Wh = average watts × hours ÷ 0.85 ÷ 0.8 (lithium) or ÷ 0.5 (lead-acid). Then, Ah = Wh ÷ the battery’s volts.
