Appliances are rated in AC watts and batteries store DC energy, so every figure has to cross the inverter. That crossing costs something, and the inverter also draws power sitting idle, which is the part most people leave out of their sizing.
The calculator below converts your AC daily energy into the DC the bank actually has to supply. Our note on how many watt hours you need covers building the load list.
Quick Answer
Divide AC watt hours by inverter efficiency to get the converted DC figure, then add idle consumption for every hour the inverter stays powered. The total is what the battery supplies, and it is before depth of discharge is applied.
Key Points
- Inverter efficiency is commonly around ninety percent, so DC demand exceeds AC demand.
- Idle draw runs whether anything is plugged in or not.
- On small systems idle can rival the actual load.
- Inverters are least efficient when lightly loaded.
- The amp hour figure is before depth of discharge, so bank capacity is larger again.
Where the Energy Goes
| Stage | What happens |
|---|---|
| Appliance rating | Stated in AC watts |
| Conversion | Roughly ten percent lost as heat |
| Idle consumption | Constant while the inverter is on |
| DC draw | What the battery actually supplies |
| Depth of discharge | Bank must be larger again |
| Cable losses | Small, but rise with current |
Conversion Is Never Free
An inverter turns direct current into alternating current through switching circuitry, and that process dissipates some of the energy as heat.
Typical pure sine units sit somewhere around ninety percent efficient at reasonable load, meaning roughly a tenth of what enters never reaches the appliance.
Higher efficiency units exist and cost more, and the difference matters most on systems running large loads for long periods.
Efficiency is quoted at a particular load level, usually a favorable one, so the figure on the specification sheet is a best case rather than an average.
Our roundup of power inverters covers what to look for.
Idle Draw Is the Overlooked Cost
It runs constantly in the background
Whether or not anything at all is switched on.
It scales up with inverter size
Larger units generally idle at a higher figure.
Overnight is where it really adds up
Small draws across many hours become real.
Switching it off recovers all of it
The simplest saving available on a small system.
Light Loads Make Efficiency Worse
Inverter efficiency is not a fixed property, and it falls away sharply at the bottom of the range.
A large inverter running a small appliance spends a significant share of its draw on its own operation rather than on the load.
This is why oversizing an inverter has a running cost as well as a purchase cost, and why matching the unit to realistic loads matters.
Where a system has one large occasional load and many small constant ones, two inverters sometimes make more sense than one large unit doing both jobs.
Running some loads directly on DC avoids the conversion entirely, which is why twelve volt appliances remain common off grid. Our roundup of 12V refrigerators covers the clearest example.
What DC Loads Avoid
Anything that runs on DC skips the inverter completely, which removes both the conversion loss and the reason to leave the inverter powered.
Lighting, fans, water pumps and refrigeration are all available in DC versions, and on small systems the savings compound.
The tradeoff is cost and availability, since DC appliances are a smaller market and generally carry a premium.
Wiring is also less forgiving. DC at twelve volts needs thicker cable over the same distance than AC does, which limits how far from the bank things can sit.
For a small cabin or van the arithmetic frequently favors DC for the constant loads and an inverter for occasional AC ones. Our note on what appliances you can run off grid covers the load list.
Surge Draw Is a Separate Question
Everything above concerns energy over a day. Peak power is a different constraint and it catches people who sized only for energy.
Motors demand several times their running watts for a moment at startup, and that spike has to come through the inverter and out of the battery simultaneously.
A bank with plenty of stored energy can still struggle if it cannot deliver current fast enough, which is a function of the battery’s discharge rating rather than its capacity.
Lead acid sags noticeably under heavy momentary load, and that voltage dip can trip an inverter’s low voltage cutoff even with charge remaining.
Checking the battery’s continuous and peak discharge ratings alongside its capacity avoids that surprise. Our note on inverter sizing covers the surge side.
Depth of Discharge Comes After This
The amp hour figure the calculator returns is what the bank has to deliver, not how large the bank needs to be.
Lead acid banks are generally kept above about half capacity, so a bank sized for this figure would need roughly double the nominal capacity.
Lithium iron phosphate tolerates deeper cycling, so the multiplier is smaller though still above one.
Reserve for cloudy days sits on top of that again, since a bank sized exactly to one day’s use has no margin at all.
Most off-grid systems that disappoint their owners were sized on one of these figures while skipping the others. Our note on what size solar battery you need covers stacking them properly.
Cable Losses Sit on Top
Energy is also lost in the cable between the battery and the inverter, and that loss rises with current.
At twelve volts the currents involved are high enough that undersized cable becomes a measurable drain rather than a rounding error.
Keeping that run short and thick is the standard advice, and it matters more than the equivalent run on the AC side.
Higher system voltage reduces the problem directly, since the same power at twice the voltage draws half the current.
Our note on what size wire you need covers sizing the run.
Before You Size a Bank
Start from your real AC loads
Nameplate watts and honestly estimated hours.
Add the conversion loss
Divide by efficiency, do not multiply.
Add idle for every hour on
Including the hours nothing is running.
Then apply the depth of discharge
The bank is larger than the demand.
Common Mistakes to Avoid
Sizing the bank from AC watt hours
The battery supplies DC, and conversion means the DC figure is always larger.
Ignoring idle consumption
An inverter left on around the clock draws power continuously. On a small system that can rival the load itself.
Buying a much larger inverter than needed
Efficiency falls at light load and idle draw rises with size, so oversizing costs energy every day.
Stopping before depth of discharge
The DC figure is what the bank delivers, not how large it has to be.
Recommended Reading
See our guide on choosing a solar inverter, our note on how many batteries you need, our roundup of inverter displays, and our comparison of LiFePO4 versus lead acid.
DC to AC FAQ
Why does my battery need more than the appliance uses?
Conversion from DC to AC loses roughly a tenth as heat, and the inverter draws power idling on top of that.
How efficient are inverters?
Commonly around ninety percent at reasonable load, though quoted figures are best cases and efficiency falls at light load.
What is idle draw?
The power an inverter consumes simply being switched on. It continues whether or not anything is plugged in.
Should I switch the inverter off?
On a small system it is the easiest saving available, particularly overnight when nothing needs AC power.
Is a bigger inverter safer?
Not free. Larger units idle higher and run less efficiently at light load, so oversizing costs energy daily.
Do DC appliances avoid all this?
They skip conversion entirely, which is why 12V refrigerators and lighting are common off grid. They cost more and need thicker cable.
Is the amp hour figure my bank size?
No. It is what the bank delivers. Apply depth of discharge and add reserve for cloudy days to get the capacity you need.
Do cable losses matter?
Between battery and inverter, yes, especially at 12V where currents are high. Keep that run short and thick.
