Watts, watt hours, amp hours and BTU

Watts, watt hours and amp hours are not three ways of saying the same thing. They answer three different questions, and a product listing that quotes one of them is not telling you the other two.

A watt is a rate: how fast energy is being used right now. A heater rated 1,500 W draws 1,500 W every second it is on. It says nothing about how long you can run it, because it says nothing about how much energy you have.

A watt hour is an amount: energy delivered over time. The same heater running for two hours uses 3,000 Wh. Two hours of 1,500 W and half an hour of 6,000 W are both 3,000 Wh, which is why a battery is rated in watt hours and an appliance is rated in watts.

An amp hour is also an amount, but an incomplete one. It only becomes energy when you multiply by voltage. A 100 Ah battery is 1,200 Wh at 12 V and 4,800 Wh at 48 V. Without the voltage, the number is a description of a box, not of a capability.

There is a fourth unit that gets mixed in when people read inverter labels: the volt-ampere, or VA. It measures apparent power, which includes current that does no useful work. Dividing watts by VA gives the power factor, and for resistive loads like a heater it is close to one, so watts and VA are nearly the same number. For motors and electronics it is lower, which is why a 1,500 VA inverter may only deliver 1,200 W to some loads. If a listing quotes VA and you need watts, the honest answer is that the conversion depends on the appliance.

The conversions inside each dimension are fixed numbers. The conversions between them are not conversions at all: they need a third quantity, usually time or voltage.

Everything in the top four rows is exact and needs nothing else. The bottom two rows are the ones that cause trouble, because they cannot be completed without a number the listing often does not give.

One more distinction matters when reading a utility bill. Electricity is sold in kWh because the meter integrates power over time; it is not measuring the instantaneous draw at any moment. That is why a 2,000 W appliance used for half an hour and a 100 W appliance used for ten hours cost the same, even though one of them would trip a 15 A circuit and the other would not. The bill is about energy; the circuit is about power, and they are answered by different numbers.

This is the table to keep. The battery is described in amp hours only, which is exactly how a marine or RV battery is sold, and the Wh content changes by a factor of four across the three common voltages.

Read it backwards and it explains a common disappointment. Someone buys a 100 Ah battery expecting to run a 1,000 Wh appliance for a day, connects it to a 12 V system, and gets 1,200 Wh of nameplate and far less of usable energy once depth of discharge is applied. The battery did what it said; the expectation was formed without the voltage.

The same relationship explains why a portable station is rated in watt hours and a car battery in amp hours. The station contains its own voltage, so the manufacturer can quote the complete figure. A battery sold for a vehicle has to work in a system the buyer is assembling, so the incomplete figure is the honest one.

An air conditioner is sold as 12,000 BTU, and a heater is sold as 12,000 BTU per hour. Those are different things, and the difference is a factor of hours. The first is a cooling capacity quoted as a rate with the "per hour" dropped, the second is explicitly a rate.

To convert a cooling rate to watts, multiply BTU per hour by 0.293. A 12,000 BTU per hour unit is about 3,516 W of cooling. It draws more than that from the wall, because a heat pump is not 100 % efficient in the way a resistance heater is, and because the rating is measured under test conditions.

The practical consequence for sizing: a battery is chosen by watt hours, so an air conditioner must first be converted from BTU per hour into watts, then multiplied by the hours you want it to run. Skipping the conversion and reading "12,000" as watt hours overstates the battery need by a factor of three or more.

Take 1,000 Wh, which is the size of a small portable station. At 24 V that is 41.7 Ah. In BTU it is 3,412. In joules it is 3.6 million. Running a 100 W load, it lasts 10 hours. Running a 2,000 W kettle, it lasts 30 minutes.

All five statements describe the same battery. Only the last two change with the appliance, which is the point: energy is fixed by the pack, runtime is set by the load. A listing that quotes a runtime without saying at what wattage has said nothing at all.

It is worth doing the same exercise with a real appliance label, because the numbers on packaging are usually quoted in the unit that flatters the product. A vacuum cleaner advertised at 1,200 W tells you the draw, not the runtime. A battery advertised at 1,000 Wh tells you the amount, not the watts it can deliver at once. The pair matters; either one alone is half a specification.

Recharging from panels adds one more unit to the chain. A 1,000 Wh daily need, at 4.5 peak sun hours and 80 % efficiency, needs about 278 W of panels. That calculation lives on the battery sizing calculator, and the panel side is on the panel count calculator.

Treating amp hours as capacity. A 100 Ah pack is 1.2 kWh at 12 V and 4.8 kWh at 48 V. Buying on amp hours alone is how people end up with a quarter of the energy they planned for.

Treating watts as watt hours. A 1,500 W heater and a 1,500 Wh battery are not comparable. One is a rate, the other an amount, and the only thing that connects them is how long you run it.

Reading BTU as power. An air conditioner's 12,000 BTU is a cooling rate, not a wattage and not an energy figure. Convert to watts first, then decide how many hours you want.

Forgetting that nameplate is not usable. Depth of discharge and conversion losses take roughly a quarter before the first appliance is switched on. A 1,000 Wh pack behaves like 750 Wh in practice.

Mixing DC and AC amps. An inverter turns 12 V DC into 120 V AC, so an AC current of 8 A is not the same 8 A as on the battery side. Converting at the battery voltage is the only safe route.

Assuming two batteries with the same amp hours are interchangeable. A 100 Ah 12 V battery and a 100 Ah 48 V battery have the same label and four times the difference in energy. The label describes current capacity; only the product with the voltage describes what you can run.

Eight units from our current US list, each shown in watt hours, in amp hours at 24 V, and in BTU, so the same capacity can be read three ways. For a wider comparison, see the portable power station ranking.

Notice the pattern in the card headers: the Wh figure is a property of the product, while the amp-hour figure is a property of the product and your system voltage. That is why one of them belongs in a specification sheet and the other belongs in an installation drawing.

Kilowatts to watts: move the decimal three places. A 1.5 kW load is 1,500 W.

Watt hours to amp hours: divide by voltage. The lower the voltage, the larger the number, which is why 12 V systems quote intimidating amp figures.

Watts to BTU per hour: multiply by 3.412. A 1,000 W heater is about 3,412 BTU per hour, and every resistance heater is the same regardless of price.

Watt hours to runtime: divide by the load, then take about three quarters of the answer for depth of discharge and losses. That last step is the one people skip.

Amp hours to usable energy: multiply by voltage, then by 0.75 to 0.85. Below that range you are planning on energy the pack will not give you.

Runtime to energy: multiply the load by the hours. This is the reverse of the rule above and the one to use when an appliance label gives you watts and your question is how much battery to buy.

Heating is always the same efficiency. Every resistance heater converts essentially all of its input to heat, so a 1,500 W heater from a discount store and one from a premium brand produce the same 5,118 BTU per hour. The difference in price buys build quality and controls, never more heat per watt.

Every one of these is reproduced by the calculator in the right-hand panel, which is the point of having it: a rule of thumb you can check in two seconds is worth more than one you have to trust.

Every conversion on this page can be done on a phone calculator in one step, and doing it once is the fastest way to stop being fooled by a listing. The three checks worth internalizing are the division by voltage, the multiplication by time, and the three-quarters rule for usable energy.

A worked check takes about twenty seconds. Take any listing that quotes amp hours, look for the voltage, multiply the two, and compare the result with the Wh figure in the same listing. If the product is advertised as 100 Ah and 1,280 Wh, the implied voltage is 12.8 V, which is a four-cell lithium pack. If the same pack were described as 24 V, the watt hours would double, and that discrepancy is exactly what a careful buyer catches before paying.

The same check works in reverse on an appliance. A label that says 1,500 W and a runtime claim of four hours implies 6,000 Wh, which is larger than most portable stations. When the two numbers cannot both be true, one of them is marketing rather than measurement.

If a number on this page disagrees with your own arithmetic, the calculator in the right-hand panel is the source of truth, and the changelog records every time one of these values changed. The page describing how corrections are published is the disclosure page, and the privacy page covers what this site collects.

Once the units are clear, the next question is which size to buy: the power station match calculator compares capacity, output and surge against a backup load.