While a wall adapter is usually isolated, it likely isn't rated to 1kV. A simple solution would be to find a DMM with logging capability and CAT III/IV 1kV isolated external power source or long battery life. Here are some candidates:
Another option is to use a simple micro with an ADC, float the whole circuit at chassis voltage, then use proper isolation techniques (more than just optoisolation -- if you're not sure I suggest another question) to communicate with a PC over your preferred serial connection (UART -> opto -> RS232 -> USB -> opto -> PC, with cable shield voltage measurement and warning, would be my choice). Note that this means you can't touch anything on the floating (hot chassis) side of the widget. This way you can eliminate the power supply isolation worries by just using a battery, and still run it easily for 6 months to a year without replacing it, following thought on power consumption and sleep modes (ie: MSP430). Also note that a sputtering machine generates electrical noise, so you may need to use RS485 with error detection/correction algorithms.
The new Microchip PICs with the "XLP" designation are very low power. I hear that the TI MSP430 are also quite low power but have not investigated that myself.
0.5% accuracy rules out using a internal R-C oscillator for wakeup. The best accessible solution for that will be a micro that is intended to run a 32768 Hz crystal like is used in wrist watches. In the PIC line, this is a micro with a timer 1 oscillator, which is most of them. The main oscillator and the CPU can be shut down, but the watch crystal and timer 1 keep running and can be used to wake the processor periodically. Without doing anything special, this will happen every 2 seconds. The firmware then counts 2 second wakeups to get to whatever longer time you want. If your processing requirements are small, one of the newer four digit PIC 16LFxxxx should do nicely. They have a internal oscillator to run the CPU from when it does wake up and are otherwise small, cheap, and low power.
As for the radio transmission, it's not as simple as just sending a couple of bytes. The other end has to identify you are transmitting, figure out the level to detect 1 from 0, etc. In practise this ususally means manchester encoding with maybe 10 bits of preamble, a start bit, the 16 data bits, then a checksum.
The chance of any one bit of a RF transmission getting messed up is high enough that you need to plan for that happening. With CRC checksum you at least have a good chance to determine it happened. You then have to decide how likely that is and what the consequences are of the data not getting thru. You could send two packets every time hoping that at least one gets thru. But if you're going to spend the power on that, you could just as well send at half the interval so that when things do work right you get better data. There is no easy answer. Reliability can not be guaranteed without two-way communication. It's a probability and cost versus risk game.
If you really want to reduce overall power, then you have to look into fancy error correction encoding schemes. Some of these won't be easy to do in a small micro. Some put most of the burden onto the receiver. There are lots of schemes. For example, one of the Venus probes of the 1970s sent the data both forwards and backwards (and probably a few more tricks). It took over a day on a high end mainframe at the time to decode the last frame before the probe went into clouds and couldn't be heard from. Again, there are lots of schemes with different tradeoffs, but consider them against the cost of a larger battery.
Added:
I had originally thought that power would be dominated by the RF transmitter, but hadn't really worked thru the numbers. I saw Clabacchio's answer where he states the opposite, so let's do the math.
Let's say the transmitter draws 20 mA average when on. This is plausible for a ISM band 434 MHz OOK transmitter. Let's say data is sent using manchester coding at 10 kHz bit rate. This is easily doable with a small PIC. I have done exactly this with a PIC 10F202 in some small active RFID tags. Let's say the total transmitted stream is 10 preamble bits, 1 start bit, 16 data bits, and 16 checksum bits, for a total of 43 bits. It takes 4.3 ms to send those bits. The transmitter will need a millisecond or two startup time during which it draws some power but less than when actually transmitting. So let's round up and say the power draw is equivalent to 5 ms at 20 mA every 5 minutes. That comes out to 333 nA average. That means the sleep current of the processor is a significant factor in overall battery life, particularly since it will be running a 32768 Hz watch crystal during that time. In fact, it looks like the sleep plus watch crystal current will be more than the average RF transmission current.
Best Answer
While others have suggested linear optocouplers and voltage dependent oscillators, I'll throw a very different method out as an answer to read high voltages with very high isolation.
I've used this in fully floating front ends up to around 56V. It should work for any voltage you can get a suitable transformer for.
In the schematic below I've created a short pulse driving a FET. When the FET is ON, the transformer ratio is the attenuator. I've previously used small audio transformer because they have high inductance primaries, but I'd suggest you could use small DC-DC converter transformers just as successfully.
The transformer used here is most like this with an 18:1 turns ratio and about 6mH primary inductance. The transformer turns ratio is of course very stable with temp/time so makes an excellent attenuator.
simulate this circuit – Schematic created using CircuitLab
You could expect waveforms like this:
With the rather low primary inductance here, the current rises to 80mA quite rapidly. If you can find a transformer with a 20-50mH primary then the peak current is reduced and the time you can activate the FET made longer.
If you are using an Arduino then by default the A/D takes 104us per conversion. The pulse width in this circuit would therefore require a sample and hold to capture the stable output voltage. But if you can find a better transformer, then you might be able to hold the FET on for more than 15us so not require the external sample/hold (the ATMega328 has about 12us sample time for the internal S/H). It all depends on what you want as an acceptable input current peak in the transformer primary.
You obviously have to provide an isolated drive for the FET, but there are plenty of pulse transformers for this application that could be driven from the Arduino.