Sounds like he has micro inverters. Micro inverters directly attach to each panel and are wired in parallel. They simply turn DC into AC and feed it to the homes electrical system and to the grid depending on load. The smart meter tracks how much power is consumed from the grid vs fed back to the grid (excess). They also do not work in a blackout, they must see grid power to turn on.
How about storing that excess energy in a battery bank? The higher the excess, the more charge current is delivered to the battery (via PWM switching). The picaxe can monitor the excess and control a charge circuit to compensate for the load. Then you can use an off the shelf inverter to power appliances from the battery bank. The benefit of this system is that not only is it simple, but also gives you battery backup for blackout protection. Your friend can also use relays to switch certain appliances between battery and grid depending on the time of day. So during the day excess is dumped to batteries. At night when there is no more excess, relays can switch certain appliances or lighting circuits to battery and when the batteries run low, switch back to grid. Examples of relayed appliances could be: refrigerator, air conditioner, well water pump (if he has), lights, washing machine etc. The same grid->battery charger can also detect cloudy or low PV producing days and keep the battery bank charged in case of a blackout.
The batteries can get expensive but its a simple way to catch the excess and you have blackout protection.
You don't want contact with the mains. OK, so is your project going to be battery powered?
If you buy a low voltage, AC output wall-wart, with UL, CE, DIN and every other safety marking you can think of, you can regard its output as at least as safe as any other appliance in your house. Then, in the low voltage output, you have a reasonable facsimile of the mains voltage waveform. The accuracy will degrade if you also rectify that output to power your project. If the last 1% accuracy is important, then why not buy two, one for the reference, and one to power the project, or a DC output one to power the project without further messing about. Your current measurement is already transformer coupled, why not have the voltage measurement transformer coupled as well?
To confirm real and apparent power calculations ...
for real power, repeatedly compute the instantaneous power as the product of the instantaneous voltage and the instantaneous current many times a cycle, then average the power
for apparent power, compute the mean voltage by averaging, and seperately the mean current by averaging, then take the product of these
... and here an average means either a) average over a long time, the more cycles the more accurate or b) sum synchronously over an integer number of whole cycles
Best Answer
The typical way of doing it would be with a clamp-on power meter.
The clamp-on DC power meter works by measuring two things in parallel:
The magnetic flux in a ferromagnetic clamp that encloses the wire with a DC current flowing in it.
The voltage across the load (or source).
Inside the meter, a measure of the flux is converted (scaled) into current, and multiplied with voltage to provide instantaneous power. The instantaneous power is time-averaged to provide average, or RMS, power.
This isn't totally non-invasive, as you need access to the battery leads - you'd clamp the meter around one of the leads, and attach the voltage probes to the battery terminals. But that's the best that you can do, unless the wheelchair has a power/energy logging function that's available to the user.
The meter itself might have a logging function that totalizes the energy, or you might attach it to an external logger. Finally, instead of using a dedicated power meter, you could use a data logger, and an external voltage-output DC current probe. The logger would log the voltage and current on two channels, and you could do the power and energy calculations yourself.