High quality moving iron meters are fairly good for ac, on dc they tend to suffer from residual magnetism, if you take a reading, reverse the leads and read again you may find a difference, if so the average will be close to correct.
Moving iron meters at the low cost end of the range can be pretty poor and it depends on what they were intended for.
Moving iron measures rms, so it may not agree with a mean reading meter on dc if there is ripple, there should be no ripple with solar unless you have a pwm charge controller operating.
The other catch is that many cheap moving iron meters are intended for battery chargers and they are scaled to the strange form factor of the peaky charging waveform.These things when used on smooth dc can be in error by up to 8% .
Flux