At https://andymaypetrophysicist.com/2018/09/19/how-constant-is-the-solar-c… … the IPCC lowered their estimate of the impact of solar variability on the Earth's climate from the already low value of 0.12 watts per square metre given in the 4th Report to just 0.05 in the 5th Report (2013). There are long term values extended over 261 years (1750-2011) and they apply to 11 year solar cycles (sunspots). The peaks of sunspot/solar cycles vary so what is being measured is the baseline (the minimums of each cycle). The Sun's output (total solar irradiance) varies at all time scales – but by how much. The amount of change on short time scales is well known but the magnitude of solar variability over longer periods of time is poorly understood. Small changes in solar constant over long periods of time can affect earth's climate significantly – see Eddy (1976) and Eddy (2009). John Eddy's 1976 paper on the Maunder Minimum is a classic he says.
Nine pages later Andy May concludes – no, we have not measured solar output accurately enough over a long enough period to definitively say solar variability would not have caused all or a significant part of the current warming observed since 1750. However, we have not proved solar variability is the cause of all or even a large portion of the warming – only that it should not be excluded as a possible cause (which is contrary to what the IPCC have said).