Get max value for a month and the difference to the previous month

I’m using PostgreSQL 9.6.

I have a table containing data from my electricity meter such as there is a row for each hour, this is just an example, so there many rows for each month:

I want to show the max meter value for each month (i.e. the value it has at the end of the month) and I want to show how much the value has changed for each month compared to the previous month.

So I want a result like this (this example is based on some other data)

I came up with this query but it contains 4 selects, is there any easier/better way to do this? It uses row_number to assign "1" to the max value for each month and then it uses lag to get the previous months value.

select date, max, max - lag "diff" from (
select date, max, lag(max) over (order by max) from (
select * from (
select  date(date_trunc('month', time)), max(value), row_number() over (partition by date(date_trunc('month', time)) order by value desc) rn
    
    from public.electricitymetering where "meterid" = '1'

group by date_trunc('month', time), value 
order by date(date_trunc('month', time)) desc ) as result
where rn = 1
    ) as temp ) as final

>Solution :

Yes, there is a simpler way, based on this example of the lag function:

with monthly_maxes as (
   select date(date_trunc('month', time)) as date, max(value) as max,
   from public.electricitymetering where "meterid" = '1';
)
select date, max, max - lag(max, 1) OVER (ORDER BY date) diff
from monthly_maxes;