Feeding? I feel dumb . . .

This is going to sound like an incredibly stupid question considering I've had horses for years but . . . How much hay, as a general rule, should you feed your horse in relation body weight? I realize the nutritional value of forages can very but I'm just looking for a place to get started. I can always change the ration if needed.
Our horses have always been on pasture or have had free choice hay off a large round bale. This summer my horse situation is going to change and I'll be feeding my horse hay over the summer and I have no idea how much I should be feeding her come March. I'm fairly confident she will need little if any grain to supplement her diet. She is a pretty easy keeper and I do not do a lot of hard riding.
If anyone has any recommendation are personal experiences they could share it would be greatly appreciated.

Don't quote me but I'm pretty sure horses need 10% of their weight in dry mass of feed.
I think...

2 1/2% of the body weight a day is a good general rule, so typically around 25 lbs. Of hay.

I've always heard that horses should consume/ be fed 1.5 - 2 % (+) of their body weight per day
(ie. 1000 lb horse/ 15-20 lbs of hay per day).
You can feed a diet of 100% roughage (hay) and no grain if it keeps proper weight on your horse.
Grain is usually fed when a horse has a 'work schedule' and needs the additional calories for the extra energy.

It depends

A super easy keeper might just need 1%
But s harder keeper might need 3% or more

2% is the "general" guideline adjusting up or down as needed

My 1000 pound mare does just fine on about 12 pounds of hay a day. She's a pretty easy keeper though.

I was told to figure a bale of hay (80-100 lbs) will last close to a week, and then up it if they lose weight. Mine go through about a bale a week apiece, but they also get a meal of complete feed pellets as one of their 3 meals/day. If temperatures drop (southern AZ, so bitter cold = 25 deg F), then I up the pellets. If it is real windy or they just seem unhappy, I up the hay (bermuda).

If I feed them alfalfa hay, they get hyper.

HAHA bitter cold is 25 deg F. I would about kill for 25 deg F. This week hasnt been too bad but last week the average high was around -8. One day it was -24 deg F. This next week its suppost to be consistently below zero again . . . Not looking forward to that.

I should have added that I feed her about 8 pounds of alfalfa and 4 of grass hay. :) She's also blanketed 24/7 unless it's sunny and above 25 degrees overnight. We get a lot of rain here in my part of Oregon and in the past, she'd lose quite a bit of weight trying to keep herself warm when she got wet so now when it's rainy or really cold, the blankets come out.