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There are several things to keep in mind when building things:
- Practical knowledge: For every unit you produce, you will gain a particular kind of practical knowledge. That practical knowledge will, in turn, make the next unit of that type that you build cheaper in terms of IC days.
- IC days: The best way to understand the actual cost of a particular item is to determine the cost in IC days. This is simply the IC cost multiplied by the number of days it takes to build the unit. For instance, a ship that costs 4 IC per day will actually cost 1460 “IC days” over the course of a full year. Compare that to a light armoured unit (6.88 IC per day) which may cost more daily IC, but only takes 180 days to finish, which equals 1240 “IC days.”
- Serial & Parallel construction: Serial production is the ideal method for creating units: units are produced one at a time, which means the next in the series benefits from practical knowledge increases. Parallel construction is used to meet immediate needs: when you need a bunch of a certain unit type at once. As a result, parallel construction is much less efficient than serial builds.
- Do not use serial production if more than one build item is going to use the practical the object uses. The reason for is this when you have multiple things queued up and they complete on the same day, they are processed sequentially and not in parallel. Thus, if you have five runs of industry that complete on the same day and there were more queued up, the next five might be 5, 4.9, 4.8, 4.7, 4.6. However, if you hadn't had them queued up and instead you simply selected five new industry the cost would be 4.5 for all five. Thus, you saved 1.5 IC by avoiding a serial build. This can have a dramatic effect in the early IC building stages when the IC cost after a first run of industry drops almost 2.5 points. This applies whenever anything in the production queue shares its practical with someone else so it's best to avoid serial queues all together (even if it does require a little more micromanagement).
- Building a unit, then upgrading it, is cheaper than building the upgraded unit from scratch, though it takes longer.
- Building several brigades is cheaper than building an entire division that contains the same brigades.
- You do not always have to finish a unit you have in production. In fact, sometimes it’s a good idea not to. You can complete most of the components (i.e. finish most of its construction time), and then leave it incomplete until you know you really need it.
- Don’t start building level 2 tanks if you’re going to research level 3 soon. Wait for model improvements and then build. It’s always possible to upgrade once you’ve researched the new techs, but that actually takes longer than simply building from scratch, and it’s a hassle.
- Production Licenses don’t give you the technical knowledge directly, but they do build your overall knowledge (having built the item), which may help you to develop that tech in the future. Note that the price of a license is not fixed, it is on a slider, so they can be sold cheaply to your allies. Buying a licensed product can save you research points better spent elsewhere and allow you to acquire unique units not otherwise available to your country. Typical lincense purchases for Germany would be landing craft and marines from Japan. You can buy licenses from the Diplomacy screen, this costs 1 diplomatic point per license. You purchase the production license with money and then the item is placed in the production queue and built with your IC. The country that is paid for the license will see the payment as an increase in its stocks of money, not IC. You can't license-build a ship bigger than a cruiser.
For more information, view the video tutorial on Production here: Hearts of Iron 3 FTM - Basic Tutorial - Episode 4 - Production
The production sliders are very important, because they’re what keeps your whole economy and military machine functioning. There’s a “need” listed by each category, and if you’re not meeting the need, the value of your funding (displayed on the right) will be shown in red (and there will probably be an alert in the upper left corner of your screen). Otherwise, it will be white. You can click on the number to make the production slider jump to the needed figure. Any of these sliders can be changed by clicking and dragging, or by using the “+” or “-“ buttons. To move the sliders faster press the “shift” key while clicking - this “increments” these buttons by a percentage. When you have the right figure set, right click on the slider to fix it in place.
The demand for supplies and reinforcements goes through an annoying 24 hour cycle but it is the figure when the time of day is 20:00 that matters, so don't bother to try to fix it all the time. At the top of the screen, you have the option to automatically set the sliders by choosing to emphasize reinforcements and upgrades or consumer goods. This will keep the slider set to the requested production figure all the time but set supplies to zero or whatever the minimum supply figure is so it is often not possible to use this feature.
While the most critical slider is production, because that’s what builds anything you want to construct, it’s easiest to determine the slider locations for the others first – determine how much you have to use on production by process of elimination. Consumer goods is a fixed amount, and unless you want to raise some extra cash, you’ll always keep this at the level of need – not more than, and most certainly not less than. Anytime consumer goods are underfunded, you’ll be gaining dissent, which no one wants.
The next categories to select are upgrades and reinforcements. As noted below, it’s not a huge problem if you cannot completely fund reinforcements or upgrades. Either you can come back to these funding categories when your economic situation improves, or you can partially fund them, which will gradually implement the einforcement or upgrades unless you are either researching very quickly (i.e. developing new upgrades) or in heavy combat, requiring continual reinforcement. But usually, you will find partial funding just spreads out the cost over a longer period, but accomplishes the same result. Both of these can be checked by periodic inspections of the numbers to make sure the demand for funding is dropping. This can keep you from overfunding too – alerts will tell you when you’re funding above the need, but a careful watch can allow you to shift funding to more important sliders by reducing the rate at which you’re partially funding upgrades and reinforcement. Don’t be surprised if you have a slight reinforcement need during peacetime. Any land province has some attrition, and so your units will slowly “leak” manpower which must be replaced (due to illness, retirements, etc.). Any province where the temperature is above 30 degrees Celsius, or below -10, your troops will experience additional attrition.
Your supplies are an interesting tradeoff – this value will probably jockey with your production value as your needs change. Despite what the alerts may tell you, there’s never a time when IC spent on supplies is wasted, unless you have an enormous stockpile and no one to buy them. A good stock of supplies is always good to have for wartime, because it offers you flexibility, and the comfort of knowing you won’t face a shortage that could bring your military operations to a halt. You can also sell supplies during peacetime or wartime, and these sales may be your most viable means of getting the money you need to buy other resources. In any case, a positive rate of supply production is always a good thing. If this value goes “red” from time to time (i.e. your army is drawing from your stockpile), that’s not necessarily a bad thing so long as you have a sufficient stockpile.
Underfunding reinforcements is wise, because otherwise you’ll always be changing the slider. If you experiment, you may be able to find an “equilibrium” point, unless you’re at war, when you’ll always have fluctuations due to casualties. Don’t get too far behind, or you’ll have to change things more substantially to catch up.
It’s cheaper to upgrade your existing units than to build new ones. These will apply all those techs you’ve researched, and give your units better performance in combat (and sometimes even in peacetime). Many scenarios start with some upgrades pending, so quickly decide how aggressive you want to be in pursuing them. If you do decide to upgrade a unit, ensure it is out of reach of any enemy action - if it is attacked it will be permanently destroyed. An upgrading unit is placed in the production build queue, so ensure it can be built before attempting to upgrade it.
You can underfund upgrades. During peacetime, you’ll still want to upgrade to prepare for war. During wartime, you should take upgrading more seriously. Either slightly underfund it, or watch carefully for the alert telling you your upgrades are complete. Otherwise you might waste some of your valuable IC on upgrades that don’t exist.
To fully upgrade a unit, through one cycle of upgrades, will take two months. It may take another two months to upgrade through a second level of techs, if you’re that far behind. You may see some increase, though, in the speed of upgrades, if you have more than one level to finish.