It's fun comparing a battery bank to your savings account. If you have a regular income your savings account doesn't have to be as big as the case of the fellow whose income is intermittent. He needs an account big enough to cover the times when there is no income. A generator is similar to a regular income while relying on solar/wind is like the fellow who gets temporary employment and has to put a lot in the bank to cover periods of unemployment. Trojan Battery calls that time period the "days of autonomy". It recommends five days of battery capacity if you are relying on solar only while saying two days of autonomy is sufficient for dependable charging. The battery consumption in both cases may be the same but the charging frequency determines your battery capacity. Thanks. Steve S.
I have to respectfully disagree with this.
Though I see the theory in what you are saying, in practice it doesn't work that way at all. In practice if you are using a generator to keep your bank charged then your bank will almost always be less than fully charged. As soon as the generator stops running the loads you have on the bank start depleting the bank and you aren't likely to restart the generator until you "need it" (meaning the bank has dropped to some specific level of charge or something like 12.1 volts, or whatever ..). So with a generator your bank is usually in some state of discharge.
With solar and wind it is just the opposite, because your solar panels are almost always charging unless there is bad weather or at night, you battery bank's natural state is charged, and it is only at night and during storms that it becomes discharged, and left on its own it will charge right back up.
I know it sounds like semantics, but it really feels different in practice. With a generator you are always kind of feeling guilty because you know you are slowly destroying your bank because you didn't get up and go out and start the generator when you should have, or didn't let it run as long as you should have to get a full charge on the bank (which can take many hours for acceptance charging). With a solar system it feels totally different, you feel like you are harming the bank if you put a huge load on it, but you know in the back of your mind that won't last and that once you stop abusing the bank it'll go back to "normal", normal being fully charged. When you walk away from your boat with a generator you're thinking, wow, I shouldn't have left without charging the bank, and I should come back in a few weeks and charge it again. With solar you walk away and think, wow, I'm sure glad I have solar, I turned the lights off and in a day or two the bank will be fully charged and it will stay that way until I get back.
I have a generator too, and I use it often if/when I run the bank down, but solar gives me peace of mind and if I had to have one or the other I'd keep the solar panels.
To use your savings account idea, using a generator feels like work, you have to do something to put the money back into the account. But solar panels feel like a trust fund, it just magically happens, and even if you run the account to zero all you have to do is stop spending for a while until it recharges. Like I said, I know it sounds like semantics, but it really feels different in practice.