About ten years ago, I joined the local generator and bought the right to use four amperes a week. After a year, my wife said she’d prefer using a hotplate (a Shabbos “plata”) instead of using the gas. We tried one out and saw that the plata didn’t cause the circuit breaker to fall. Therefore, we assumed that we were within the four amps that we purchased and we continued to pay the monthly charge for the use of four amps assuming that was the amount we were using. Recently, we moved to a new apartment and again bought four amps. However, the circuit breaker fell. We spoke with the organizer of the generator and told him what we were using. He told us that for what we were using we required six amps. We checked with the organizer of our previous generator and he agreed that because of our hotplate we required six amps. He explained that the reason the circuit breaker never fell is that I had a six amp circuit breaker. Even though I only bought four amps, since at the time they only had six amp circuit breakers in stock, that is what they installed. I am an avreich and can’t afford the extra two amps and would have continued using gas had I known that I would need to pay an extra fifty percent each week. Do I owe anything to the organizer in my previous neighborhood?
Before we answer your question, we should analyze your situation so that we can compare it with analogous situations. In your situation, you inadvertently enjoyed the benefit of six amps. Moreover, had you known that you would be charged for them, you would not have used the extra two amps. There are various cases in the Gemara and poskim which are similar to yours, where someone inadvertently used something which he would not have used had he known the true facts.
One case in the Gemara (Kesubos 34B, Bava Kama 112A) is orphans whose father had borrowed a cow. When the father passed away he did not alert the orphans to the fact that the cow was borrowed. The orphans slaughtered the cow and ate the meat thinking that the cow was theirs. The Gemara rules that they have to pay the true owner only two thirds of the value of the cow. Rashi (Bava Kama) explains that the reason they pay only two thirds is because if the orphans had known that the cow wasn’t theirs they wouldn’t have slaughtered it and would have eaten something cheaper.
The Rashbam (Bava Basra 146B) explains that the reason they pay two thirds is that we assume that even one who normally does not eat the better and more expensive product would eat it and spend the money if he could buy it for two thirds of the normal price.
It is important to understand what they are paying for. Tosafos (Bava Kama 27B) writes that the orphans do not have to pay damages to the owner of the cow since their mistake is excusable. Therefore, as far as damages are concerned their damages are classified as an oness for which Tosafos maintain that a person is not liable and we don’t say odom mu’ad le’olam.
According to the version of Tosafos of the Rashash, Tosafos say they have to pay for the benefit they derive. Even if one does not have this version of Tosafos we see from the Gemara (Bava Kama 20A) that paying two thirds of the full price is the amount one who is not liable for damages pays for the benefit he derived. The Gemara says this in reference to an animal that ate another person’s food that was located in the public thoroughfare. The Torah says the animal’s owner is not liable for damages. However, the Gemara writes that the owner has to pay for the benefit he derived and the Gemara says the amount he must pay is two thirds of the full-price.
Thus, we have established a very important principle: that when one damages and at the same time benefits from the damage that he did, if the damage is excusable he does not pay for the damages but he must pay for the benefit that he derived.
Another case that is discussed in Shulchan Aruch (246, 17) is where a person invited someone and later on asked to be paid for the food that his guest ate. The Rama writes that if it is clear that originally the host meant to give a present and only later due to a souring of relations he asked to be paid, then he is entitled to nothing. However, if that was not the case he is entitled to payment. The Ketsos (246, 1) explains that the guest does not have to pay for damages since he was given permission to eat. The amount that he must pay is only for the benefit he derived from eating the meal.
There is another case that is discussed in Shulchan Aruch (363, 10) from where we can derive another important principle. The Shulchan Aruch rules that one who rented a property at a cheap price from someone who pretended the property was his, must pay the full price, which the owner normally charged, to the true owner. The Ketsos (363, 7) asks why he is required to pay the full price since he thought he had to pay less. It should be equivalent to the orphans who slaughtered the animal their father left them. The Ulam Hamishpot answers that when one rents a house he should find out who the owner is. Therefore, it is not an oness, like orphans who blamelessly assumed the cow belonged to their father.
While there are many who do not agree in this case with the Ulam Hamishpot, nevertheless, the principle is correct. It is only where the damage is totally excusable that one has to pay only for the benefit he derived.
It would seem that your situation is comparable to the cow of the orphans since you had no reason to suspect that you were given a six ampere fuse when you only ordered four amps.
Having established that you do not have to pay the full price of the extra two amperes, we have to determine how much you are required to pay.
We have seen that you have to pay for all the benefits you derived from use of the generator. One benefit is that you saved on your gas bill. The second benefit is, as you wrote, that your wife preferred a hotplate to gas. We saw that concerning food Chazal estimated that normally one who does not eat the more expensive food, would eat and pay for the more expensive food if he could purchase it at a one third discount. Therefore, you have to estimate how much extra you would have paid the generator each month in order to use a hotplate instead of gas and pay that, in addition to the cost of gas, to the generator organizer. Of course, you needn’t pay more than the cost of two amps.
We should note that it is clear from the Gemara (Bava Basra 4B) that this has to be determined on an individual basis. We see this because there is a case where the Gemara states that one who derived benefit can say, “I would have been satisfied with a flimsy low-cost wall.”
In conclusion: You are obligated to pay for all the benefits you derived from having used the generator for your hotplate, but in no case should it be more than the original cost of an additional two amperes.