it sounds like the mowers engine is defently an upgrade, being an over head valve design, also v-belts are extreamly ineffcient, so if your new mower has fewer belts, or better designed tensioners you are going to save alot of energy that way as well.
we have a Toro Care-free electric mower that has a small deep cycle battery, I think it's about $30 for a battery, and this one is on it's 3rd year, we've pluged the charger in to a kill-a-watt meter (check your libery) and if I remember right it costs less then 15 cents to charge it from fully drained, we mostly use it to trim around the garden, trees, and other small areas, it's a great mower, extreamly simple, you have a battery, a motor with the blade on it, a switch to turn it on, and a thermo overload protector, keep the blade sharp and it will mow all kinds of tall grass.
we also just picked up a sickle bar mower that should alow us to cut the grass less often, because it dosn't care how long it is, it just shears it off at the set hight, and has a 4.5hp engine that runs at a fast idle as it hardly has any load even cutting thick tall grass and weeds.
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