Hi
Depends what you want it to do-if it's just to cover the alarm etc then a little portable one propped up inside your window should keep it topped up once it's fully charge but won't charge it if low. If you want it to use while you're away off hookup then would suggest a solar panel on the roof, which will continuously charge all the daylight hours and even replenish a low battery (but not totally flat)
I have a 100 watt panel on the roof & a 110a battery and this gives us enough power for lights, tv, showers fans & pump on a daily basis but gets low if we have TV on more than about 3 hrs. However it will recharge in about 2 hrs in daylight so is back up by morning.
Our dealer had a special offer of £349 for the 100watt panel, regulator etc including fitting & VAT which so far has proved value for money.
We are off to Spain soon so will be interesting to find out if the electricity usage is less this year than previous years with this unit and not using the charger/hair straighteners/TV on mains etc.
Hope this helps
Mike